push v0.1.3 version commit bd2ea47

detectron2/modeling/meta_arch/rcnn.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import logging
+import numpy as np
+import torch
+from torch import nn
+
+from detectron2.structures import ImageList
+from detectron2.utils.events import get_event_storage
+from detectron2.utils.logger import log_first_n
+
+from ..backbone import build_backbone
+from ..postprocessing import detector_postprocess
+from ..proposal_generator import build_proposal_generator
+from ..roi_heads import build_roi_heads
+from .build import META_ARCH_REGISTRY
+
+__all__ = ["GeneralizedRCNN", "ProposalNetwork"]
+
+
+@META_ARCH_REGISTRY.register()
+class GeneralizedRCNN(nn.Module):
+    """
+    Generalized R-CNN. Any models that contains the following three components:
+    1. Per-image feature extraction (aka backbone)
+    2. Region proposal generation
+    3. Per-region feature extraction and prediction
+    """
+
+    def __init__(self, cfg):
+        super().__init__()
+
+        self.backbone = build_backbone(cfg)
+        self.proposal_generator = build_proposal_generator(cfg, self.backbone.output_shape())
+        self.roi_heads = build_roi_heads(cfg, self.backbone.output_shape())
+        self.vis_period = cfg.VIS_PERIOD
+        self.input_format = cfg.INPUT.FORMAT
+
+        assert len(cfg.MODEL.PIXEL_MEAN) == len(cfg.MODEL.PIXEL_STD)
+        self.register_buffer("pixel_mean", torch.Tensor(cfg.MODEL.PIXEL_MEAN).view(-1, 1, 1))
+        self.register_buffer("pixel_std", torch.Tensor(cfg.MODEL.PIXEL_STD).view(-1, 1, 1))
+
+    @property
+    def device(self):
+        return self.pixel_mean.device
+
+    def visualize_training(self, batched_inputs, proposals):
+        """
+        A function used to visualize images and proposals. It shows ground truth
+        bounding boxes on the original image and up to 20 predicted object
+        proposals on the original image. Users can implement different
+        visualization functions for different models.
+
+        Args:
+            batched_inputs (list): a list that contains input to the model.
+            proposals (list): a list that contains predicted proposals. Both
+                batched_inputs and proposals should have the same length.
+        """
+        from detectron2.utils.visualizer import Visualizer
+
+        storage = get_event_storage()
+        max_vis_prop = 20
+
+        for input, prop in zip(batched_inputs, proposals):
+            img = input["image"].cpu().numpy()
+            assert img.shape[0] == 3, "Images should have 3 channels."
+            if self.input_format == "BGR":
+                img = img[::-1, :, :]
+            img = img.transpose(1, 2, 0)
+            v_gt = Visualizer(img, None)
+            v_gt = v_gt.overlay_instances(boxes=input["instances"].gt_boxes)
+            anno_img = v_gt.get_image()
+            box_size = min(len(prop.proposal_boxes), max_vis_prop)
+            v_pred = Visualizer(img, None)
+            v_pred = v_pred.overlay_instances(
+                boxes=prop.proposal_boxes[0:box_size].tensor.cpu().numpy()
+            )
+            prop_img = v_pred.get_image()
+            vis_img = np.concatenate((anno_img, prop_img), axis=1)
+            vis_img = vis_img.transpose(2, 0, 1)
+            vis_name = "Left: GT bounding boxes;  Right: Predicted proposals"
+            storage.put_image(vis_name, vis_img)
+            break  # only visualize one image in a batch
+
+    def forward(self, batched_inputs):
+        """
+        Args:
+            batched_inputs: a list, batched outputs of :class:`DatasetMapper` .
+                Each item in the list contains the inputs for one image.
+                For now, each item in the list is a dict that contains:
+
+                * image: Tensor, image in (C, H, W) format.
+                * instances (optional): groundtruth :class:`Instances`
+                * proposals (optional): :class:`Instances`, precomputed proposals.
+
+                Other information that's included in the original dicts, such as:
+
+                * "height", "width" (int): the output resolution of the model, used in inference.
+                  See :meth:`postprocess` for details.
+
+        Returns:
+            list[dict]:
+                Each dict is the output for one input image.
+                The dict contains one key "instances" whose value is a :class:`Instances`.
+                The :class:`Instances` object has the following keys:
+                "pred_boxes", "pred_classes", "scores", "pred_masks", "pred_keypoints"
+        """
+        if not self.training:
+            return self.inference(batched_inputs)
+
+        images = self.preprocess_image(batched_inputs)
+        if "instances" in batched_inputs[0]:
+            gt_instances = [x["instances"].to(self.device) for x in batched_inputs]
+        elif "targets" in batched_inputs[0]:
+            log_first_n(
+                logging.WARN, "'targets' in the model inputs is now renamed to 'instances'!", n=10
+            )
+            gt_instances = [x["targets"].to(self.device) for x in batched_inputs]
+        else:
+            gt_instances = None
+
+        features = self.backbone(images.tensor)
+
+        if self.proposal_generator:
+            proposals, proposal_losses = self.proposal_generator(images, features, gt_instances)
+        else:
+            assert "proposals" in batched_inputs[0]
+            proposals = [x["proposals"].to(self.device) for x in batched_inputs]
+            proposal_losses = {}
+
+        _, detector_losses = self.roi_heads(images, features, proposals, gt_instances)
+        if self.vis_period > 0:
+            storage = get_event_storage()
+            if storage.iter % self.vis_period == 0:
+                self.visualize_training(batched_inputs, proposals)
+
+        losses = {}
+        losses.update(detector_losses)
+        losses.update(proposal_losses)
+        return losses
+
+    def inference(self, batched_inputs, detected_instances=None, do_postprocess=True):
+        """
+        Run inference on the given inputs.
+
+        Args:
+            batched_inputs (list[dict]): same as in :meth:`forward`
+            detected_instances (None or list[Instances]): if not None, it
+                contains an `Instances` object per image. The `Instances`
+                object contains "pred_boxes" and "pred_classes" which are
+                known boxes in the image.
+                The inference will then skip the detection of bounding boxes,
+                and only predict other per-ROI outputs.
+            do_postprocess (bool): whether to apply post-processing on the outputs.
+
+        Returns:
+            same as in :meth:`forward`.
+        """
+        assert not self.training
+
+        images = self.preprocess_image(batched_inputs)
+        features = self.backbone(images.tensor)
+
+        if detected_instances is None:
+            if self.proposal_generator:
+                proposals, _ = self.proposal_generator(images, features, None)
+            else:
+                assert "proposals" in batched_inputs[0]
+                proposals = [x["proposals"].to(self.device) for x in batched_inputs]
+
+            results, _ = self.roi_heads(images, features, proposals, None)
+        else:
+            detected_instances = [x.to(self.device) for x in detected_instances]
+            results = self.roi_heads.forward_with_given_boxes(features, detected_instances)
+
+        if do_postprocess:
+            return GeneralizedRCNN._postprocess(results, batched_inputs, images.image_sizes)
+        else:
+            return results
+
+    def preprocess_image(self, batched_inputs):
+        """
+        Normalize, pad and batch the input images.
+        """
+        images = [x["image"].to(self.device) for x in batched_inputs]
+        images = [(x - self.pixel_mean) / self.pixel_std for x in images]
+        images = ImageList.from_tensors(images, self.backbone.size_divisibility)
+        return images
+
+    @staticmethod
+    def _postprocess(instances, batched_inputs, image_sizes):
+        """
+        Rescale the output instances to the target size.
+        """
+        # note: private function; subject to changes
+        processed_results = []
+        for results_per_image, input_per_image, image_size in zip(
+            instances, batched_inputs, image_sizes
+        ):
+            height = input_per_image.get("height", image_size[0])
+            width = input_per_image.get("width", image_size[1])
+            r = detector_postprocess(results_per_image, height, width)
+            processed_results.append({"instances": r})
+        return processed_results
+
+
+@META_ARCH_REGISTRY.register()
+class ProposalNetwork(nn.Module):
+    """
+    A meta architecture that only predicts object proposals.
+    """
+
+    def __init__(self, cfg):
+        super().__init__()
+        self.backbone = build_backbone(cfg)
+        self.proposal_generator = build_proposal_generator(cfg, self.backbone.output_shape())
+
+        self.register_buffer("pixel_mean", torch.Tensor(cfg.MODEL.PIXEL_MEAN).view(-1, 1, 1))
+        self.register_buffer("pixel_std", torch.Tensor(cfg.MODEL.PIXEL_STD).view(-1, 1, 1))
+
+    @property
+    def device(self):
+        return self.pixel_mean.device
+
+    def forward(self, batched_inputs):
+        """
+        Args:
+            Same as in :class:`GeneralizedRCNN.forward`
+
+        Returns:
+            list[dict]:
+                Each dict is the output for one input image.
+                The dict contains one key "proposals" whose value is a
+                :class:`Instances` with keys "proposal_boxes" and "objectness_logits".
+        """
+        images = [x["image"].to(self.device) for x in batched_inputs]
+        images = [(x - self.pixel_mean) / self.pixel_std for x in images]
+        images = ImageList.from_tensors(images, self.backbone.size_divisibility)
+        features = self.backbone(images.tensor)
+
+        if "instances" in batched_inputs[0]:
+            gt_instances = [x["instances"].to(self.device) for x in batched_inputs]
+        elif "targets" in batched_inputs[0]:
+            log_first_n(
+                logging.WARN, "'targets' in the model inputs is now renamed to 'instances'!", n=10
+            )
+            gt_instances = [x["targets"].to(self.device) for x in batched_inputs]
+        else:
+            gt_instances = None
+        proposals, proposal_losses = self.proposal_generator(images, features, gt_instances)
+        # In training, the proposals are not useful at all but we generate them anyway.
+        # This makes RPN-only models about 5% slower.
+        if self.training:
+            return proposal_losses
+
+        processed_results = []
+        for results_per_image, input_per_image, image_size in zip(
+            proposals, batched_inputs, images.image_sizes
+        ):
+            height = input_per_image.get("height", image_size[0])
+            width = input_per_image.get("width", image_size[1])
+            r = detector_postprocess(results_per_image, height, width)
+            processed_results.append({"proposals": r})
+        return processed_results

detectron2/modeling/meta_arch/retinanet.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import logging
+import math
+import numpy as np
+from typing import List
+import torch
+from fvcore.nn import sigmoid_focal_loss_jit, smooth_l1_loss
+from torch import nn
+
+from detectron2.layers import ShapeSpec, batched_nms, cat
+from detectron2.structures import Boxes, ImageList, Instances, pairwise_iou
+from detectron2.utils.events import get_event_storage
+from detectron2.utils.logger import log_first_n
+
+from ..anchor_generator import build_anchor_generator
+from ..backbone import build_backbone
+from ..box_regression import Box2BoxTransform
+from ..matcher import Matcher
+from ..postprocessing import detector_postprocess
+from .build import META_ARCH_REGISTRY
+
+__all__ = ["RetinaNet"]
+
+
+def permute_to_N_HWA_K(tensor, K):
+    """
+    Transpose/reshape a tensor from (N, (A x K), H, W) to (N, (HxWxA), K)
+    """
+    assert tensor.dim() == 4, tensor.shape
+    N, _, H, W = tensor.shape
+    tensor = tensor.view(N, -1, K, H, W)
+    tensor = tensor.permute(0, 3, 4, 1, 2)
+    tensor = tensor.reshape(N, -1, K)  # Size=(N,HWA,K)
+    return tensor
+
+
+def permute_all_cls_and_box_to_N_HWA_K_and_concat(box_cls, box_delta, num_classes=80):
+    """
+    Rearrange the tensor layout from the network output, i.e.:
+    list[Tensor]: #lvl tensors of shape (N, A x K, Hi, Wi)
+    to per-image predictions, i.e.:
+    Tensor: of shape (N x sum(Hi x Wi x A), K)
+    """
+    # for each feature level, permute the outputs to make them be in the
+    # same format as the labels. Note that the labels are computed for
+    # all feature levels concatenated, so we keep the same representation
+    # for the objectness and the box_delta
+    box_cls_flattened = [permute_to_N_HWA_K(x, num_classes) for x in box_cls]
+    box_delta_flattened = [permute_to_N_HWA_K(x, 4) for x in box_delta]
+    # concatenate on the first dimension (representing the feature levels), to
+    # take into account the way the labels were generated (with all feature maps
+    # being concatenated as well)
+    box_cls = cat(box_cls_flattened, dim=1).view(-1, num_classes)
+    box_delta = cat(box_delta_flattened, dim=1).view(-1, 4)
+    return box_cls, box_delta
+
+
+@META_ARCH_REGISTRY.register()
+class RetinaNet(nn.Module):
+    """
+    Implement RetinaNet in :paper:`RetinaNet`.
+    """
+
+    def __init__(self, cfg):
+        super().__init__()
+
+        # fmt: off
+        self.num_classes              = cfg.MODEL.RETINANET.NUM_CLASSES
+        self.in_features              = cfg.MODEL.RETINANET.IN_FEATURES
+        # Loss parameters:
+        self.focal_loss_alpha         = cfg.MODEL.RETINANET.FOCAL_LOSS_ALPHA
+        self.focal_loss_gamma         = cfg.MODEL.RETINANET.FOCAL_LOSS_GAMMA
+        self.smooth_l1_loss_beta      = cfg.MODEL.RETINANET.SMOOTH_L1_LOSS_BETA
+        # Inference parameters:
+        self.score_threshold          = cfg.MODEL.RETINANET.SCORE_THRESH_TEST
+        self.topk_candidates          = cfg.MODEL.RETINANET.TOPK_CANDIDATES_TEST
+        self.nms_threshold            = cfg.MODEL.RETINANET.NMS_THRESH_TEST
+        self.max_detections_per_image = cfg.TEST.DETECTIONS_PER_IMAGE
+        # Vis parameters
+        self.vis_period               = cfg.VIS_PERIOD
+        self.input_format             = cfg.INPUT.FORMAT
+        # fmt: on
+
+        self.backbone = build_backbone(cfg)
+
+        backbone_shape = self.backbone.output_shape()
+        feature_shapes = [backbone_shape[f] for f in self.in_features]
+        self.head = RetinaNetHead(cfg, feature_shapes)
+        self.anchor_generator = build_anchor_generator(cfg, feature_shapes)
+
+        # Matching and loss
+        self.box2box_transform = Box2BoxTransform(weights=cfg.MODEL.RPN.BBOX_REG_WEIGHTS)
+        self.matcher = Matcher(
+            cfg.MODEL.RETINANET.IOU_THRESHOLDS,
+            cfg.MODEL.RETINANET.IOU_LABELS,
+            allow_low_quality_matches=True,
+        )
+
+        self.register_buffer("pixel_mean", torch.Tensor(cfg.MODEL.PIXEL_MEAN).view(-1, 1, 1))
+        self.register_buffer("pixel_std", torch.Tensor(cfg.MODEL.PIXEL_STD).view(-1, 1, 1))
+
+        """
+        In Detectron1, loss is normalized by number of foreground samples in the batch.
+        When batch size is 1 per GPU, #foreground has a large variance and
+        using it lead to lower performance. Here we maintain an EMA of #foreground to
+        stabilize the normalizer.
+        """
+        self.loss_normalizer = 100  # initialize with any reasonable #fg that's not too small
+        self.loss_normalizer_momentum = 0.9
+
+    @property
+    def device(self):
+        return self.pixel_mean.device
+
+    def visualize_training(self, batched_inputs, results):
+        """
+        A function used to visualize ground truth images and final network predictions.
+        It shows ground truth bounding boxes on the original image and up to 20
+        predicted object bounding boxes on the original image.
+
+        Args:
+            batched_inputs (list): a list that contains input to the model.
+            results (List[Instances]): a list of #images elements.
+        """
+        from detectron2.utils.visualizer import Visualizer
+
+        assert len(batched_inputs) == len(
+            results
+        ), "Cannot visualize inputs and results of different sizes"
+        storage = get_event_storage()
+        max_boxes = 20
+
+        image_index = 0  # only visualize a single image
+        img = batched_inputs[image_index]["image"].cpu().numpy()
+        assert img.shape[0] == 3, "Images should have 3 channels."
+        if self.input_format == "BGR":
+            img = img[::-1, :, :]
+        img = img.transpose(1, 2, 0)
+        v_gt = Visualizer(img, None)
+        v_gt = v_gt.overlay_instances(boxes=batched_inputs[image_index]["instances"].gt_boxes)
+        anno_img = v_gt.get_image()
+        processed_results = detector_postprocess(results[image_index], img.shape[0], img.shape[1])
+        predicted_boxes = processed_results.pred_boxes.tensor.detach().cpu().numpy()
+
+        v_pred = Visualizer(img, None)
+        v_pred = v_pred.overlay_instances(boxes=predicted_boxes[0:max_boxes])
+        prop_img = v_pred.get_image()
+        vis_img = np.vstack((anno_img, prop_img))
+        vis_img = vis_img.transpose(2, 0, 1)
+        vis_name = f"Top: GT bounding boxes; Bottom: {max_boxes} Highest Scoring Results"
+        storage.put_image(vis_name, vis_img)
+
+    def forward(self, batched_inputs):
+        """
+        Args:
+            batched_inputs: a list, batched outputs of :class:`DatasetMapper` .
+                Each item in the list contains the inputs for one image.
+                For now, each item in the list is a dict that contains:
+
+                * image: Tensor, image in (C, H, W) format.
+                * instances: Instances
+
+                Other information that's included in the original dicts, such as:
+
+                * "height", "width" (int): the output resolution of the model, used in inference.
+                  See :meth:`postprocess` for details.
+        Returns:
+            dict[str: Tensor]:
+                mapping from a named loss to a tensor storing the loss. Used during training only.
+        """
+        images = self.preprocess_image(batched_inputs)
+        if "instances" in batched_inputs[0]:
+            gt_instances = [x["instances"].to(self.device) for x in batched_inputs]
+        elif "targets" in batched_inputs[0]:
+            log_first_n(
+                logging.WARN, "'targets' in the model inputs is now renamed to 'instances'!", n=10
+            )
+            gt_instances = [x["targets"].to(self.device) for x in batched_inputs]
+        else:
+            gt_instances = None
+
+        features = self.backbone(images.tensor)
+        features = [features[f] for f in self.in_features]
+        box_cls, box_delta = self.head(features)
+        anchors = self.anchor_generator(features)
+
+        if self.training:
+            gt_classes, gt_anchors_reg_deltas = self.get_ground_truth(anchors, gt_instances)
+            losses = self.losses(gt_classes, gt_anchors_reg_deltas, box_cls, box_delta)
+
+            if self.vis_period > 0:
+                storage = get_event_storage()
+                if storage.iter % self.vis_period == 0:
+                    results = self.inference(box_cls, box_delta, anchors, images.image_sizes)
+                    self.visualize_training(batched_inputs, results)
+
+            return losses
+        else:
+            results = self.inference(box_cls, box_delta, anchors, images.image_sizes)
+            processed_results = []
+            for results_per_image, input_per_image, image_size in zip(
+                results, batched_inputs, images.image_sizes
+            ):
+                height = input_per_image.get("height", image_size[0])
+                width = input_per_image.get("width", image_size[1])
+                r = detector_postprocess(results_per_image, height, width)
+                processed_results.append({"instances": r})
+            return processed_results
+
+    def losses(self, gt_classes, gt_anchors_deltas, pred_class_logits, pred_anchor_deltas):
+        """
+        Args:
+            For `gt_classes` and `gt_anchors_deltas` parameters, see
+                :meth:`RetinaNet.get_ground_truth`.
+            Their shapes are (N, R) and (N, R, 4), respectively, where R is
+            the total number of anchors across levels, i.e. sum(Hi x Wi x A)
+            For `pred_class_logits` and `pred_anchor_deltas`, see
+                :meth:`RetinaNetHead.forward`.
+
+        Returns:
+            dict[str, Tensor]:
+                mapping from a named loss to a scalar tensor
+                storing the loss. Used during training only. The dict keys are:
+                "loss_cls" and "loss_box_reg"
+        """
+        pred_class_logits, pred_anchor_deltas = permute_all_cls_and_box_to_N_HWA_K_and_concat(
+            pred_class_logits, pred_anchor_deltas, self.num_classes
+        )  # Shapes: (N x R, K) and (N x R, 4), respectively.
+
+        gt_classes = gt_classes.flatten()
+        gt_anchors_deltas = gt_anchors_deltas.view(-1, 4)
+
+        valid_idxs = gt_classes >= 0
+        foreground_idxs = (gt_classes >= 0) & (gt_classes != self.num_classes)
+        num_foreground = foreground_idxs.sum().item()
+        get_event_storage().put_scalar("num_foreground", num_foreground)
+        self.loss_normalizer = (
+            self.loss_normalizer_momentum * self.loss_normalizer
+            + (1 - self.loss_normalizer_momentum) * num_foreground
+        )
+
+        gt_classes_target = torch.zeros_like(pred_class_logits)
+        gt_classes_target[foreground_idxs, gt_classes[foreground_idxs]] = 1
+
+        # logits loss
+        loss_cls = sigmoid_focal_loss_jit(
+            pred_class_logits[valid_idxs],
+            gt_classes_target[valid_idxs],
+            alpha=self.focal_loss_alpha,
+            gamma=self.focal_loss_gamma,
+            reduction="sum",
+        ) / max(1, self.loss_normalizer)
+
+        # regression loss
+        loss_box_reg = smooth_l1_loss(
+            pred_anchor_deltas[foreground_idxs],
+            gt_anchors_deltas[foreground_idxs],
+            beta=self.smooth_l1_loss_beta,
+            reduction="sum",
+        ) / max(1, self.loss_normalizer)
+
+        return {"loss_cls": loss_cls, "loss_box_reg": loss_box_reg}
+
+    @torch.no_grad()
+    def get_ground_truth(self, anchors, targets):
+        """
+        Args:
+            anchors (list[Boxes]): A list of #feature level Boxes.
+                The Boxes contains anchors of this image on the specific feature level.
+            targets (list[Instances]): a list of N `Instances`s. The i-th
+                `Instances` contains the ground-truth per-instance annotations
+                for the i-th input image.  Specify `targets` during training only.
+
+        Returns:
+            gt_classes (Tensor):
+                An integer tensor of shape (N, R) storing ground-truth labels for each anchor.
+                R is the total number of anchors, i.e. the sum of Hi x Wi x A for all levels.
+                Anchors with an IoU with some target higher than the foreground threshold
+                are assigned their corresponding label in the [0, K-1] range.
+                Anchors whose IoU are below the background threshold are assigned
+                the label "K". Anchors whose IoU are between the foreground and background
+                thresholds are assigned a label "-1", i.e. ignore.
+            gt_anchors_deltas (Tensor):
+                Shape (N, R, 4).
+                The last dimension represents ground-truth box2box transform
+                targets (dx, dy, dw, dh) that map each anchor to its matched ground-truth box.
+                The values in the tensor are meaningful only when the corresponding
+                anchor is labeled as foreground.
+        """
+        gt_classes = []
+        gt_anchors_deltas = []
+        anchors = Boxes.cat(anchors)  # Rx4
+
+        for targets_per_image in targets:
+            match_quality_matrix = pairwise_iou(targets_per_image.gt_boxes, anchors)
+            gt_matched_idxs, anchor_labels = self.matcher(match_quality_matrix)
+
+            has_gt = len(targets_per_image) > 0
+            if has_gt:
+                # ground truth box regression
+                matched_gt_boxes = targets_per_image.gt_boxes[gt_matched_idxs]
+                gt_anchors_reg_deltas_i = self.box2box_transform.get_deltas(
+                    anchors.tensor, matched_gt_boxes.tensor
+                )
+
+                gt_classes_i = targets_per_image.gt_classes[gt_matched_idxs]
+                # Anchors with label 0 are treated as background.
+                gt_classes_i[anchor_labels == 0] = self.num_classes
+                # Anchors with label -1 are ignored.
+                gt_classes_i[anchor_labels == -1] = -1
+            else:
+                gt_classes_i = torch.zeros_like(gt_matched_idxs) + self.num_classes
+                gt_anchors_reg_deltas_i = torch.zeros_like(anchors.tensor)
+
+            gt_classes.append(gt_classes_i)
+            gt_anchors_deltas.append(gt_anchors_reg_deltas_i)
+
+        return torch.stack(gt_classes), torch.stack(gt_anchors_deltas)
+
+    def inference(self, box_cls, box_delta, anchors, image_sizes):
+        """
+        Arguments:
+            box_cls, box_delta: Same as the output of :meth:`RetinaNetHead.forward`
+            anchors (list[Boxes]): A list of #feature level Boxes.
+                The Boxes contain anchors of this image on the specific feature level.
+            image_sizes (List[torch.Size]): the input image sizes
+
+        Returns:
+            results (List[Instances]): a list of #images elements.
+        """
+        results = []
+
+        box_cls = [permute_to_N_HWA_K(x, self.num_classes) for x in box_cls]
+        box_delta = [permute_to_N_HWA_K(x, 4) for x in box_delta]
+        # list[Tensor], one per level, each has shape (N, Hi x Wi x A, K or 4)
+
+        for img_idx, image_size in enumerate(image_sizes):
+            box_cls_per_image = [box_cls_per_level[img_idx] for box_cls_per_level in box_cls]
+            box_reg_per_image = [box_reg_per_level[img_idx] for box_reg_per_level in box_delta]
+            results_per_image = self.inference_single_image(
+                box_cls_per_image, box_reg_per_image, anchors, tuple(image_size)
+            )
+            results.append(results_per_image)
+        return results
+
+    def inference_single_image(self, box_cls, box_delta, anchors, image_size):
+        """
+        Single-image inference. Return bounding-box detection results by thresholding
+        on scores and applying non-maximum suppression (NMS).
+
+        Arguments:
+            box_cls (list[Tensor]): list of #feature levels. Each entry contains
+                tensor of size (H x W x A, K)
+            box_delta (list[Tensor]): Same shape as 'box_cls' except that K becomes 4.
+            anchors (list[Boxes]): list of #feature levels. Each entry contains
+                a Boxes object, which contains all the anchors for that
+                image in that feature level.
+            image_size (tuple(H, W)): a tuple of the image height and width.
+
+        Returns:
+            Same as `inference`, but for only one image.
+        """
+        boxes_all = []
+        scores_all = []
+        class_idxs_all = []
+
+        # Iterate over every feature level
+        for box_cls_i, box_reg_i, anchors_i in zip(box_cls, box_delta, anchors):
+            # (HxWxAxK,)
+            box_cls_i = box_cls_i.flatten().sigmoid_()
+
+            # Keep top k top scoring indices only.
+            num_topk = min(self.topk_candidates, box_reg_i.size(0))
+            # torch.sort is actually faster than .topk (at least on GPUs)
+            predicted_prob, topk_idxs = box_cls_i.sort(descending=True)
+            predicted_prob = predicted_prob[:num_topk]
+            topk_idxs = topk_idxs[:num_topk]
+
+            # filter out the proposals with low confidence score
+            keep_idxs = predicted_prob > self.score_threshold
+            predicted_prob = predicted_prob[keep_idxs]
+            topk_idxs = topk_idxs[keep_idxs]
+
+            anchor_idxs = topk_idxs // self.num_classes
+            classes_idxs = topk_idxs % self.num_classes
+
+            box_reg_i = box_reg_i[anchor_idxs]
+            anchors_i = anchors_i[anchor_idxs]
+            # predict boxes
+            predicted_boxes = self.box2box_transform.apply_deltas(box_reg_i, anchors_i.tensor)
+
+            boxes_all.append(predicted_boxes)
+            scores_all.append(predicted_prob)
+            class_idxs_all.append(classes_idxs)
+
+        boxes_all, scores_all, class_idxs_all = [
+            cat(x) for x in [boxes_all, scores_all, class_idxs_all]
+        ]
+        keep = batched_nms(boxes_all, scores_all, class_idxs_all, self.nms_threshold)
+        keep = keep[: self.max_detections_per_image]
+
+        result = Instances(image_size)
+        result.pred_boxes = Boxes(boxes_all[keep])
+        result.scores = scores_all[keep]
+        result.pred_classes = class_idxs_all[keep]
+        return result
+
+    def preprocess_image(self, batched_inputs):
+        """
+        Normalize, pad and batch the input images.
+        """
+        images = [x["image"].to(self.device) for x in batched_inputs]
+        images = [(x - self.pixel_mean) / self.pixel_std for x in images]
+        images = ImageList.from_tensors(images, self.backbone.size_divisibility)
+        return images
+
+
+class RetinaNetHead(nn.Module):
+    """
+    The head used in RetinaNet for object classification and box regression.
+    It has two subnets for the two tasks, with a common structure but separate parameters.
+    """
+
+    def __init__(self, cfg, input_shape: List[ShapeSpec]):
+        super().__init__()
+        # fmt: off
+        in_channels      = input_shape[0].channels
+        num_classes      = cfg.MODEL.RETINANET.NUM_CLASSES
+        num_convs        = cfg.MODEL.RETINANET.NUM_CONVS
+        prior_prob       = cfg.MODEL.RETINANET.PRIOR_PROB
+        num_anchors      = build_anchor_generator(cfg, input_shape).num_cell_anchors
+        # fmt: on
+        assert (
+            len(set(num_anchors)) == 1
+        ), "Using different number of anchors between levels is not currently supported!"
+        num_anchors = num_anchors[0]
+
+        cls_subnet = []
+        bbox_subnet = []
+        for _ in range(num_convs):
+            cls_subnet.append(
+                nn.Conv2d(in_channels, in_channels, kernel_size=3, stride=1, padding=1)
+            )
+            cls_subnet.append(nn.ReLU())
+            bbox_subnet.append(
+                nn.Conv2d(in_channels, in_channels, kernel_size=3, stride=1, padding=1)
+            )
+            bbox_subnet.append(nn.ReLU())
+
+        self.cls_subnet = nn.Sequential(*cls_subnet)
+        self.bbox_subnet = nn.Sequential(*bbox_subnet)
+        self.cls_score = nn.Conv2d(
+            in_channels, num_anchors * num_classes, kernel_size=3, stride=1, padding=1
+        )
+        self.bbox_pred = nn.Conv2d(in_channels, num_anchors * 4, kernel_size=3, stride=1, padding=1)
+
+        # Initialization
+        for modules in [self.cls_subnet, self.bbox_subnet, self.cls_score, self.bbox_pred]:
+            for layer in modules.modules():
+                if isinstance(layer, nn.Conv2d):
+                    torch.nn.init.normal_(layer.weight, mean=0, std=0.01)
+                    torch.nn.init.constant_(layer.bias, 0)
+
+        # Use prior in model initialization to improve stability
+        bias_value = -(math.log((1 - prior_prob) / prior_prob))
+        torch.nn.init.constant_(self.cls_score.bias, bias_value)
+
+    def forward(self, features):
+        """
+        Arguments:
+            features (list[Tensor]): FPN feature map tensors in high to low resolution.
+                Each tensor in the list correspond to different feature levels.
+
+        Returns:
+            logits (list[Tensor]): #lvl tensors, each has shape (N, AxK, Hi, Wi).
+                The tensor predicts the classification probability
+                at each spatial position for each of the A anchors and K object
+                classes.
+            bbox_reg (list[Tensor]): #lvl tensors, each has shape (N, Ax4, Hi, Wi).
+                The tensor predicts 4-vector (dx,dy,dw,dh) box
+                regression values for every anchor. These values are the
+                relative offset between the anchor and the ground truth box.
+        """
+        logits = []
+        bbox_reg = []
+        for feature in features:
+            logits.append(self.cls_score(self.cls_subnet(feature)))
+            bbox_reg.append(self.bbox_pred(self.bbox_subnet(feature)))
+        return logits, bbox_reg

detectron2/modeling/meta_arch/semantic_seg.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import numpy as np
+from typing import Dict
+import fvcore.nn.weight_init as weight_init
+import torch
+from torch import nn
+from torch.nn import functional as F
+
+from detectron2.layers import Conv2d, ShapeSpec
+from detectron2.structures import ImageList
+from detectron2.utils.registry import Registry
+
+from ..backbone import build_backbone
+from ..postprocessing import sem_seg_postprocess
+from .build import META_ARCH_REGISTRY
+
+__all__ = ["SemanticSegmentor", "SEM_SEG_HEADS_REGISTRY", "SemSegFPNHead", "build_sem_seg_head"]
+
+
+SEM_SEG_HEADS_REGISTRY = Registry("SEM_SEG_HEADS")
+SEM_SEG_HEADS_REGISTRY.__doc__ = """
+Registry for semantic segmentation heads, which make semantic segmentation predictions
+from feature maps.
+"""
+
+
+@META_ARCH_REGISTRY.register()
+class SemanticSegmentor(nn.Module):
+    """
+    Main class for semantic segmentation architectures.
+    """
+
+    def __init__(self, cfg):
+        super().__init__()
+        self.backbone = build_backbone(cfg)
+        self.sem_seg_head = build_sem_seg_head(cfg, self.backbone.output_shape())
+        self.register_buffer("pixel_mean", torch.Tensor(cfg.MODEL.PIXEL_MEAN).view(-1, 1, 1))
+        self.register_buffer("pixel_std", torch.Tensor(cfg.MODEL.PIXEL_STD).view(-1, 1, 1))
+
+    @property
+    def device(self):
+        return self.pixel_mean.device
+
+    def forward(self, batched_inputs):
+        """
+        Args:
+            batched_inputs: a list, batched outputs of :class:`DatasetMapper`.
+                Each item in the list contains the inputs for one image.
+
+                For now, each item in the list is a dict that contains:
+
+                   * "image": Tensor, image in (C, H, W) format.
+                   * "sem_seg": semantic segmentation ground truth
+                   * Other information that's included in the original dicts, such as:
+                     "height", "width" (int): the output resolution of the model, used in inference.
+                     See :meth:`postprocess` for details.
+
+        Returns:
+            list[dict]:
+              Each dict is the output for one input image.
+              The dict contains one key "sem_seg" whose value is a
+              Tensor that represents the
+              per-pixel segmentation prediced by the head.
+              The prediction has shape KxHxW that represents the logits of
+              each class for each pixel.
+        """
+        images = [x["image"].to(self.device) for x in batched_inputs]
+        images = [(x - self.pixel_mean) / self.pixel_std for x in images]
+        images = ImageList.from_tensors(images, self.backbone.size_divisibility)
+
+        features = self.backbone(images.tensor)
+
+        if "sem_seg" in batched_inputs[0]:
+            targets = [x["sem_seg"].to(self.device) for x in batched_inputs]
+            targets = ImageList.from_tensors(
+                targets, self.backbone.size_divisibility, self.sem_seg_head.ignore_value
+            ).tensor
+        else:
+            targets = None
+        results, losses = self.sem_seg_head(features, targets)
+
+        if self.training:
+            return losses
+
+        processed_results = []
+        for result, input_per_image, image_size in zip(results, batched_inputs, images.image_sizes):
+            height = input_per_image.get("height")
+            width = input_per_image.get("width")
+            r = sem_seg_postprocess(result, image_size, height, width)
+            processed_results.append({"sem_seg": r})
+        return processed_results
+
+
+def build_sem_seg_head(cfg, input_shape):
+    """
+    Build a semantic segmentation head from `cfg.MODEL.SEM_SEG_HEAD.NAME`.
+    """
+    name = cfg.MODEL.SEM_SEG_HEAD.NAME
+    return SEM_SEG_HEADS_REGISTRY.get(name)(cfg, input_shape)
+
+
+@SEM_SEG_HEADS_REGISTRY.register()
+class SemSegFPNHead(nn.Module):
+    """
+    A semantic segmentation head described in :paper:`PanopticFPN`.
+    It takes FPN features as input and merges information from all
+    levels of the FPN into single output.
+    """
+
+    def __init__(self, cfg, input_shape: Dict[str, ShapeSpec]):
+        super().__init__()
+
+        # fmt: off
+        self.in_features      = cfg.MODEL.SEM_SEG_HEAD.IN_FEATURES
+        feature_strides       = {k: v.stride for k, v in input_shape.items()}
+        feature_channels      = {k: v.channels for k, v in input_shape.items()}
+        self.ignore_value     = cfg.MODEL.SEM_SEG_HEAD.IGNORE_VALUE
+        num_classes           = cfg.MODEL.SEM_SEG_HEAD.NUM_CLASSES
+        conv_dims             = cfg.MODEL.SEM_SEG_HEAD.CONVS_DIM
+        self.common_stride    = cfg.MODEL.SEM_SEG_HEAD.COMMON_STRIDE
+        norm                  = cfg.MODEL.SEM_SEG_HEAD.NORM
+        self.loss_weight      = cfg.MODEL.SEM_SEG_HEAD.LOSS_WEIGHT
+        # fmt: on
+
+        self.scale_heads = []
+        for in_feature in self.in_features:
+            head_ops = []
+            head_length = max(
+                1, int(np.log2(feature_strides[in_feature]) - np.log2(self.common_stride))
+            )
+            for k in range(head_length):
+                norm_module = nn.GroupNorm(32, conv_dims) if norm == "GN" else None
+                conv = Conv2d(
+                    feature_channels[in_feature] if k == 0 else conv_dims,
+                    conv_dims,
+                    kernel_size=3,
+                    stride=1,
+                    padding=1,
+                    bias=not norm,
+                    norm=norm_module,
+                    activation=F.relu,
+                )
+                weight_init.c2_msra_fill(conv)
+                head_ops.append(conv)
+                if feature_strides[in_feature] != self.common_stride:
+                    head_ops.append(
+                        nn.Upsample(scale_factor=2, mode="bilinear", align_corners=False)
+                    )
+            self.scale_heads.append(nn.Sequential(*head_ops))
+            self.add_module(in_feature, self.scale_heads[-1])
+        self.predictor = Conv2d(conv_dims, num_classes, kernel_size=1, stride=1, padding=0)
+        weight_init.c2_msra_fill(self.predictor)
+
+    def forward(self, features, targets=None):
+        """
+        Returns:
+            In training, returns (None, dict of losses)
+            In inference, returns (CxHxW logits, {})
+        """
+        x = self.layers(features)
+        if self.training:
+            return None, self.losses(x, targets)
+        else:
+            x = F.interpolate(
+                x, scale_factor=self.common_stride, mode="bilinear", align_corners=False
+            )
+            return x, {}
+
+    def layers(self, features):
+        for i, f in enumerate(self.in_features):
+            if i == 0:
+                x = self.scale_heads[i](features[f])
+            else:
+                x = x + self.scale_heads[i](features[f])
+        x = self.predictor(x)
+        return x
+
+    def losses(self, predictions, targets):
+        predictions = F.interpolate(
+            predictions, scale_factor=self.common_stride, mode="bilinear", align_corners=False
+        )
+        loss = F.cross_entropy(
+            predictions, targets, reduction="mean", ignore_index=self.ignore_value
+        )
+        losses = {"loss_sem_seg": loss * self.loss_weight}
+        return losses

detectron2/modeling/poolers.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+import math
+import sys
+import torch
+from torch import nn
+from torchvision.ops import RoIPool
+
+from detectron2.layers import ROIAlign, ROIAlignRotated, cat
+
+__all__ = ["ROIPooler"]
+
+
+def assign_boxes_to_levels(box_lists, min_level, max_level, canonical_box_size, canonical_level):
+    """
+    Map each box in `box_lists` to a feature map level index and return the assignment
+    vector.
+
+    Args:
+        box_lists (list[Boxes] | list[RotatedBoxes]): A list of N Boxes or N RotatedBoxes,
+            where N is the number of images in the batch.
+        min_level (int): Smallest feature map level index. The input is considered index 0,
+            the output of stage 1 is index 1, and so.
+        max_level (int): Largest feature map level index.
+        canonical_box_size (int): A canonical box size in pixels (sqrt(box area)).
+        canonical_level (int): The feature map level index on which a canonically-sized box
+            should be placed.
+
+    Returns:
+        A tensor of length M, where M is the total number of boxes aggregated over all
+            N batch images. The memory layout corresponds to the concatenation of boxes
+            from all images. Each element is the feature map index, as an offset from
+            `self.min_level`, for the corresponding box (so value i means the box is at
+            `self.min_level + i`).
+    """
+    eps = sys.float_info.epsilon
+    box_sizes = torch.sqrt(cat([boxes.area() for boxes in box_lists]))
+    # Eqn.(1) in FPN paper
+    level_assignments = torch.floor(
+        canonical_level + torch.log2(box_sizes / canonical_box_size + eps)
+    )
+    # clamp level to (min, max), in case the box size is too large or too small
+    # for the available feature maps
+    level_assignments = torch.clamp(level_assignments, min=min_level, max=max_level)
+    return level_assignments.to(torch.int64) - min_level
+
+
+def convert_boxes_to_pooler_format(box_lists):
+    """
+    Convert all boxes in `box_lists` to the low-level format used by ROI pooling ops
+    (see description under Returns).
+
+    Args:
+        box_lists (list[Boxes] | list[RotatedBoxes]):
+            A list of N Boxes or N RotatedBoxes, where N is the number of images in the batch.
+
+    Returns:
+        When input is list[Boxes]:
+            A tensor of shape (M, 5), where M is the total number of boxes aggregated over all
+            N batch images.
+            The 5 columns are (batch index, x0, y0, x1, y1), where batch index
+            is the index in [0, N) identifying which batch image the box with corners at
+            (x0, y0, x1, y1) comes from.
+        When input is list[RotatedBoxes]:
+            A tensor of shape (M, 6), where M is the total number of boxes aggregated over all
+            N batch images.
+            The 6 columns are (batch index, x_ctr, y_ctr, width, height, angle_degrees),
+            where batch index is the index in [0, N) identifying which batch image the
+            rotated box (x_ctr, y_ctr, width, height, angle_degrees) comes from.
+    """
+
+    def fmt_box_list(box_tensor, batch_index):
+        repeated_index = torch.full(
+            (len(box_tensor), 1), batch_index, dtype=box_tensor.dtype, device=box_tensor.device
+        )
+        return cat((repeated_index, box_tensor), dim=1)
+
+    pooler_fmt_boxes = cat(
+        [fmt_box_list(box_list.tensor, i) for i, box_list in enumerate(box_lists)], dim=0
+    )
+
+    return pooler_fmt_boxes
+
+
+class ROIPooler(nn.Module):
+    """
+    Region of interest feature map pooler that supports pooling from one or more
+    feature maps.
+    """
+
+    def __init__(
+        self,
+        output_size,
+        scales,
+        sampling_ratio,
+        pooler_type,
+        canonical_box_size=224,
+        canonical_level=4,
+    ):
+        """
+        Args:
+            output_size (int, tuple[int] or list[int]): output size of the pooled region,
+                e.g., 14 x 14. If tuple or list is given, the length must be 2.
+            scales (list[float]): The scale for each low-level pooling op relative to
+                the input image. For a feature map with stride s relative to the input
+                image, scale is defined as a 1 / s. The stride must be power of 2.
+                When there are multiple scales, they must form a pyramid, i.e. they must be
+                a monotically decreasing geometric sequence with a factor of 1/2.
+            sampling_ratio (int): The `sampling_ratio` parameter for the ROIAlign op.
+            pooler_type (string): Name of the type of pooling operation that should be applied.
+                For instance, "ROIPool" or "ROIAlignV2".
+            canonical_box_size (int): A canonical box size in pixels (sqrt(box area)). The default
+                is heuristically defined as 224 pixels in the FPN paper (based on ImageNet
+                pre-training).
+            canonical_level (int): The feature map level index from which a canonically-sized box
+                should be placed. The default is defined as level 4 (stride=16) in the FPN paper,
+                i.e., a box of size 224x224 will be placed on the feature with stride=16.
+                The box placement for all boxes will be determined from their sizes w.r.t
+                canonical_box_size. For example, a box whose area is 4x that of a canonical box
+                should be used to pool features from feature level ``canonical_level+1``.
+
+                Note that the actual input feature maps given to this module may not have
+                sufficiently many levels for the input boxes. If the boxes are too large or too
+                small for the input feature maps, the closest level will be used.
+        """
+        super().__init__()
+
+        if isinstance(output_size, int):
+            output_size = (output_size, output_size)
+        assert len(output_size) == 2
+        assert isinstance(output_size[0], int) and isinstance(output_size[1], int)
+        self.output_size = output_size
+
+        if pooler_type == "ROIAlign":
+            self.level_poolers = nn.ModuleList(
+                ROIAlign(
+                    output_size, spatial_scale=scale, sampling_ratio=sampling_ratio, aligned=False
+                )
+                for scale in scales
+            )
+        elif pooler_type == "ROIAlignV2":
+            self.level_poolers = nn.ModuleList(
+                ROIAlign(
+                    output_size, spatial_scale=scale, sampling_ratio=sampling_ratio, aligned=True
+                )
+                for scale in scales
+            )
+        elif pooler_type == "ROIPool":
+            self.level_poolers = nn.ModuleList(
+                RoIPool(output_size, spatial_scale=scale) for scale in scales
+            )
+        elif pooler_type == "ROIAlignRotated":
+            self.level_poolers = nn.ModuleList(
+                ROIAlignRotated(output_size, spatial_scale=scale, sampling_ratio=sampling_ratio)
+                for scale in scales
+            )
+        else:
+            raise ValueError("Unknown pooler type: {}".format(pooler_type))
+
+        # Map scale (defined as 1 / stride) to its feature map level under the
+        # assumption that stride is a power of 2.
+        min_level = -(math.log2(scales[0]))
+        max_level = -(math.log2(scales[-1]))
+        assert math.isclose(min_level, int(min_level)) and math.isclose(
+            max_level, int(max_level)
+        ), "Featuremap stride is not power of 2!"
+        self.min_level = int(min_level)
+        self.max_level = int(max_level)
+        assert (
+            len(scales) == self.max_level - self.min_level + 1
+        ), "[ROIPooler] Sizes of input featuremaps do not form a pyramid!"
+        assert 0 < self.min_level and self.min_level <= self.max_level
+        self.canonical_level = canonical_level
+        assert canonical_box_size > 0
+        self.canonical_box_size = canonical_box_size
+
+    def forward(self, x, box_lists):
+        """
+        Args:
+            x (list[Tensor]): A list of feature maps of NCHW shape, with scales matching those
+                used to construct this module.
+            box_lists (list[Boxes] | list[RotatedBoxes]):
+                A list of N Boxes or N RotatedBoxes, where N is the number of images in the batch.
+                The box coordinates are defined on the original image and
+                will be scaled by the `scales` argument of :class:`ROIPooler`.
+
+        Returns:
+            Tensor:
+                A tensor of shape (M, C, output_size, output_size) where M is the total number of
+                boxes aggregated over all N batch images and C is the number of channels in `x`.
+        """
+        num_level_assignments = len(self.level_poolers)
+
+        assert isinstance(x, list) and isinstance(
+            box_lists, list
+        ), "Arguments to pooler must be lists"
+        assert (
+            len(x) == num_level_assignments
+        ), "unequal value, num_level_assignments={}, but x is list of {} Tensors".format(
+            num_level_assignments, len(x)
+        )
+
+        assert len(box_lists) == x[0].size(
+            0
+        ), "unequal value, x[0] batch dim 0 is {}, but box_list has length {}".format(
+            x[0].size(0), len(box_lists)
+        )
+
+        pooler_fmt_boxes = convert_boxes_to_pooler_format(box_lists)
+
+        if num_level_assignments == 1:
+            return self.level_poolers[0](x[0], pooler_fmt_boxes)
+
+        level_assignments = assign_boxes_to_levels(
+            box_lists, self.min_level, self.max_level, self.canonical_box_size, self.canonical_level
+        )
+
+        num_boxes = len(pooler_fmt_boxes)
+        num_channels = x[0].shape[1]
+        output_size = self.output_size[0]
+
+        dtype, device = x[0].dtype, x[0].device
+        output = torch.zeros(
+            (num_boxes, num_channels, output_size, output_size), dtype=dtype, device=device
+        )
+
+        for level, (x_level, pooler) in enumerate(zip(x, self.level_poolers)):
+            inds = torch.nonzero(level_assignments == level, as_tuple=True)[0]
+            pooler_fmt_boxes_level = pooler_fmt_boxes[inds]
+            output[inds] = pooler(x_level, pooler_fmt_boxes_level)
+
+        return output

detectron2/modeling/postprocessing.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+from torch.nn import functional as F
+
+from detectron2.layers import paste_masks_in_image
+from detectron2.structures import Instances
+from detectron2.utils.memory import retry_if_cuda_oom
+
+
+def detector_postprocess(results, output_height, output_width, mask_threshold=0.5):
+    """
+    Resize the output instances.
+    The input images are often resized when entering an object detector.
+    As a result, we often need the outputs of the detector in a different
+    resolution from its inputs.
+
+    This function will resize the raw outputs of an R-CNN detector
+    to produce outputs according to the desired output resolution.
+
+    Args:
+        results (Instances): the raw outputs from the detector.
+            `results.image_size` contains the input image resolution the detector sees.
+            This object might be modified in-place.
+        output_height, output_width: the desired output resolution.
+
+    Returns:
+        Instances: the resized output from the model, based on the output resolution
+    """
+    scale_x, scale_y = (output_width / results.image_size[1], output_height / results.image_size[0])
+    results = Instances((output_height, output_width), **results.get_fields())
+
+    if results.has("pred_boxes"):
+        output_boxes = results.pred_boxes
+    elif results.has("proposal_boxes"):
+        output_boxes = results.proposal_boxes
+
+    output_boxes.scale(scale_x, scale_y)
+    output_boxes.clip(results.image_size)
+
+    results = results[output_boxes.nonempty()]
+
+    if results.has("pred_masks"):
+        results.pred_masks = retry_if_cuda_oom(paste_masks_in_image)(
+            results.pred_masks[:, 0, :, :],  # N, 1, M, M
+            results.pred_boxes,
+            results.image_size,
+            threshold=mask_threshold,
+        )
+
+    if results.has("pred_keypoints"):
+        results.pred_keypoints[:, :, 0] *= scale_x
+        results.pred_keypoints[:, :, 1] *= scale_y
+
+    return results
+
+
+def sem_seg_postprocess(result, img_size, output_height, output_width):
+    """
+    Return semantic segmentation predictions in the original resolution.
+
+    The input images are often resized when entering semantic segmentor. Moreover, in same
+    cases, they also padded inside segmentor to be divisible by maximum network stride.
+    As a result, we often need the predictions of the segmentor in a different
+    resolution from its inputs.
+
+    Args:
+        result (Tensor): semantic segmentation prediction logits. A tensor of shape (C, H, W),
+            where C is the number of classes, and H, W are the height and width of the prediction.
+        img_size (tuple): image size that segmentor is taking as input.
+        output_height, output_width: the desired output resolution.
+
+    Returns:
+        semantic segmentation prediction (Tensor): A tensor of the shape
+            (C, output_height, output_width) that contains per-pixel soft predictions.
+    """
+    result = result[:, : img_size[0], : img_size[1]].expand(1, -1, -1, -1)
+    result = F.interpolate(
+        result, size=(output_height, output_width), mode="bilinear", align_corners=False
+    )[0]
+    return result

detectron2/modeling/proposal_generator/__init__.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+from .build import PROPOSAL_GENERATOR_REGISTRY, build_proposal_generator
+from .rpn import RPN_HEAD_REGISTRY, build_rpn_head, RPN

detectron2/modeling/proposal_generator/build.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+from detectron2.utils.registry import Registry
+
+PROPOSAL_GENERATOR_REGISTRY = Registry("PROPOSAL_GENERATOR")
+PROPOSAL_GENERATOR_REGISTRY.__doc__ = """
+Registry for proposal generator, which produces object proposals from feature maps.
+
+The registered object will be called with `obj(cfg, input_shape)`.
+The call should return a `nn.Module` object.
+"""
+
+from . import rpn, rrpn  # noqa F401 isort:skip
+
+
+def build_proposal_generator(cfg, input_shape):
+    """
+    Build a proposal generator from `cfg.MODEL.PROPOSAL_GENERATOR.NAME`.
+    The name can be "PrecomputedProposals" to use no proposal generator.
+    """
+    name = cfg.MODEL.PROPOSAL_GENERATOR.NAME
+    if name == "PrecomputedProposals":
+        return None
+
+    return PROPOSAL_GENERATOR_REGISTRY.get(name)(cfg, input_shape)

detectron2/modeling/proposal_generator/proposal_utils.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import math
+import torch
+
+from detectron2.structures import Instances
+
+
+def add_ground_truth_to_proposals(gt_boxes, proposals):
+    """
+    Call `add_ground_truth_to_proposals_single_image` for all images.
+
+    Args:
+        gt_boxes(list[Boxes]): list of N elements. Element i is a Boxes
+            representing the gound-truth for image i.
+        proposals (list[Instances]): list of N elements. Element i is a Instances
+            representing the proposals for image i.
+
+    Returns:
+        list[Instances]: list of N Instances. Each is the proposals for the image,
+            with field "proposal_boxes" and "objectness_logits".
+    """
+    assert gt_boxes is not None
+
+    assert len(proposals) == len(gt_boxes)
+    if len(proposals) == 0:
+        return proposals
+
+    return [
+        add_ground_truth_to_proposals_single_image(gt_boxes_i, proposals_i)
+        for gt_boxes_i, proposals_i in zip(gt_boxes, proposals)
+    ]
+
+
+def add_ground_truth_to_proposals_single_image(gt_boxes, proposals):
+    """
+    Augment `proposals` with ground-truth boxes from `gt_boxes`.
+
+    Args:
+        Same as `add_ground_truth_to_proposals`, but with gt_boxes and proposals
+        per image.
+
+    Returns:
+        Same as `add_ground_truth_to_proposals`, but for only one image.
+    """
+    device = proposals.objectness_logits.device
+    # Concatenating gt_boxes with proposals requires them to have the same fields
+    # Assign all ground-truth boxes an objectness logit corresponding to P(object) \approx 1.
+    gt_logit_value = math.log((1.0 - 1e-10) / (1 - (1.0 - 1e-10)))
+
+    gt_logits = gt_logit_value * torch.ones(len(gt_boxes), device=device)
+    gt_proposal = Instances(proposals.image_size)
+
+    gt_proposal.proposal_boxes = gt_boxes
+    gt_proposal.objectness_logits = gt_logits
+    new_proposals = Instances.cat([proposals, gt_proposal])
+
+    return new_proposals

detectron2/modeling/proposal_generator/rpn.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+from typing import Dict, List
+import torch
+import torch.nn.functional as F
+from torch import nn
+
+from detectron2.config import configurable
+from detectron2.layers import ShapeSpec
+from detectron2.structures import Boxes, Instances, pairwise_iou
+from detectron2.utils.memory import retry_if_cuda_oom
+from detectron2.utils.registry import Registry
+
+from ..anchor_generator import build_anchor_generator
+from ..box_regression import Box2BoxTransform
+from ..matcher import Matcher
+from ..sampling import subsample_labels
+from .build import PROPOSAL_GENERATOR_REGISTRY
+from .rpn_outputs import RPNOutputs, find_top_rpn_proposals
+
+RPN_HEAD_REGISTRY = Registry("RPN_HEAD")
+RPN_HEAD_REGISTRY.__doc__ = """
+Registry for RPN heads, which take feature maps and perform
+objectness classification and bounding box regression for anchors.
+
+The registered object will be called with `obj(cfg, input_shape)`.
+The call should return a `nn.Module` object.
+"""
+
+
+def build_rpn_head(cfg, input_shape):
+    """
+    Build an RPN head defined by `cfg.MODEL.RPN.HEAD_NAME`.
+    """
+    name = cfg.MODEL.RPN.HEAD_NAME
+    return RPN_HEAD_REGISTRY.get(name)(cfg, input_shape)
+
+
+@RPN_HEAD_REGISTRY.register()
+class StandardRPNHead(nn.Module):
+    """
+    Standard RPN classification and regression heads described in :paper:`Faster R-CNN`.
+    Uses a 3x3 conv to produce a shared hidden state from which one 1x1 conv predicts
+    objectness logits for each anchor and a second 1x1 conv predicts bounding-box deltas
+    specifying how to deform each anchor into an object proposal.
+    """
+
+    @configurable
+    def __init__(self, *, in_channels: int, num_anchors: int, box_dim: int = 4):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            in_channels (int): number of input feature channels. When using multiple
+                input features, they must have the same number of channels.
+            num_anchors (int): number of anchors to predict for *each spatial position*
+                on the feature map. The total number of anchors for each
+                feature map will be `num_anchors * H * W`.
+            box_dim (int): dimension of a box, which is also the number of box regression
+                predictions to make for each anchor. An axis aligned box has
+                box_dim=4, while a rotated box has box_dim=5.
+        """
+        super().__init__()
+        # 3x3 conv for the hidden representation
+        self.conv = nn.Conv2d(in_channels, in_channels, kernel_size=3, stride=1, padding=1)
+        # 1x1 conv for predicting objectness logits
+        self.objectness_logits = nn.Conv2d(in_channels, num_anchors, kernel_size=1, stride=1)
+        # 1x1 conv for predicting box2box transform deltas
+        self.anchor_deltas = nn.Conv2d(in_channels, num_anchors * box_dim, kernel_size=1, stride=1)
+
+        for l in [self.conv, self.objectness_logits, self.anchor_deltas]:
+            nn.init.normal_(l.weight, std=0.01)
+            nn.init.constant_(l.bias, 0)
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        # Standard RPN is shared across levels:
+        in_channels = [s.channels for s in input_shape]
+        assert len(set(in_channels)) == 1, "Each level must have the same channel!"
+        in_channels = in_channels[0]
+
+        # RPNHead should take the same input as anchor generator
+        # NOTE: it assumes that creating an anchor generator does not have unwanted side effect.
+        anchor_generator = build_anchor_generator(cfg, input_shape)
+        num_anchors = anchor_generator.num_anchors
+        box_dim = anchor_generator.box_dim
+        assert (
+            len(set(num_anchors)) == 1
+        ), "Each level must have the same number of anchors per spatial position"
+        return {"in_channels": in_channels, "num_anchors": num_anchors[0], "box_dim": box_dim}
+
+    def forward(self, features):
+        """
+        Args:
+            features (list[Tensor]): list of feature maps
+
+        Returns:
+            list[Tensor]: A list of L elements.
+                Element i is a tensor of shape (N, A, Hi, Wi) representing
+                the predicted objectness logits for all anchors. A is the number of cell anchors.
+            list[Tensor]: A list of L elements. Element i is a tensor of shape
+                (N, A*box_dim, Hi, Wi) representing the predicted "deltas" used to transform anchors
+                to proposals.
+        """
+        pred_objectness_logits = []
+        pred_anchor_deltas = []
+        for x in features:
+            t = F.relu(self.conv(x))
+            pred_objectness_logits.append(self.objectness_logits(t))
+            pred_anchor_deltas.append(self.anchor_deltas(t))
+        return pred_objectness_logits, pred_anchor_deltas
+
+
+@PROPOSAL_GENERATOR_REGISTRY.register()
+class RPN(nn.Module):
+    """
+    Region Proposal Network, introduced by :paper:`Faster R-CNN`.
+    """
+
+    def __init__(self, cfg, input_shape: Dict[str, ShapeSpec]):
+        super().__init__()
+
+        # fmt: off
+        self.min_box_side_len     = cfg.MODEL.PROPOSAL_GENERATOR.MIN_SIZE
+        self.in_features          = cfg.MODEL.RPN.IN_FEATURES
+        self.nms_thresh           = cfg.MODEL.RPN.NMS_THRESH
+        self.batch_size_per_image = cfg.MODEL.RPN.BATCH_SIZE_PER_IMAGE
+        self.positive_fraction    = cfg.MODEL.RPN.POSITIVE_FRACTION
+        self.smooth_l1_beta       = cfg.MODEL.RPN.SMOOTH_L1_BETA
+        self.loss_weight          = cfg.MODEL.RPN.LOSS_WEIGHT
+        # fmt: on
+
+        # Map from self.training state to train/test settings
+        self.pre_nms_topk = {
+            True: cfg.MODEL.RPN.PRE_NMS_TOPK_TRAIN,
+            False: cfg.MODEL.RPN.PRE_NMS_TOPK_TEST,
+        }
+        self.post_nms_topk = {
+            True: cfg.MODEL.RPN.POST_NMS_TOPK_TRAIN,
+            False: cfg.MODEL.RPN.POST_NMS_TOPK_TEST,
+        }
+        self.boundary_threshold = cfg.MODEL.RPN.BOUNDARY_THRESH
+
+        self.anchor_generator = build_anchor_generator(
+            cfg, [input_shape[f] for f in self.in_features]
+        )
+        self.box2box_transform = Box2BoxTransform(weights=cfg.MODEL.RPN.BBOX_REG_WEIGHTS)
+        self.anchor_matcher = Matcher(
+            cfg.MODEL.RPN.IOU_THRESHOLDS, cfg.MODEL.RPN.IOU_LABELS, allow_low_quality_matches=True
+        )
+        self.rpn_head = build_rpn_head(cfg, [input_shape[f] for f in self.in_features])
+
+    def _subsample_labels(self, label):
+        """
+        Randomly sample a subset of positive and negative examples, and overwrite
+        the label vector to the ignore value (-1) for all elements that are not
+        included in the sample.
+
+        Args:
+            labels (Tensor): a vector of -1, 0, 1. Will be modified in-place and returned.
+        """
+        pos_idx, neg_idx = subsample_labels(
+            label, self.batch_size_per_image, self.positive_fraction, 0
+        )
+        # Fill with the ignore label (-1), then set positive and negative labels
+        label.fill_(-1)
+        label.scatter_(0, pos_idx, 1)
+        label.scatter_(0, neg_idx, 0)
+        return label
+
+    @torch.no_grad()
+    def label_and_sample_anchors(self, anchors: List[Boxes], gt_instances: List[Instances]):
+        """
+        Args:
+            anchors (list[Boxes]): anchors for each feature map.
+            gt_instances: the ground-truth instances for each image.
+
+        Returns:
+            list[Tensor]:
+                List of #img tensors. i-th element is a vector of labels whose length is
+                the total number of anchors across feature maps. Label values are in {-1, 0, 1},
+                with meanings: -1 = ignore; 0 = negative class; 1 = positive class.
+            list[Tensor]:
+                i-th element is a Nx4 tensor, where N is the total number of anchors across
+                feature maps.  The values are the matched gt boxes for each anchor.
+                Values are undefined for those anchors not labeled as 1.
+        """
+        anchors = Boxes.cat(anchors)
+
+        gt_boxes = [x.gt_boxes for x in gt_instances]
+        image_sizes = [x.image_size for x in gt_instances]
+        del gt_instances
+
+        gt_labels = []
+        matched_gt_boxes = []
+        for image_size_i, gt_boxes_i in zip(image_sizes, gt_boxes):
+            """
+            image_size_i: (h, w) for the i-th image
+            gt_boxes_i: ground-truth boxes for i-th image
+            """
+
+            match_quality_matrix = retry_if_cuda_oom(pairwise_iou)(gt_boxes_i, anchors)
+            matched_idxs, gt_labels_i = retry_if_cuda_oom(self.anchor_matcher)(match_quality_matrix)
+            # Matching is memory-expensive and may result in CPU tensors. But the result is small
+            gt_labels_i = gt_labels_i.to(device=gt_boxes_i.device)
+            del match_quality_matrix
+
+            if self.boundary_threshold >= 0:
+                # Discard anchors that go out of the boundaries of the image
+                # NOTE: This is legacy functionality that is turned off by default in Detectron2
+                anchors_inside_image = anchors.inside_box(image_size_i, self.boundary_threshold)
+                gt_labels_i[~anchors_inside_image] = -1
+
+            # A vector of labels (-1, 0, 1) for each anchor
+            gt_labels_i = self._subsample_labels(gt_labels_i)
+
+            if len(gt_boxes_i) == 0:
+                # These values won't be used anyway since the anchor is labeled as background
+                matched_gt_boxes_i = torch.zeros_like(anchors.tensor)
+            else:
+                # TODO wasted indexing computation for ignored boxes
+                matched_gt_boxes_i = gt_boxes_i[matched_idxs].tensor
+
+            gt_labels.append(gt_labels_i)  # N,AHW
+            matched_gt_boxes.append(matched_gt_boxes_i)
+        return gt_labels, matched_gt_boxes
+
+    def forward(self, images, features, gt_instances=None):
+        """
+        Args:
+            images (ImageList): input images of length `N`
+            features (dict[str: Tensor]): input data as a mapping from feature
+                map name to tensor. Axis 0 represents the number of images `N` in
+                the input data; axes 1-3 are channels, height, and width, which may
+                vary between feature maps (e.g., if a feature pyramid is used).
+            gt_instances (list[Instances], optional): a length `N` list of `Instances`s.
+                Each `Instances` stores ground-truth instances for the corresponding image.
+
+        Returns:
+            proposals: list[Instances]: contains fields "proposal_boxes", "objectness_logits"
+            loss: dict[Tensor] or None
+        """
+        features = [features[f] for f in self.in_features]
+        pred_objectness_logits, pred_anchor_deltas = self.rpn_head(features)
+        anchors = self.anchor_generator(features)
+
+        if self.training:
+            gt_labels, gt_boxes = self.label_and_sample_anchors(anchors, gt_instances)
+        else:
+            gt_labels, gt_boxes = None, None
+
+        outputs = RPNOutputs(
+            self.box2box_transform,
+            self.batch_size_per_image,
+            images,
+            pred_objectness_logits,
+            pred_anchor_deltas,
+            anchors,
+            gt_labels,
+            gt_boxes,
+            self.smooth_l1_beta,
+        )
+
+        if self.training:
+            losses = {k: v * self.loss_weight for k, v in outputs.losses().items()}
+        else:
+            losses = {}
+
+        with torch.no_grad():
+            # Find the top proposals by applying NMS and removing boxes that
+            # are too small. The proposals are treated as fixed for approximate
+            # joint training with roi heads. This approach ignores the derivative
+            # w.r.t. the proposal boxes’ coordinates that are also network
+            # responses, so is approximate.
+            proposals = find_top_rpn_proposals(
+                outputs.predict_proposals(),
+                outputs.predict_objectness_logits(),
+                images,
+                self.nms_thresh,
+                self.pre_nms_topk[self.training],
+                self.post_nms_topk[self.training],
+                self.min_box_side_len,
+                self.training,
+            )
+
+        return proposals, losses

detectron2/modeling/proposal_generator/rpn_outputs.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import itertools
+import logging
+import torch
+import torch.nn.functional as F
+from fvcore.nn import smooth_l1_loss
+
+from detectron2.layers import batched_nms, cat
+from detectron2.structures import Boxes, Instances
+from detectron2.utils.events import get_event_storage
+
+logger = logging.getLogger(__name__)
+
+# TODO: comments for future refactoring of this module
+#
+# From @rbg:
+# This code involves a significant amount of tensor reshaping and permuting. Look for
+# ways to simplify this.
+
+"""
+Shape shorthand in this module:
+
+    N: number of images in the minibatch
+    L: number of feature maps per image on which RPN is run
+    A: number of cell anchors (must be the same for all feature maps)
+    Hi, Wi: height and width of the i-th feature map
+    4: size of the box parameterization
+
+Naming convention:
+
+    objectness: refers to the binary classification of an anchor as object vs. not
+    object.
+
+    deltas: refers to the 4-d (dx, dy, dw, dh) deltas that parameterize the box2box
+    transform (see :class:`box_regression.Box2BoxTransform`).
+
+    pred_objectness_logits: predicted objectness scores in [-inf, +inf]; use
+        sigmoid(pred_objectness_logits) to estimate P(object).
+
+    gt_labels: ground-truth binary classification labels for objectness
+
+    pred_anchor_deltas: predicted box2box transform deltas
+
+    gt_anchor_deltas: ground-truth box2box transform deltas
+"""
+
+
+def find_top_rpn_proposals(
+    proposals,
+    pred_objectness_logits,
+    images,
+    nms_thresh,
+    pre_nms_topk,
+    post_nms_topk,
+    min_box_side_len,
+    training,
+):
+    """
+    For each feature map, select the `pre_nms_topk` highest scoring proposals,
+    apply NMS, clip proposals, and remove small boxes. Return the `post_nms_topk`
+    highest scoring proposals among all the feature maps if `training` is True,
+    otherwise, returns the highest `post_nms_topk` scoring proposals for each
+    feature map.
+
+    Args:
+        proposals (list[Tensor]): A list of L tensors. Tensor i has shape (N, Hi*Wi*A, 4).
+            All proposal predictions on the feature maps.
+        pred_objectness_logits (list[Tensor]): A list of L tensors. Tensor i has shape (N, Hi*Wi*A).
+        images (ImageList): Input images as an :class:`ImageList`.
+        nms_thresh (float): IoU threshold to use for NMS
+        pre_nms_topk (int): number of top k scoring proposals to keep before applying NMS.
+            When RPN is run on multiple feature maps (as in FPN) this number is per
+            feature map.
+        post_nms_topk (int): number of top k scoring proposals to keep after applying NMS.
+            When RPN is run on multiple feature maps (as in FPN) this number is total,
+            over all feature maps.
+        min_box_side_len (float): minimum proposal box side length in pixels (absolute units
+            wrt input images).
+        training (bool): True if proposals are to be used in training, otherwise False.
+            This arg exists only to support a legacy bug; look for the "NB: Legacy bug ..."
+            comment.
+
+    Returns:
+        proposals (list[Instances]): list of N Instances. The i-th Instances
+            stores post_nms_topk object proposals for image i, sorted by their
+            objectness score in descending order.
+    """
+    image_sizes = images.image_sizes  # in (h, w) order
+    num_images = len(image_sizes)
+    device = proposals[0].device
+
+    # 1. Select top-k anchor for every level and every image
+    topk_scores = []  # #lvl Tensor, each of shape N x topk
+    topk_proposals = []
+    level_ids = []  # #lvl Tensor, each of shape (topk,)
+    batch_idx = torch.arange(num_images, device=device)
+    for level_id, proposals_i, logits_i in zip(
+        itertools.count(), proposals, pred_objectness_logits
+    ):
+        Hi_Wi_A = logits_i.shape[1]
+        num_proposals_i = min(pre_nms_topk, Hi_Wi_A)
+
+        # sort is faster than topk (https://github.com/pytorch/pytorch/issues/22812)
+        # topk_scores_i, topk_idx = logits_i.topk(num_proposals_i, dim=1)
+        logits_i, idx = logits_i.sort(descending=True, dim=1)
+        topk_scores_i = logits_i[batch_idx, :num_proposals_i]
+        topk_idx = idx[batch_idx, :num_proposals_i]
+
+        # each is N x topk
+        topk_proposals_i = proposals_i[batch_idx[:, None], topk_idx]  # N x topk x 4
+
+        topk_proposals.append(topk_proposals_i)
+        topk_scores.append(topk_scores_i)
+        level_ids.append(torch.full((num_proposals_i,), level_id, dtype=torch.int64, device=device))
+
+    # 2. Concat all levels together
+    topk_scores = cat(topk_scores, dim=1)
+    topk_proposals = cat(topk_proposals, dim=1)
+    level_ids = cat(level_ids, dim=0)
+
+    # 3. For each image, run a per-level NMS, and choose topk results.
+    results = []
+    for n, image_size in enumerate(image_sizes):
+        boxes = Boxes(topk_proposals[n])
+        scores_per_img = topk_scores[n]
+        lvl = level_ids
+
+        valid_mask = torch.isfinite(boxes.tensor).all(dim=1) & torch.isfinite(scores_per_img)
+        if not valid_mask.all():
+            if training:
+                raise FloatingPointError(
+                    "Predicted boxes or scores contain Inf/NaN. Training has diverged."
+                )
+            boxes = boxes[valid_mask]
+            scores_per_img = scores_per_img[valid_mask]
+            lvl = lvl[valid_mask]
+        boxes.clip(image_size)
+
+        # filter empty boxes
+        keep = boxes.nonempty(threshold=min_box_side_len)
+        if keep.sum().item() != len(boxes):
+            boxes, scores_per_img, lvl = boxes[keep], scores_per_img[keep], lvl[keep]
+
+        keep = batched_nms(boxes.tensor, scores_per_img, lvl, nms_thresh)
+        # In Detectron1, there was different behavior during training vs. testing.
+        # (https://github.com/facebookresearch/Detectron/issues/459)
+        # During training, topk is over the proposals from *all* images in the training batch.
+        # During testing, it is over the proposals for each image separately.
+        # As a result, the training behavior becomes batch-dependent,
+        # and the configuration "POST_NMS_TOPK_TRAIN" end up relying on the batch size.
+        # This bug is addressed in Detectron2 to make the behavior independent of batch size.
+        keep = keep[:post_nms_topk]  # keep is already sorted
+
+        res = Instances(image_size)
+        res.proposal_boxes = boxes[keep]
+        res.objectness_logits = scores_per_img[keep]
+        results.append(res)
+    return results
+
+
+def rpn_losses(
+    gt_labels, gt_anchor_deltas, pred_objectness_logits, pred_anchor_deltas, smooth_l1_beta
+):
+    """
+    Args:
+        gt_labels (Tensor): shape (N,), each element in {-1, 0, 1} representing
+            ground-truth objectness labels with: -1 = ignore; 0 = not object; 1 = object.
+        gt_anchor_deltas (Tensor): shape (N, box_dim), row i represents ground-truth
+            box2box transform targets (dx, dy, dw, dh) or (dx, dy, dw, dh, da) that map anchor i to
+            its matched ground-truth box.
+        pred_objectness_logits (Tensor): shape (N,), each element is a predicted objectness
+            logit.
+        pred_anchor_deltas (Tensor): shape (N, box_dim), each row is a predicted box2box
+            transform (dx, dy, dw, dh) or (dx, dy, dw, dh, da)
+        smooth_l1_beta (float): The transition point between L1 and L2 loss in
+            the smooth L1 loss function. When set to 0, the loss becomes L1. When
+            set to +inf, the loss becomes constant 0.
+
+    Returns:
+        objectness_loss, localization_loss, both unnormalized (summed over samples).
+    """
+    pos_masks = gt_labels == 1
+    localization_loss = smooth_l1_loss(
+        pred_anchor_deltas[pos_masks], gt_anchor_deltas[pos_masks], smooth_l1_beta, reduction="sum"
+    )
+
+    valid_masks = gt_labels >= 0
+    objectness_loss = F.binary_cross_entropy_with_logits(
+        pred_objectness_logits[valid_masks],
+        gt_labels[valid_masks].to(torch.float32),
+        reduction="sum",
+    )
+    return objectness_loss, localization_loss
+
+
+class RPNOutputs(object):
+    def __init__(
+        self,
+        box2box_transform,
+        batch_size_per_image,
+        images,
+        pred_objectness_logits,
+        pred_anchor_deltas,
+        anchors,
+        gt_labels=None,
+        gt_boxes=None,
+        smooth_l1_beta=0.0,
+    ):
+        """
+        Args:
+            box2box_transform (Box2BoxTransform): :class:`Box2BoxTransform` instance for
+                anchor-proposal transformations.
+            images (ImageList): :class:`ImageList` instance representing N input images
+            batch_size_per_image (int): number of proposals to sample when training
+            pred_objectness_logits (list[Tensor]): A list of L elements.
+                Element i is a tensor of shape (N, A, Hi, Wi) representing
+                the predicted objectness logits for anchors.
+            pred_anchor_deltas (list[Tensor]): A list of L elements. Element i is a tensor of shape
+                (N, A*4 or 5, Hi, Wi) representing the predicted "deltas" used to transform anchors
+                to proposals.
+            anchors (list[Boxes or RotatedBoxes]): A list of Boxes/RotatedBoxes storing the all
+                the anchors for each feature map. See :meth:`AnchorGenerator.forward`.
+            gt_labels (list[Tensor]): Available on in training.
+                See :meth:`RPN.label_and_sample_anchors`.
+            gt_boxes (list[Boxes or RotatedBoxes]): Available on in training.
+                See :meth:`RPN.label_and_sample_anchors`.
+            smooth_l1_beta (float): The transition point between L1 and L2 loss in
+                the smooth L1 loss function. When set to 0, the loss becomes L1. When
+                set to +inf, the loss becomes constant 0.
+        """
+        self.box2box_transform = box2box_transform
+        self.batch_size_per_image = batch_size_per_image
+
+        B = anchors[0].tensor.size(1)  # box dimension (4 or 5)
+        self.pred_objectness_logits = [
+            # Reshape: (N, A, Hi, Wi) -> (N, Hi, Wi, A) -> (N, Hi*Wi*A)
+            score.permute(0, 2, 3, 1).flatten(1)
+            for score in pred_objectness_logits
+        ]
+
+        self.pred_anchor_deltas = [
+            # Reshape: (N, A*B, Hi, Wi) -> (N, A, B, Hi, Wi) -> (N, Hi, Wi, A, B)
+            #          -> (N, Hi*Wi*A, B)
+            x.view(x.shape[0], -1, B, x.shape[-2], x.shape[-1])
+            .permute(0, 3, 4, 1, 2)
+            .flatten(1, -2)
+            for x in pred_anchor_deltas
+        ]
+
+        self.anchors = anchors
+
+        self.gt_boxes = gt_boxes
+        self.gt_labels = gt_labels
+
+        self.num_images = len(images)
+        self.smooth_l1_beta = smooth_l1_beta
+
+    def losses(self):
+        """
+        Return the losses from a set of RPN predictions and their associated ground-truth.
+
+        Returns:
+            dict[loss name -> loss value]: A dict mapping from loss name to loss value.
+                Loss names are: `loss_rpn_cls` for objectness classification and
+                `loss_rpn_loc` for proposal localization.
+        """
+        gt_labels = torch.stack(self.gt_labels)
+        anchors = self.anchors[0].cat(self.anchors).tensor  # Ax(4 or 5)
+        gt_anchor_deltas = [self.box2box_transform.get_deltas(anchors, k) for k in self.gt_boxes]
+        gt_anchor_deltas = torch.stack(gt_anchor_deltas)
+
+        # Log the number of positive/negative anchors per-image that's used in training
+        num_pos_anchors = (gt_labels == 1).sum().item()
+        num_neg_anchors = (gt_labels == 0).sum().item()
+        storage = get_event_storage()
+        storage.put_scalar("rpn/num_pos_anchors", num_pos_anchors / self.num_images)
+        storage.put_scalar("rpn/num_neg_anchors", num_neg_anchors / self.num_images)
+
+        objectness_loss, localization_loss = rpn_losses(
+            gt_labels,
+            gt_anchor_deltas,
+            # concat on the Hi*Wi*A dimension
+            cat(self.pred_objectness_logits, dim=1),
+            cat(self.pred_anchor_deltas, dim=1),
+            self.smooth_l1_beta,
+        )
+        normalizer = self.batch_size_per_image * self.num_images
+        return {
+            "loss_rpn_cls": objectness_loss / normalizer,
+            "loss_rpn_loc": localization_loss / normalizer,
+        }
+
+    def predict_proposals(self):
+        """
+        Transform anchors into proposals by applying the predicted anchor deltas.
+
+        Returns:
+            proposals (list[Tensor]): A list of L tensors. Tensor i has shape
+                (N, Hi*Wi*A, B), where B is box dimension (4 or 5).
+        """
+        proposals = []
+        # For each feature map
+        for anchors_i, pred_anchor_deltas_i in zip(self.anchors, self.pred_anchor_deltas):
+            B = anchors_i.tensor.size(1)
+            N = self.num_images
+            pred_anchor_deltas_i = pred_anchor_deltas_i.reshape(-1, B)
+            # Expand anchors to shape (N*Hi*Wi*A, B)
+            anchors_i = anchors_i.tensor.unsqueeze(0).expand(N, -1, -1).reshape(-1, B)
+            proposals_i = self.box2box_transform.apply_deltas(pred_anchor_deltas_i, anchors_i)
+            # Append feature map proposals with shape (N, Hi*Wi*A, B)
+            proposals.append(proposals_i.view(N, -1, B))
+        return proposals
+
+    def predict_objectness_logits(self):
+        """
+        Return objectness logits in the same format as the proposals returned by
+        :meth:`predict_proposals`.
+
+        Returns:
+            pred_objectness_logits (list[Tensor]): A list of L tensors. Tensor i has shape
+                (N, Hi*Wi*A).
+        """
+        return self.pred_objectness_logits

detectron2/modeling/proposal_generator/rrpn.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import itertools
+import logging
+from typing import Dict, List
+import torch
+
+from detectron2.layers import ShapeSpec, batched_nms_rotated, cat
+from detectron2.structures import Instances, RotatedBoxes, pairwise_iou_rotated
+from detectron2.utils.memory import retry_if_cuda_oom
+
+from ..box_regression import Box2BoxTransformRotated
+from .build import PROPOSAL_GENERATOR_REGISTRY
+from .rpn import RPN
+from .rpn_outputs import RPNOutputs
+
+logger = logging.getLogger(__name__)
+
+
+def find_top_rrpn_proposals(
+    proposals,
+    pred_objectness_logits,
+    images,
+    nms_thresh,
+    pre_nms_topk,
+    post_nms_topk,
+    min_box_side_len,
+    training,
+):
+    """
+    For each feature map, select the `pre_nms_topk` highest scoring proposals,
+    apply NMS, clip proposals, and remove small boxes. Return the `post_nms_topk`
+    highest scoring proposals among all the feature maps if `training` is True,
+    otherwise, returns the highest `post_nms_topk` scoring proposals for each
+    feature map.
+
+    Args:
+        proposals (list[Tensor]): A list of L tensors. Tensor i has shape (N, Hi*Wi*A, 5).
+            All proposal predictions on the feature maps.
+        pred_objectness_logits (list[Tensor]): A list of L tensors. Tensor i has shape (N, Hi*Wi*A).
+        images (ImageList): Input images as an :class:`ImageList`.
+        nms_thresh (float): IoU threshold to use for NMS
+        pre_nms_topk (int): number of top k scoring proposals to keep before applying NMS.
+            When RRPN is run on multiple feature maps (as in FPN) this number is per
+            feature map.
+        post_nms_topk (int): number of top k scoring proposals to keep after applying NMS.
+            When RRPN is run on multiple feature maps (as in FPN) this number is total,
+            over all feature maps.
+        min_box_side_len (float): minimum proposal box side length in pixels (absolute units
+            wrt input images).
+        training (bool): True if proposals are to be used in training, otherwise False.
+            This arg exists only to support a legacy bug; look for the "NB: Legacy bug ..."
+            comment.
+
+    Returns:
+        proposals (list[Instances]): list of N Instances. The i-th Instances
+            stores post_nms_topk object proposals for image i.
+    """
+    image_sizes = images.image_sizes  # in (h, w) order
+    num_images = len(image_sizes)
+    device = proposals[0].device
+
+    # 1. Select top-k anchor for every level and every image
+    topk_scores = []  # #lvl Tensor, each of shape N x topk
+    topk_proposals = []
+    level_ids = []  # #lvl Tensor, each of shape (topk,)
+    batch_idx = torch.arange(num_images, device=device)
+    for level_id, proposals_i, logits_i in zip(
+        itertools.count(), proposals, pred_objectness_logits
+    ):
+        Hi_Wi_A = logits_i.shape[1]
+        num_proposals_i = min(pre_nms_topk, Hi_Wi_A)
+
+        # sort is faster than topk (https://github.com/pytorch/pytorch/issues/22812)
+        # topk_scores_i, topk_idx = logits_i.topk(num_proposals_i, dim=1)
+        logits_i, idx = logits_i.sort(descending=True, dim=1)
+        topk_scores_i = logits_i[batch_idx, :num_proposals_i]
+        topk_idx = idx[batch_idx, :num_proposals_i]
+
+        # each is N x topk
+        topk_proposals_i = proposals_i[batch_idx[:, None], topk_idx]  # N x topk x 5
+
+        topk_proposals.append(topk_proposals_i)
+        topk_scores.append(topk_scores_i)
+        level_ids.append(torch.full((num_proposals_i,), level_id, dtype=torch.int64, device=device))
+
+    # 2. Concat all levels together
+    topk_scores = cat(topk_scores, dim=1)
+    topk_proposals = cat(topk_proposals, dim=1)
+    level_ids = cat(level_ids, dim=0)
+
+    # 3. For each image, run a per-level NMS, and choose topk results.
+    results = []
+    for n, image_size in enumerate(image_sizes):
+        boxes = RotatedBoxes(topk_proposals[n])
+        scores_per_img = topk_scores[n]
+        valid_mask = torch.isfinite(boxes.tensor).all(dim=1) & torch.isfinite(scores_per_img)
+        if not valid_mask.all():
+            boxes = boxes[valid_mask]
+            scores_per_img = scores_per_img[valid_mask]
+        boxes.clip(image_size)
+
+        # filter empty boxes
+        keep = boxes.nonempty(threshold=min_box_side_len)
+        lvl = level_ids
+        if keep.sum().item() != len(boxes):
+            boxes, scores_per_img, lvl = (boxes[keep], scores_per_img[keep], level_ids[keep])
+
+        keep = batched_nms_rotated(boxes.tensor, scores_per_img, lvl, nms_thresh)
+        # In Detectron1, there was different behavior during training vs. testing.
+        # (https://github.com/facebookresearch/Detectron/issues/459)
+        # During training, topk is over the proposals from *all* images in the training batch.
+        # During testing, it is over the proposals for each image separately.
+        # As a result, the training behavior becomes batch-dependent,
+        # and the configuration "POST_NMS_TOPK_TRAIN" end up relying on the batch size.
+        # This bug is addressed in Detectron2 to make the behavior independent of batch size.
+        keep = keep[:post_nms_topk]
+
+        res = Instances(image_size)
+        res.proposal_boxes = boxes[keep]
+        res.objectness_logits = scores_per_img[keep]
+        results.append(res)
+    return results
+
+
+@PROPOSAL_GENERATOR_REGISTRY.register()
+class RRPN(RPN):
+    """
+    Rotated Region Proposal Network described in :paper:`RRPN`.
+    """
+
+    def __init__(self, cfg, input_shape: Dict[str, ShapeSpec]):
+        super().__init__(cfg, input_shape)
+        self.box2box_transform = Box2BoxTransformRotated(weights=cfg.MODEL.RPN.BBOX_REG_WEIGHTS)
+        if self.boundary_threshold >= 0:
+            raise NotImplementedError(
+                "boundary_threshold is a legacy option not implemented for RRPN."
+            )
+
+    @torch.no_grad()
+    def label_and_sample_anchors(self, anchors: List[RotatedBoxes], gt_instances: List[Instances]):
+        """
+        Args:
+            anchors (list[RotatedBoxes]): anchors for each feature map.
+            gt_instances: the ground-truth instances for each image.
+
+        Returns:
+            list[Tensor]:
+                List of #img tensors. i-th element is a vector of labels whose length is
+                the total number of anchors across feature maps. Label values are in {-1, 0, 1},
+                with meanings: -1 = ignore; 0 = negative class; 1 = positive class.
+            list[Tensor]:
+                i-th element is a Nx5 tensor, where N is the total number of anchors across
+                feature maps.  The values are the matched gt boxes for each anchor.
+                Values are undefined for those anchors not labeled as 1.
+        """
+        anchors = RotatedBoxes.cat(anchors)
+
+        gt_boxes = [x.gt_boxes for x in gt_instances]
+        del gt_instances
+
+        gt_labels = []
+        matched_gt_boxes = []
+        for gt_boxes_i in gt_boxes:
+            """
+            gt_boxes_i: ground-truth boxes for i-th image
+            """
+            match_quality_matrix = retry_if_cuda_oom(pairwise_iou_rotated)(gt_boxes_i, anchors)
+            matched_idxs, gt_labels_i = retry_if_cuda_oom(self.anchor_matcher)(match_quality_matrix)
+            # Matching is memory-expensive and may result in CPU tensors. But the result is small
+            gt_labels_i = gt_labels_i.to(device=gt_boxes_i.device)
+
+            # A vector of labels (-1, 0, 1) for each anchor
+            gt_labels_i = self._subsample_labels(gt_labels_i)
+
+            if len(gt_boxes_i) == 0:
+                # These values won't be used anyway since the anchor is labeled as background
+                matched_gt_boxes_i = torch.zeros_like(anchors.tensor)
+            else:
+                # TODO wasted indexing computation for ignored boxes
+                matched_gt_boxes_i = gt_boxes_i[matched_idxs].tensor
+
+            gt_labels.append(gt_labels_i)  # N,AHW
+            matched_gt_boxes.append(matched_gt_boxes_i)
+        return gt_labels, matched_gt_boxes
+
+    def forward(self, images, features, gt_instances=None):
+        # same signature as RPN.forward
+        features = [features[f] for f in self.in_features]
+        pred_objectness_logits, pred_anchor_deltas = self.rpn_head(features)
+        anchors = self.anchor_generator(features)
+
+        if self.training:
+            gt_labels, gt_boxes = self.label_and_sample_anchors(anchors, gt_instances)
+        else:
+            gt_labels, gt_boxes = None, None
+
+        outputs = RPNOutputs(
+            self.box2box_transform,
+            self.batch_size_per_image,
+            images,
+            pred_objectness_logits,
+            pred_anchor_deltas,
+            anchors,
+            gt_labels,
+            gt_boxes,
+            self.smooth_l1_beta,
+        )
+
+        if self.training:
+            losses = {k: v * self.loss_weight for k, v in outputs.losses().items()}
+        else:
+            losses = {}
+
+        with torch.no_grad():
+            # Find the top proposals by applying NMS and removing boxes that
+            # are too small. The proposals are treated as fixed for approximate
+            # joint training with roi heads. This approach ignores the derivative
+            # w.r.t. the proposal boxes’ coordinates that are also network
+            # responses, so is approximate.
+
+            # Note: this line is the only difference v.s. RPN.forward
+            proposals = find_top_rrpn_proposals(
+                outputs.predict_proposals(),
+                outputs.predict_objectness_logits(),
+                images,
+                self.nms_thresh,
+                self.pre_nms_topk[self.training],
+                self.post_nms_topk[self.training],
+                self.min_box_side_len,
+                self.training,
+            )
+
+        return proposals, losses

detectron2/modeling/roi_heads/__init__.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+from .box_head import ROI_BOX_HEAD_REGISTRY, build_box_head
+from .keypoint_head import ROI_KEYPOINT_HEAD_REGISTRY, build_keypoint_head, BaseKeypointRCNNHead
+from .mask_head import ROI_MASK_HEAD_REGISTRY, build_mask_head, BaseMaskRCNNHead
+from .roi_heads import (
+    ROI_HEADS_REGISTRY,
+    ROIHeads,
+    Res5ROIHeads,
+    StandardROIHeads,
+    build_roi_heads,
+    select_foreground_proposals,
+)
+from .rotated_fast_rcnn import RROIHeads
+from .fast_rcnn import FastRCNNOutputLayers
+
+from . import cascade_rcnn  # isort:skip

detectron2/modeling/roi_heads/box_head.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import numpy as np
+from typing import List
+import fvcore.nn.weight_init as weight_init
+import torch
+from torch import nn
+from torch.nn import functional as F
+
+from detectron2.config import configurable
+from detectron2.layers import Conv2d, Linear, ShapeSpec, get_norm
+from detectron2.utils.registry import Registry
+
+ROI_BOX_HEAD_REGISTRY = Registry("ROI_BOX_HEAD")
+ROI_BOX_HEAD_REGISTRY.__doc__ = """
+Registry for box heads, which make box predictions from per-region features.
+
+The registered object will be called with `obj(cfg, input_shape)`.
+"""
+
+
+@ROI_BOX_HEAD_REGISTRY.register()
+class FastRCNNConvFCHead(nn.Module):
+    """
+    A head with several 3x3 conv layers (each followed by norm & relu) and then
+    several fc layers (each followed by relu).
+    """
+
+    @configurable
+    def __init__(
+        self, input_shape: ShapeSpec, *, conv_dims: List[int], fc_dims: List[int], conv_norm=""
+    ):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            input_shape (ShapeSpec): shape of the input feature.
+            conv_dims (list[int]): the output dimensions of the conv layers
+            fc_dims (list[int]): the output dimensions of the fc layers
+            conv_norm (str or callable): normalization for the conv layers.
+                See :func:`detectron2.layers.get_norm` for supported types.
+        """
+        super().__init__()
+        assert len(conv_dims) + len(fc_dims) > 0
+
+        self._output_size = (input_shape.channels, input_shape.height, input_shape.width)
+
+        self.conv_norm_relus = []
+        for k, conv_dim in enumerate(conv_dims):
+            conv = Conv2d(
+                self._output_size[0],
+                conv_dim,
+                kernel_size=3,
+                padding=1,
+                bias=not conv_norm,
+                norm=get_norm(conv_norm, conv_dim),
+                activation=F.relu,
+            )
+            self.add_module("conv{}".format(k + 1), conv)
+            self.conv_norm_relus.append(conv)
+            self._output_size = (conv_dim, self._output_size[1], self._output_size[2])
+
+        self.fcs = []
+        for k, fc_dim in enumerate(fc_dims):
+            fc = Linear(np.prod(self._output_size), fc_dim)
+            self.add_module("fc{}".format(k + 1), fc)
+            self.fcs.append(fc)
+            self._output_size = fc_dim
+
+        for layer in self.conv_norm_relus:
+            weight_init.c2_msra_fill(layer)
+        for layer in self.fcs:
+            weight_init.c2_xavier_fill(layer)
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        num_conv = cfg.MODEL.ROI_BOX_HEAD.NUM_CONV
+        conv_dim = cfg.MODEL.ROI_BOX_HEAD.CONV_DIM
+        num_fc = cfg.MODEL.ROI_BOX_HEAD.NUM_FC
+        fc_dim = cfg.MODEL.ROI_BOX_HEAD.FC_DIM
+        return {
+            "input_shape": input_shape,
+            "conv_dims": [conv_dim] * num_conv,
+            "fc_dims": [fc_dim] * num_fc,
+            "conv_norm": cfg.MODEL.ROI_BOX_HEAD.NORM,
+        }
+
+    def forward(self, x):
+        for layer in self.conv_norm_relus:
+            x = layer(x)
+        if len(self.fcs):
+            if x.dim() > 2:
+                x = torch.flatten(x, start_dim=1)
+            for layer in self.fcs:
+                x = F.relu(layer(x))
+        return x
+
+    @property
+    def output_shape(self):
+        """
+        Returns:
+            ShapeSpec: the output feature shape
+        """
+        o = self._output_size
+        if isinstance(o, int):
+            return ShapeSpec(channels=o)
+        else:
+            return ShapeSpec(channels=o[0], height=o[1], width=o[2])
+
+
+def build_box_head(cfg, input_shape):
+    """
+    Build a box head defined by `cfg.MODEL.ROI_BOX_HEAD.NAME`.
+    """
+    name = cfg.MODEL.ROI_BOX_HEAD.NAME
+    return ROI_BOX_HEAD_REGISTRY.get(name)(cfg, input_shape)

detectron2/modeling/roi_heads/cascade_rcnn.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+from typing import List
+import torch
+from torch import nn
+from torch.autograd.function import Function
+
+from detectron2.config import configurable
+from detectron2.layers import ShapeSpec
+from detectron2.structures import Boxes, Instances, pairwise_iou
+from detectron2.utils.events import get_event_storage
+
+from ..box_regression import Box2BoxTransform
+from ..matcher import Matcher
+from ..poolers import ROIPooler
+from .box_head import build_box_head
+from .fast_rcnn import FastRCNNOutputLayers, fast_rcnn_inference
+from .roi_heads import ROI_HEADS_REGISTRY, StandardROIHeads
+
+
+class _ScaleGradient(Function):
+    @staticmethod
+    def forward(ctx, input, scale):
+        ctx.scale = scale
+        return input
+
+    @staticmethod
+    def backward(ctx, grad_output):
+        return grad_output * ctx.scale, None
+
+
+@ROI_HEADS_REGISTRY.register()
+class CascadeROIHeads(StandardROIHeads):
+    """
+    Implement :paper:`Cascade R-CNN`.
+    """
+
+    @configurable
+    def __init__(
+        self,
+        *,
+        box_in_features: List[str],
+        box_pooler: ROIPooler,
+        box_heads: List[nn.Module],
+        box_predictors: List[nn.Module],
+        proposal_matchers: List[Matcher],
+        **kwargs,
+    ):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            box_pooler (ROIPooler): pooler that extracts region features from given boxes
+            box_heads (list[nn.Module]): box head for each cascade stage
+            box_predictors (list[nn.Module]): box predictor for each cascade stage
+            proposal_matchers (list[Matcher]): matcher with different IoU thresholds to
+                match boxes with ground truth for each stage. The first matcher matches
+                RPN proposals with ground truth, the other matchers use boxes predicted
+                by the previous stage as proposals and match them with ground truth.
+        """
+        assert "proposal_matcher" not in kwargs, (
+            "CascadeROIHeads takes 'proposal_matchers=' for each stage instead "
+            "of one 'proposal_matcher='."
+        )
+        # The first matcher matches RPN proposals with ground truth, done in the base class
+        kwargs["proposal_matcher"] = proposal_matchers[0]
+        num_stages = self.num_cascade_stages = len(box_heads)
+        box_heads = nn.ModuleList(box_heads)
+        box_predictors = nn.ModuleList(box_predictors)
+        assert len(box_predictors) == num_stages, f"{len(box_predictors)} != {num_stages}!"
+        assert len(proposal_matchers) == num_stages, f"{len(proposal_matchers)} != {num_stages}!"
+        super().__init__(
+            box_in_features=box_in_features,
+            box_pooler=box_pooler,
+            box_head=box_heads,
+            box_predictor=box_predictors,
+            **kwargs,
+        )
+        self.proposal_matchers = proposal_matchers
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        ret = super().from_config(cfg, input_shape)
+        ret.pop("proposal_matcher")
+        return ret
+
+    @classmethod
+    def _init_box_head(cls, cfg, input_shape):
+        # fmt: off
+        in_features              = cfg.MODEL.ROI_HEADS.IN_FEATURES
+        pooler_resolution        = cfg.MODEL.ROI_BOX_HEAD.POOLER_RESOLUTION
+        pooler_scales            = tuple(1.0 / input_shape[k].stride for k in in_features)
+        sampling_ratio           = cfg.MODEL.ROI_BOX_HEAD.POOLER_SAMPLING_RATIO
+        pooler_type              = cfg.MODEL.ROI_BOX_HEAD.POOLER_TYPE
+        cascade_bbox_reg_weights = cfg.MODEL.ROI_BOX_CASCADE_HEAD.BBOX_REG_WEIGHTS
+        cascade_ious             = cfg.MODEL.ROI_BOX_CASCADE_HEAD.IOUS
+        assert len(cascade_bbox_reg_weights) == len(cascade_ious)
+        assert cfg.MODEL.ROI_BOX_HEAD.CLS_AGNOSTIC_BBOX_REG,  \
+            "CascadeROIHeads only support class-agnostic regression now!"
+        assert cascade_ious[0] == cfg.MODEL.ROI_HEADS.IOU_THRESHOLDS[0]
+        # fmt: on
+
+        in_channels = [input_shape[f].channels for f in in_features]
+        # Check all channel counts are equal
+        assert len(set(in_channels)) == 1, in_channels
+        in_channels = in_channels[0]
+
+        box_pooler = ROIPooler(
+            output_size=pooler_resolution,
+            scales=pooler_scales,
+            sampling_ratio=sampling_ratio,
+            pooler_type=pooler_type,
+        )
+        pooled_shape = ShapeSpec(
+            channels=in_channels, width=pooler_resolution, height=pooler_resolution
+        )
+
+        box_heads, box_predictors, proposal_matchers = [], [], []
+        for match_iou, bbox_reg_weights in zip(cascade_ious, cascade_bbox_reg_weights):
+            box_head = build_box_head(cfg, pooled_shape)
+            box_heads.append(box_head)
+            box_predictors.append(
+                FastRCNNOutputLayers(
+                    cfg,
+                    box_head.output_shape,
+                    box2box_transform=Box2BoxTransform(weights=bbox_reg_weights),
+                )
+            )
+            proposal_matchers.append(Matcher([match_iou], [0, 1], allow_low_quality_matches=False))
+        return {
+            "box_in_features": in_features,
+            "box_pooler": box_pooler,
+            "box_heads": box_heads,
+            "box_predictors": box_predictors,
+            "proposal_matchers": proposal_matchers,
+        }
+
+    def forward(self, images, features, proposals, targets=None):
+        del images
+        if self.training:
+            proposals = self.label_and_sample_proposals(proposals, targets)
+
+        if self.training:
+            # Need targets to box head
+            losses = self._forward_box(features, proposals, targets)
+            losses.update(self._forward_mask(features, proposals))
+            losses.update(self._forward_keypoint(features, proposals))
+            return proposals, losses
+        else:
+            pred_instances = self._forward_box(features, proposals)
+            pred_instances = self.forward_with_given_boxes(features, pred_instances)
+            return pred_instances, {}
+
+    def _forward_box(self, features, proposals, targets=None):
+        """
+        Args:
+            features, targets: the same as in
+                Same as in :meth:`ROIHeads.forward`.
+            proposals (list[Instances]): the per-image object proposals with
+                their matching ground truth.
+                Each has fields "proposal_boxes", and "objectness_logits",
+                "gt_classes", "gt_boxes".
+        """
+        features = [features[f] for f in self.box_in_features]
+        head_outputs = []  # (predictor, predictions, proposals)
+        prev_pred_boxes = None
+        image_sizes = [x.image_size for x in proposals]
+        for k in range(self.num_cascade_stages):
+            if k > 0:
+                # The output boxes of the previous stage are used to create the input
+                # proposals of the next stage.
+                proposals = self._create_proposals_from_boxes(prev_pred_boxes, image_sizes)
+                if self.training:
+                    proposals = self._match_and_label_boxes(proposals, k, targets)
+            predictions = self._run_stage(features, proposals, k)
+            prev_pred_boxes = self.box_predictor[k].predict_boxes(predictions, proposals)
+            head_outputs.append((self.box_predictor[k], predictions, proposals))
+
+        if self.training:
+            losses = {}
+            storage = get_event_storage()
+            for stage, (predictor, predictions, proposals) in enumerate(head_outputs):
+                with storage.name_scope("stage{}".format(stage)):
+                    stage_losses = predictor.losses(predictions, proposals)
+                losses.update({k + "_stage{}".format(stage): v for k, v in stage_losses.items()})
+            return losses
+        else:
+            # Each is a list[Tensor] of length #image. Each tensor is Ri x (K+1)
+            scores_per_stage = [h[0].predict_probs(h[1], h[2]) for h in head_outputs]
+
+            # Average the scores across heads
+            scores = [
+                sum(list(scores_per_image)) * (1.0 / self.num_cascade_stages)
+                for scores_per_image in zip(*scores_per_stage)
+            ]
+            # Use the boxes of the last head
+            predictor, predictions, proposals = head_outputs[-1]
+            boxes = predictor.predict_boxes(predictions, proposals)
+            pred_instances, _ = fast_rcnn_inference(
+                boxes,
+                scores,
+                image_sizes,
+                predictor.test_score_thresh,
+                predictor.test_nms_thresh,
+                predictor.test_topk_per_image,
+            )
+            return pred_instances
+
+    @torch.no_grad()
+    def _match_and_label_boxes(self, proposals, stage, targets):
+        """
+        Match proposals with groundtruth using the matcher at the given stage.
+        Label the proposals as foreground or background based on the match.
+
+        Args:
+            proposals (list[Instances]): One Instances for each image, with
+                the field "proposal_boxes".
+            stage (int): the current stage
+            targets (list[Instances]): the ground truth instances
+
+        Returns:
+            list[Instances]: the same proposals, but with fields "gt_classes" and "gt_boxes"
+        """
+        num_fg_samples, num_bg_samples = [], []
+        for proposals_per_image, targets_per_image in zip(proposals, targets):
+            match_quality_matrix = pairwise_iou(
+                targets_per_image.gt_boxes, proposals_per_image.proposal_boxes
+            )
+            # proposal_labels are 0 or 1
+            matched_idxs, proposal_labels = self.proposal_matchers[stage](match_quality_matrix)
+            if len(targets_per_image) > 0:
+                gt_classes = targets_per_image.gt_classes[matched_idxs]
+                # Label unmatched proposals (0 label from matcher) as background (label=num_classes)
+                gt_classes[proposal_labels == 0] = self.num_classes
+                gt_boxes = targets_per_image.gt_boxes[matched_idxs]
+            else:
+                gt_classes = torch.zeros_like(matched_idxs) + self.num_classes
+                gt_boxes = Boxes(
+                    targets_per_image.gt_boxes.tensor.new_zeros((len(proposals_per_image), 4))
+                )
+            proposals_per_image.gt_classes = gt_classes
+            proposals_per_image.gt_boxes = gt_boxes
+
+            num_fg_samples.append((proposal_labels == 1).sum().item())
+            num_bg_samples.append(proposal_labels.numel() - num_fg_samples[-1])
+
+        # Log the number of fg/bg samples in each stage
+        storage = get_event_storage()
+        storage.put_scalar(
+            "stage{}/roi_head/num_fg_samples".format(stage),
+            sum(num_fg_samples) / len(num_fg_samples),
+        )
+        storage.put_scalar(
+            "stage{}/roi_head/num_bg_samples".format(stage),
+            sum(num_bg_samples) / len(num_bg_samples),
+        )
+        return proposals
+
+    def _run_stage(self, features, proposals, stage):
+        """
+        Args:
+            features (list[Tensor]): #lvl input features to ROIHeads
+            proposals (list[Instances]): #image Instances, with the field "proposal_boxes"
+            stage (int): the current stage
+
+        Returns:
+            Same output as `FastRCNNOutputLayers.forward()`.
+        """
+        box_features = self.box_pooler(features, [x.proposal_boxes for x in proposals])
+        # The original implementation averages the losses among heads,
+        # but scale up the parameter gradients of the heads.
+        # This is equivalent to adding the losses among heads,
+        # but scale down the gradients on features.
+        box_features = _ScaleGradient.apply(box_features, 1.0 / self.num_cascade_stages)
+        box_features = self.box_head[stage](box_features)
+        return self.box_predictor[stage](box_features)
+
+    def _create_proposals_from_boxes(self, boxes, image_sizes):
+        """
+        Args:
+            boxes (list[Tensor]): per-image predicted boxes, each of shape Ri x 4
+            image_sizes (list[tuple]): list of image shapes in (h, w)
+
+        Returns:
+            list[Instances]: per-image proposals with the given boxes.
+        """
+        # Just like RPN, the proposals should not have gradients
+        boxes = [Boxes(b.detach()) for b in boxes]
+        proposals = []
+        for boxes_per_image, image_size in zip(boxes, image_sizes):
+            boxes_per_image.clip(image_size)
+            if self.training:
+                # do not filter empty boxes at inference time,
+                # because the scores from each stage need to be aligned and added later
+                boxes_per_image = boxes_per_image[boxes_per_image.nonempty()]
+            prop = Instances(image_size)
+            prop.proposal_boxes = boxes_per_image
+            proposals.append(prop)
+        return proposals

detectron2/modeling/roi_heads/fast_rcnn.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import logging
+import torch
+from fvcore.nn import smooth_l1_loss
+from torch import nn
+from torch.nn import functional as F
+
+from detectron2.config import configurable
+from detectron2.layers import Linear, ShapeSpec, batched_nms, cat
+from detectron2.modeling.box_regression import Box2BoxTransform, apply_deltas_broadcast
+from detectron2.structures import Boxes, Instances
+from detectron2.utils.events import get_event_storage
+
+__all__ = ["fast_rcnn_inference", "FastRCNNOutputLayers"]
+
+
+logger = logging.getLogger(__name__)
+
+"""
+Shape shorthand in this module:
+
+    N: number of images in the minibatch
+    R: number of ROIs, combined over all images, in the minibatch
+    Ri: number of ROIs in image i
+    K: number of foreground classes. E.g.,there are 80 foreground classes in COCO.
+
+Naming convention:
+
+    deltas: refers to the 4-d (dx, dy, dw, dh) deltas that parameterize the box2box
+    transform (see :class:`box_regression.Box2BoxTransform`).
+
+    pred_class_logits: predicted class scores in [-inf, +inf]; use
+        softmax(pred_class_logits) to estimate P(class).
+
+    gt_classes: ground-truth classification labels in [0, K], where [0, K) represent
+        foreground object classes and K represents the background class.
+
+    pred_proposal_deltas: predicted box2box transform deltas for transforming proposals
+        to detection box predictions.
+
+    gt_proposal_deltas: ground-truth box2box transform deltas
+"""
+
+
+def fast_rcnn_inference(boxes, scores, image_shapes, score_thresh, nms_thresh, topk_per_image):
+    """
+    Call `fast_rcnn_inference_single_image` for all images.
+
+    Args:
+        boxes (list[Tensor]): A list of Tensors of predicted class-specific or class-agnostic
+            boxes for each image. Element i has shape (Ri, K * 4) if doing
+            class-specific regression, or (Ri, 4) if doing class-agnostic
+            regression, where Ri is the number of predicted objects for image i.
+            This is compatible with the output of :meth:`FastRCNNOutputLayers.predict_boxes`.
+        scores (list[Tensor]): A list of Tensors of predicted class scores for each image.
+            Element i has shape (Ri, K + 1), where Ri is the number of predicted objects
+            for image i. Compatible with the output of :meth:`FastRCNNOutputLayers.predict_probs`.
+        image_shapes (list[tuple]): A list of (width, height) tuples for each image in the batch.
+        score_thresh (float): Only return detections with a confidence score exceeding this
+            threshold.
+        nms_thresh (float):  The threshold to use for box non-maximum suppression. Value in [0, 1].
+        topk_per_image (int): The number of top scoring detections to return. Set < 0 to return
+            all detections.
+
+    Returns:
+        instances: (list[Instances]): A list of N instances, one for each image in the batch,
+            that stores the topk most confidence detections.
+        kept_indices: (list[Tensor]): A list of 1D tensor of length of N, each element indicates
+            the corresponding boxes/scores index in [0, Ri) from the input, for image i.
+    """
+    result_per_image = [
+        fast_rcnn_inference_single_image(
+            boxes_per_image, scores_per_image, image_shape, score_thresh, nms_thresh, topk_per_image
+        )
+        for scores_per_image, boxes_per_image, image_shape in zip(scores, boxes, image_shapes)
+    ]
+    return [x[0] for x in result_per_image], [x[1] for x in result_per_image]
+
+
+def fast_rcnn_inference_single_image(
+    boxes, scores, image_shape, score_thresh, nms_thresh, topk_per_image
+):
+    """
+    Single-image inference. Return bounding-box detection results by thresholding
+    on scores and applying non-maximum suppression (NMS).
+
+    Args:
+        Same as `fast_rcnn_inference`, but with boxes, scores, and image shapes
+        per image.
+
+    Returns:
+        Same as `fast_rcnn_inference`, but for only one image.
+    """
+    valid_mask = torch.isfinite(boxes).all(dim=1) & torch.isfinite(scores).all(dim=1)
+    if not valid_mask.all():
+        boxes = boxes[valid_mask]
+        scores = scores[valid_mask]
+
+    scores = scores[:, :-1]
+    num_bbox_reg_classes = boxes.shape[1] // 4
+    # Convert to Boxes to use the `clip` function ...
+    boxes = Boxes(boxes.reshape(-1, 4))
+    boxes.clip(image_shape)
+    boxes = boxes.tensor.view(-1, num_bbox_reg_classes, 4)  # R x C x 4
+
+    # Filter results based on detection scores
+    filter_mask = scores > score_thresh  # R x K
+    # R' x 2. First column contains indices of the R predictions;
+    # Second column contains indices of classes.
+    filter_inds = filter_mask.nonzero()
+    if num_bbox_reg_classes == 1:
+        boxes = boxes[filter_inds[:, 0], 0]
+    else:
+        boxes = boxes[filter_mask]
+    scores = scores[filter_mask]
+
+    # Apply per-class NMS
+    keep = batched_nms(boxes, scores, filter_inds[:, 1], nms_thresh)
+    if topk_per_image >= 0:
+        keep = keep[:topk_per_image]
+    boxes, scores, filter_inds = boxes[keep], scores[keep], filter_inds[keep]
+
+    result = Instances(image_shape)
+    result.pred_boxes = Boxes(boxes)
+    result.scores = scores
+    result.pred_classes = filter_inds[:, 1]
+    return result, filter_inds[:, 0]
+
+
+class FastRCNNOutputs(object):
+    """
+    A class that stores information about outputs of a Fast R-CNN head.
+    It provides methods that are used to decode the outputs of a Fast R-CNN head.
+    """
+
+    def __init__(
+        self,
+        box2box_transform,
+        pred_class_logits,
+        pred_proposal_deltas,
+        proposals,
+        smooth_l1_beta=0,
+    ):
+        """
+        Args:
+            box2box_transform (Box2BoxTransform/Box2BoxTransformRotated):
+                box2box transform instance for proposal-to-detection transformations.
+            pred_class_logits (Tensor): A tensor of shape (R, K + 1) storing the predicted class
+                logits for all R predicted object instances.
+                Each row corresponds to a predicted object instance.
+            pred_proposal_deltas (Tensor): A tensor of shape (R, K * B) or (R, B) for
+                class-specific or class-agnostic regression. It stores the predicted deltas that
+                transform proposals into final box detections.
+                B is the box dimension (4 or 5).
+                When B is 4, each row is [dx, dy, dw, dh (, ....)].
+                When B is 5, each row is [dx, dy, dw, dh, da (, ....)].
+            proposals (list[Instances]): A list of N Instances, where Instances i stores the
+                proposals for image i, in the field "proposal_boxes".
+                When training, each Instances must have ground-truth labels
+                stored in the field "gt_classes" and "gt_boxes".
+                The total number of all instances must be equal to R.
+            smooth_l1_beta (float): The transition point between L1 and L2 loss in
+                the smooth L1 loss function. When set to 0, the loss becomes L1. When
+                set to +inf, the loss becomes constant 0.
+        """
+        self.box2box_transform = box2box_transform
+        self.num_preds_per_image = [len(p) for p in proposals]
+        self.pred_class_logits = pred_class_logits
+        self.pred_proposal_deltas = pred_proposal_deltas
+        self.smooth_l1_beta = smooth_l1_beta
+        self.image_shapes = [x.image_size for x in proposals]
+
+        if len(proposals):
+            box_type = type(proposals[0].proposal_boxes)
+            # cat(..., dim=0) concatenates over all images in the batch
+            self.proposals = box_type.cat([p.proposal_boxes for p in proposals])
+            assert (
+                not self.proposals.tensor.requires_grad
+            ), "Proposals should not require gradients!"
+
+            # The following fields should exist only when training.
+            if proposals[0].has("gt_boxes"):
+                self.gt_boxes = box_type.cat([p.gt_boxes for p in proposals])
+                assert proposals[0].has("gt_classes")
+                self.gt_classes = cat([p.gt_classes for p in proposals], dim=0)
+        else:
+            self.proposals = Boxes(torch.zeros(0, 4, device=self.pred_proposal_deltas.device))
+        self._no_instances = len(proposals) == 0  # no instances found
+
+    def _log_accuracy(self):
+        """
+        Log the accuracy metrics to EventStorage.
+        """
+        num_instances = self.gt_classes.numel()
+        pred_classes = self.pred_class_logits.argmax(dim=1)
+        bg_class_ind = self.pred_class_logits.shape[1] - 1
+
+        fg_inds = (self.gt_classes >= 0) & (self.gt_classes < bg_class_ind)
+        num_fg = fg_inds.nonzero().numel()
+        fg_gt_classes = self.gt_classes[fg_inds]
+        fg_pred_classes = pred_classes[fg_inds]
+
+        num_false_negative = (fg_pred_classes == bg_class_ind).nonzero().numel()
+        num_accurate = (pred_classes == self.gt_classes).nonzero().numel()
+        fg_num_accurate = (fg_pred_classes == fg_gt_classes).nonzero().numel()
+
+        storage = get_event_storage()
+        if num_instances > 0:
+            storage.put_scalar("fast_rcnn/cls_accuracy", num_accurate / num_instances)
+            if num_fg > 0:
+                storage.put_scalar("fast_rcnn/fg_cls_accuracy", fg_num_accurate / num_fg)
+                storage.put_scalar("fast_rcnn/false_negative", num_false_negative / num_fg)
+
+    def softmax_cross_entropy_loss(self):
+        """
+        Compute the softmax cross entropy loss for box classification.
+
+        Returns:
+            scalar Tensor
+        """
+        if self._no_instances:
+            return 0.0 * self.pred_class_logits.sum()
+        else:
+            self._log_accuracy()
+            return F.cross_entropy(self.pred_class_logits, self.gt_classes, reduction="mean")
+
+    def smooth_l1_loss(self):
+        """
+        Compute the smooth L1 loss for box regression.
+
+        Returns:
+            scalar Tensor
+        """
+        if self._no_instances:
+            return 0.0 * self.pred_proposal_deltas.sum()
+        gt_proposal_deltas = self.box2box_transform.get_deltas(
+            self.proposals.tensor, self.gt_boxes.tensor
+        )
+        box_dim = gt_proposal_deltas.size(1)  # 4 or 5
+        cls_agnostic_bbox_reg = self.pred_proposal_deltas.size(1) == box_dim
+        device = self.pred_proposal_deltas.device
+
+        bg_class_ind = self.pred_class_logits.shape[1] - 1
+
+        # Box delta loss is only computed between the prediction for the gt class k
+        # (if 0 <= k < bg_class_ind) and the target; there is no loss defined on predictions
+        # for non-gt classes and background.
+        # Empty fg_inds produces a valid loss of zero as long as the size_average
+        # arg to smooth_l1_loss is False (otherwise it uses torch.mean internally
+        # and would produce a nan loss).
+        fg_inds = torch.nonzero(
+            (self.gt_classes >= 0) & (self.gt_classes < bg_class_ind), as_tuple=True
+        )[0]
+        if cls_agnostic_bbox_reg:
+            # pred_proposal_deltas only corresponds to foreground class for agnostic
+            gt_class_cols = torch.arange(box_dim, device=device)
+        else:
+            fg_gt_classes = self.gt_classes[fg_inds]
+            # pred_proposal_deltas for class k are located in columns [b * k : b * k + b],
+            # where b is the dimension of box representation (4 or 5)
+            # Note that compared to Detectron1,
+            # we do not perform bounding box regression for background classes.
+            gt_class_cols = box_dim * fg_gt_classes[:, None] + torch.arange(box_dim, device=device)
+
+        loss_box_reg = smooth_l1_loss(
+            self.pred_proposal_deltas[fg_inds[:, None], gt_class_cols],
+            gt_proposal_deltas[fg_inds],
+            self.smooth_l1_beta,
+            reduction="sum",
+        )
+        # The loss is normalized using the total number of regions (R), not the number
+        # of foreground regions even though the box regression loss is only defined on
+        # foreground regions. Why? Because doing so gives equal training influence to
+        # each foreground example. To see how, consider two different minibatches:
+        #  (1) Contains a single foreground region
+        #  (2) Contains 100 foreground regions
+        # If we normalize by the number of foreground regions, the single example in
+        # minibatch (1) will be given 100 times as much influence as each foreground
+        # example in minibatch (2). Normalizing by the total number of regions, R,
+        # means that the single example in minibatch (1) and each of the 100 examples
+        # in minibatch (2) are given equal influence.
+        loss_box_reg = loss_box_reg / self.gt_classes.numel()
+        return loss_box_reg
+
+    def _predict_boxes(self):
+        """
+        Returns:
+            Tensor: A Tensors of predicted class-specific or class-agnostic boxes
+                for all images in a batch. Element i has shape (Ri, K * B) or (Ri, B), where Ri is
+                the number of predicted objects for image i and B is the box dimension (4 or 5)
+        """
+        return apply_deltas_broadcast(
+            self.box2box_transform, self.pred_proposal_deltas, self.proposals.tensor
+        )
+
+    """
+    A subclass is expected to have the following methods because
+    they are used to query information about the head predictions.
+    """
+
+    def losses(self):
+        """
+        Compute the default losses for box head in Fast(er) R-CNN,
+        with softmax cross entropy loss and smooth L1 loss.
+
+        Returns:
+            A dict of losses (scalar tensors) containing keys "loss_cls" and "loss_box_reg".
+        """
+        return {
+            "loss_cls": self.softmax_cross_entropy_loss(),
+            "loss_box_reg": self.smooth_l1_loss(),
+        }
+
+    def predict_boxes(self):
+        """
+        Deprecated
+        """
+        return self._predict_boxes().split(self.num_preds_per_image, dim=0)
+
+    def predict_probs(self):
+        """
+        Deprecated
+        """
+        probs = F.softmax(self.pred_class_logits, dim=-1)
+        return probs.split(self.num_preds_per_image, dim=0)
+
+    def inference(self, score_thresh, nms_thresh, topk_per_image):
+        """
+        Deprecated
+        """
+        boxes = self.predict_boxes()
+        scores = self.predict_probs()
+        image_shapes = self.image_shapes
+        return fast_rcnn_inference(
+            boxes, scores, image_shapes, score_thresh, nms_thresh, topk_per_image
+        )
+
+
+class FastRCNNOutputLayers(nn.Module):
+    """
+    Two linear layers for predicting Fast R-CNN outputs:
+      (1) proposal-to-detection box regression deltas
+      (2) classification scores
+    """
+
+    @configurable
+    def __init__(
+        self,
+        input_shape,
+        *,
+        box2box_transform,
+        num_classes,
+        cls_agnostic_bbox_reg=False,
+        smooth_l1_beta=0.0,
+        test_score_thresh=0.0,
+        test_nms_thresh=0.5,
+        test_topk_per_image=100,
+    ):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            input_shape (ShapeSpec): shape of the input feature to this module
+            box2box_transform (Box2BoxTransform or Box2BoxTransformRotated):
+            num_classes (int): number of foreground classes
+            cls_agnostic_bbox_reg (bool): whether to use class agnostic for bbox regression
+            smooth_l1_beta (float): transition point from L1 to L2 loss.
+            test_score_thresh (float): threshold to filter predictions results.
+            test_nms_thresh (float): NMS threshold for prediction results.
+            test_topk_per_image (int): number of top predictions to produce per image.
+        """
+        super().__init__()
+        if isinstance(input_shape, int):  # some backward compatibility
+            input_shape = ShapeSpec(channels=input_shape)
+        input_size = input_shape.channels * (input_shape.width or 1) * (input_shape.height or 1)
+        # The prediction layer for num_classes foreground classes and one background class
+        # (hence + 1)
+        self.cls_score = Linear(input_size, num_classes + 1)
+        num_bbox_reg_classes = 1 if cls_agnostic_bbox_reg else num_classes
+        box_dim = len(box2box_transform.weights)
+        self.bbox_pred = Linear(input_size, num_bbox_reg_classes * box_dim)
+
+        nn.init.normal_(self.cls_score.weight, std=0.01)
+        nn.init.normal_(self.bbox_pred.weight, std=0.001)
+        for l in [self.cls_score, self.bbox_pred]:
+            nn.init.constant_(l.bias, 0)
+
+        self.box2box_transform = box2box_transform
+        self.smooth_l1_beta = smooth_l1_beta
+        self.test_score_thresh = test_score_thresh
+        self.test_nms_thresh = test_nms_thresh
+        self.test_topk_per_image = test_topk_per_image
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        return {
+            "input_shape": input_shape,
+            "box2box_transform": Box2BoxTransform(weights=cfg.MODEL.ROI_BOX_HEAD.BBOX_REG_WEIGHTS),
+            # fmt: off
+            "num_classes"           : cfg.MODEL.ROI_HEADS.NUM_CLASSES,
+            "cls_agnostic_bbox_reg" : cfg.MODEL.ROI_BOX_HEAD.CLS_AGNOSTIC_BBOX_REG,
+            "smooth_l1_beta"        : cfg.MODEL.ROI_BOX_HEAD.SMOOTH_L1_BETA,
+            "test_score_thresh"     : cfg.MODEL.ROI_HEADS.SCORE_THRESH_TEST,
+            "test_nms_thresh"       : cfg.MODEL.ROI_HEADS.NMS_THRESH_TEST,
+            "test_topk_per_image"   : cfg.TEST.DETECTIONS_PER_IMAGE
+            # fmt: on
+        }
+
+    def forward(self, x):
+        """
+        Returns:
+            Tensor: Nx(K+1) scores for each box
+            Tensor: Nx4 or Nx(Kx4) bounding box regression deltas.
+        """
+        if x.dim() > 2:
+            x = torch.flatten(x, start_dim=1)
+        scores = self.cls_score(x)
+        proposal_deltas = self.bbox_pred(x)
+        return scores, proposal_deltas
+
+    # TODO: move the implementation to this class.
+    def losses(self, predictions, proposals):
+        """
+        Args:
+            predictions: return values of :meth:`forward()`.
+            proposals (list[Instances]): proposals that match the features
+                that were used to compute predictions.
+        """
+        scores, proposal_deltas = predictions
+        return FastRCNNOutputs(
+            self.box2box_transform, scores, proposal_deltas, proposals, self.smooth_l1_beta
+        ).losses()
+
+    def inference(self, predictions, proposals):
+        """
+        Returns:
+            list[Instances]: same as `fast_rcnn_inference`.
+            list[Tensor]: same as `fast_rcnn_inference`.
+        """
+        boxes = self.predict_boxes(predictions, proposals)
+        scores = self.predict_probs(predictions, proposals)
+        image_shapes = [x.image_size for x in proposals]
+        return fast_rcnn_inference(
+            boxes,
+            scores,
+            image_shapes,
+            self.test_score_thresh,
+            self.test_nms_thresh,
+            self.test_topk_per_image,
+        )
+
+    def predict_boxes_for_gt_classes(self, predictions, proposals):
+        """
+        Returns:
+            list[Tensor]: A list of Tensors of predicted boxes for GT classes in case of
+                class-specific box head. Element i of the list has shape (Ri, B), where Ri is
+                the number of predicted objects for image i and B is the box dimension (4 or 5)
+        """
+        if not len(proposals):
+            return []
+        scores, proposal_deltas = predictions
+        proposal_boxes = [p.proposal_boxes for p in proposals]
+        proposal_boxes = proposal_boxes[0].cat(proposal_boxes).tensor
+        N, B = proposal_boxes.shape
+        predict_boxes = apply_deltas_broadcast(
+            self.box2box_transform, proposal_deltas, proposal_boxes
+        )  # Nx(KxB)
+
+        K = predict_boxes.shape[1] // B
+        if K > 1:
+            gt_classes = torch.cat([p.gt_classes for p in proposals], dim=0)
+            # Some proposals are ignored or have a background class. Their gt_classes
+            # cannot be used as index.
+            gt_classes = gt_classes.clamp_(0, K - 1)
+
+            predict_boxes = predict_boxes.view(N, K, B)[
+                torch.arange(N, dtype=torch.long, device=predict_boxes.device), gt_classes
+            ]
+        num_prop_per_image = [len(p) for p in proposals]
+        return predict_boxes.split(num_prop_per_image)
+
+    def predict_boxes(self, predictions, proposals):
+        """
+        Returns:
+            list[Tensor]: A list of Tensors of predicted class-specific or class-agnostic boxes
+                for each image. Element i has shape (Ri, K * B) or (Ri, B), where Ri is
+                the number of predicted objects for image i and B is the box dimension (4 or 5)
+        """
+        if not len(proposals):
+            return []
+        _, proposal_deltas = predictions
+        num_prop_per_image = [len(p) for p in proposals]
+        proposal_boxes = [p.proposal_boxes for p in proposals]
+        proposal_boxes = proposal_boxes[0].cat(proposal_boxes).tensor
+        predict_boxes = apply_deltas_broadcast(
+            self.box2box_transform, proposal_deltas, proposal_boxes
+        )  # Nx(KxB)
+        return predict_boxes.split(num_prop_per_image)
+
+    def predict_probs(self, predictions, proposals):
+        """
+        Returns:
+            list[Tensor]: A list of Tensors of predicted class probabilities for each image.
+                Element i has shape (Ri, K + 1), where Ri is the number of predicted objects
+                for image i.
+        """
+        scores, _ = predictions
+        num_inst_per_image = [len(p) for p in proposals]
+        probs = F.softmax(scores, dim=-1)
+        return probs.split(num_inst_per_image, dim=0)

detectron2/modeling/roi_heads/keypoint_head.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+from typing import List
+import torch
+from torch import nn
+from torch.nn import functional as F
+
+from detectron2.config import configurable
+from detectron2.layers import Conv2d, ConvTranspose2d, cat, interpolate
+from detectron2.structures import Instances, heatmaps_to_keypoints
+from detectron2.utils.events import get_event_storage
+from detectron2.utils.registry import Registry
+
+_TOTAL_SKIPPED = 0
+
+ROI_KEYPOINT_HEAD_REGISTRY = Registry("ROI_KEYPOINT_HEAD")
+ROI_KEYPOINT_HEAD_REGISTRY.__doc__ = """
+Registry for keypoint heads, which make keypoint predictions from per-region features.
+
+The registered object will be called with `obj(cfg, input_shape)`.
+"""
+
+
+def build_keypoint_head(cfg, input_shape):
+    """
+    Build a keypoint head from `cfg.MODEL.ROI_KEYPOINT_HEAD.NAME`.
+    """
+    name = cfg.MODEL.ROI_KEYPOINT_HEAD.NAME
+    return ROI_KEYPOINT_HEAD_REGISTRY.get(name)(cfg, input_shape)
+
+
+def keypoint_rcnn_loss(pred_keypoint_logits, instances, normalizer):
+    """
+    Arguments:
+        pred_keypoint_logits (Tensor): A tensor of shape (N, K, S, S) where N is the total number
+            of instances in the batch, K is the number of keypoints, and S is the side length
+            of the keypoint heatmap. The values are spatial logits.
+        instances (list[Instances]): A list of M Instances, where M is the batch size.
+            These instances are predictions from the model
+            that are in 1:1 correspondence with pred_keypoint_logits.
+            Each Instances should contain a `gt_keypoints` field containing a `structures.Keypoint`
+            instance.
+        normalizer (float): Normalize the loss by this amount.
+            If not specified, we normalize by the number of visible keypoints in the minibatch.
+
+    Returns a scalar tensor containing the loss.
+    """
+    heatmaps = []
+    valid = []
+
+    keypoint_side_len = pred_keypoint_logits.shape[2]
+    for instances_per_image in instances:
+        if len(instances_per_image) == 0:
+            continue
+        keypoints = instances_per_image.gt_keypoints
+        heatmaps_per_image, valid_per_image = keypoints.to_heatmap(
+            instances_per_image.proposal_boxes.tensor, keypoint_side_len
+        )
+        heatmaps.append(heatmaps_per_image.view(-1))
+        valid.append(valid_per_image.view(-1))
+
+    if len(heatmaps):
+        keypoint_targets = cat(heatmaps, dim=0)
+        valid = cat(valid, dim=0).to(dtype=torch.uint8)
+        valid = torch.nonzero(valid).squeeze(1)
+
+    # torch.mean (in binary_cross_entropy_with_logits) doesn't
+    # accept empty tensors, so handle it separately
+    if len(heatmaps) == 0 or valid.numel() == 0:
+        global _TOTAL_SKIPPED
+        _TOTAL_SKIPPED += 1
+        storage = get_event_storage()
+        storage.put_scalar("kpts_num_skipped_batches", _TOTAL_SKIPPED, smoothing_hint=False)
+        return pred_keypoint_logits.sum() * 0
+
+    N, K, H, W = pred_keypoint_logits.shape
+    pred_keypoint_logits = pred_keypoint_logits.view(N * K, H * W)
+
+    keypoint_loss = F.cross_entropy(
+        pred_keypoint_logits[valid], keypoint_targets[valid], reduction="sum"
+    )
+
+    # If a normalizer isn't specified, normalize by the number of visible keypoints in the minibatch
+    if normalizer is None:
+        normalizer = valid.numel()
+    keypoint_loss /= normalizer
+
+    return keypoint_loss
+
+
+def keypoint_rcnn_inference(pred_keypoint_logits, pred_instances):
+    """
+    Post process each predicted keypoint heatmap in `pred_keypoint_logits` into (x, y, score)
+        and add it to the `pred_instances` as a `pred_keypoints` field.
+
+    Args:
+        pred_keypoint_logits (Tensor): A tensor of shape (R, K, S, S) where R is the total number
+           of instances in the batch, K is the number of keypoints, and S is the side length of
+           the keypoint heatmap. The values are spatial logits.
+        pred_instances (list[Instances]): A list of N Instances, where N is the number of images.
+
+    Returns:
+        None. Each element in pred_instances will contain an extra "pred_keypoints" field.
+            The field is a tensor of shape (#instance, K, 3) where the last
+            dimension corresponds to (x, y, score).
+            The scores are larger than 0.
+    """
+    # flatten all bboxes from all images together (list[Boxes] -> Rx4 tensor)
+    bboxes_flat = cat([b.pred_boxes.tensor for b in pred_instances], dim=0)
+
+    keypoint_results = heatmaps_to_keypoints(pred_keypoint_logits.detach(), bboxes_flat.detach())
+    num_instances_per_image = [len(i) for i in pred_instances]
+    keypoint_results = keypoint_results[:, :, [0, 1, 3]].split(num_instances_per_image, dim=0)
+
+    for keypoint_results_per_image, instances_per_image in zip(keypoint_results, pred_instances):
+        # keypoint_results_per_image is (num instances)x(num keypoints)x(x, y, score)
+        instances_per_image.pred_keypoints = keypoint_results_per_image
+
+
+class BaseKeypointRCNNHead(nn.Module):
+    """
+    Implement the basic Keypoint R-CNN losses and inference logic described in :paper:`Mask R-CNN`.
+    """
+
+    @configurable
+    def __init__(self, *, num_keypoints, loss_weight=1.0, loss_normalizer=1.0):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            num_keypoints (int): number of keypoints to predict
+            loss_weight (float): weight to multiple on the keypoint loss
+            loss_normalizer (float or str):
+                If float, divide the loss by `loss_normalizer * #images`.
+                If 'visible', the loss is normalized by the total number of
+                    visible keypoints across images.
+        """
+        super().__init__()
+        self.num_keypoints = num_keypoints
+        self.loss_weight = loss_weight
+        assert loss_normalizer == "visible" or isinstance(loss_normalizer, float), loss_normalizer
+        self.loss_normalizer = loss_normalizer
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        ret = {
+            "loss_weight": cfg.MODEL.ROI_KEYPOINT_HEAD.LOSS_WEIGHT,
+            "num_keypoints": cfg.MODEL.ROI_KEYPOINT_HEAD.NUM_KEYPOINTS,
+        }
+        normalize_by_visible = (
+            cfg.MODEL.ROI_KEYPOINT_HEAD.NORMALIZE_LOSS_BY_VISIBLE_KEYPOINTS
+        )  # noqa
+        if not normalize_by_visible:
+            batch_size_per_image = cfg.MODEL.ROI_HEADS.BATCH_SIZE_PER_IMAGE
+            positive_sample_fraction = cfg.MODEL.ROI_HEADS.POSITIVE_FRACTION
+            ret["loss_normalizer"] = (
+                ret["num_keypoints"] * batch_size_per_image * positive_sample_fraction
+            )
+        else:
+            ret["loss_normalizer"] = "visible"
+        return ret
+
+    def forward(self, x, instances: List[Instances]):
+        """
+        Args:
+            x: input region feature(s) provided by :class:`ROIHeads`.
+            instances (list[Instances]): contains the boxes & labels corresponding
+                to the input features.
+                Exact format is up to its caller to decide.
+                Typically, this is the foreground instances in training, with
+                "proposal_boxes" field and other gt annotations.
+                In inference, it contains boxes that are already predicted.
+
+        Returns:
+            A dict of losses if in training. The predicted "instances" if in inference.
+        """
+        x = self.layers(x)
+        if self.training:
+            num_images = len(instances)
+            normalizer = (
+                None if self.loss_normalizer == "visible" else num_images * self.loss_normalizer
+            )
+            return {
+                "loss_keypoint": keypoint_rcnn_loss(x, instances, normalizer=normalizer)
+                * self.loss_weight
+            }
+        else:
+            keypoint_rcnn_inference(x, instances)
+            return instances
+
+    def layers(self, x):
+        """
+        Neural network layers that makes predictions from regional input features.
+        """
+        raise NotImplementedError
+
+
+@ROI_KEYPOINT_HEAD_REGISTRY.register()
+class KRCNNConvDeconvUpsampleHead(BaseKeypointRCNNHead):
+    """
+    A standard keypoint head containing a series of 3x3 convs, followed by
+    a transpose convolution and bilinear interpolation for upsampling.
+    """
+
+    @configurable
+    def __init__(self, input_shape, *, num_keypoints, conv_dims, **kwargs):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            input_shape (ShapeSpec): shape of the input feature
+            conv_dims: an iterable of output channel counts for each conv in the head
+                         e.g. (512, 512, 512) for three convs outputting 512 channels.
+        """
+        super().__init__(num_keypoints=num_keypoints, **kwargs)
+
+        # default up_scale to 2 (this can be made an option)
+        up_scale = 2
+        in_channels = input_shape.channels
+
+        self.blocks = []
+        for idx, layer_channels in enumerate(conv_dims, 1):
+            module = Conv2d(in_channels, layer_channels, 3, stride=1, padding=1)
+            self.add_module("conv_fcn{}".format(idx), module)
+            self.blocks.append(module)
+            in_channels = layer_channels
+
+        deconv_kernel = 4
+        self.score_lowres = ConvTranspose2d(
+            in_channels, num_keypoints, deconv_kernel, stride=2, padding=deconv_kernel // 2 - 1
+        )
+        self.up_scale = up_scale
+
+        for name, param in self.named_parameters():
+            if "bias" in name:
+                nn.init.constant_(param, 0)
+            elif "weight" in name:
+                # Caffe2 implementation uses MSRAFill, which in fact
+                # corresponds to kaiming_normal_ in PyTorch
+                nn.init.kaiming_normal_(param, mode="fan_out", nonlinearity="relu")
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        ret = super().from_config(cfg, input_shape)
+        ret["input_shape"] = input_shape
+        ret["conv_dims"] = cfg.MODEL.ROI_KEYPOINT_HEAD.CONV_DIMS
+        return ret
+
+    def layers(self, x):
+        for layer in self.blocks:
+            x = F.relu(layer(x))
+        x = self.score_lowres(x)
+        x = interpolate(x, scale_factor=self.up_scale, mode="bilinear", align_corners=False)
+        return x

detectron2/modeling/roi_heads/mask_head.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+from typing import List
+import fvcore.nn.weight_init as weight_init
+import torch
+from torch import nn
+from torch.nn import functional as F
+
+from detectron2.config import configurable
+from detectron2.layers import Conv2d, ConvTranspose2d, ShapeSpec, cat, get_norm
+from detectron2.structures import Instances
+from detectron2.utils.events import get_event_storage
+from detectron2.utils.registry import Registry
+
+ROI_MASK_HEAD_REGISTRY = Registry("ROI_MASK_HEAD")
+ROI_MASK_HEAD_REGISTRY.__doc__ = """
+Registry for mask heads, which predicts instance masks given
+per-region features.
+
+The registered object will be called with `obj(cfg, input_shape)`.
+"""
+
+
+def mask_rcnn_loss(pred_mask_logits, instances, vis_period=0):
+    """
+    Compute the mask prediction loss defined in the Mask R-CNN paper.
+
+    Args:
+        pred_mask_logits (Tensor): A tensor of shape (B, C, Hmask, Wmask) or (B, 1, Hmask, Wmask)
+            for class-specific or class-agnostic, where B is the total number of predicted masks
+            in all images, C is the number of foreground classes, and Hmask, Wmask are the height
+            and width of the mask predictions. The values are logits.
+        instances (list[Instances]): A list of N Instances, where N is the number of images
+            in the batch. These instances are in 1:1
+            correspondence with the pred_mask_logits. The ground-truth labels (class, box, mask,
+            ...) associated with each instance are stored in fields.
+        vis_period (int): the period (in steps) to dump visualization.
+
+    Returns:
+        mask_loss (Tensor): A scalar tensor containing the loss.
+    """
+    cls_agnostic_mask = pred_mask_logits.size(1) == 1
+    total_num_masks = pred_mask_logits.size(0)
+    mask_side_len = pred_mask_logits.size(2)
+    assert pred_mask_logits.size(2) == pred_mask_logits.size(3), "Mask prediction must be square!"
+
+    gt_classes = []
+    gt_masks = []
+    for instances_per_image in instances:
+        if len(instances_per_image) == 0:
+            continue
+        if not cls_agnostic_mask:
+            gt_classes_per_image = instances_per_image.gt_classes.to(dtype=torch.int64)
+            gt_classes.append(gt_classes_per_image)
+
+        gt_masks_per_image = instances_per_image.gt_masks.crop_and_resize(
+            instances_per_image.proposal_boxes.tensor, mask_side_len
+        ).to(device=pred_mask_logits.device)
+        # A tensor of shape (N, M, M), N=#instances in the image; M=mask_side_len
+        gt_masks.append(gt_masks_per_image)
+
+    if len(gt_masks) == 0:
+        return pred_mask_logits.sum() * 0
+
+    gt_masks = cat(gt_masks, dim=0)
+
+    if cls_agnostic_mask:
+        pred_mask_logits = pred_mask_logits[:, 0]
+    else:
+        indices = torch.arange(total_num_masks)
+        gt_classes = cat(gt_classes, dim=0)
+        pred_mask_logits = pred_mask_logits[indices, gt_classes]
+
+    if gt_masks.dtype == torch.bool:
+        gt_masks_bool = gt_masks
+    else:
+        # Here we allow gt_masks to be float as well (depend on the implementation of rasterize())
+        gt_masks_bool = gt_masks > 0.5
+    gt_masks = gt_masks.to(dtype=torch.float32)
+
+    # Log the training accuracy (using gt classes and 0.5 threshold)
+    mask_incorrect = (pred_mask_logits > 0.0) != gt_masks_bool
+    mask_accuracy = 1 - (mask_incorrect.sum().item() / max(mask_incorrect.numel(), 1.0))
+    num_positive = gt_masks_bool.sum().item()
+    false_positive = (mask_incorrect & ~gt_masks_bool).sum().item() / max(
+        gt_masks_bool.numel() - num_positive, 1.0
+    )
+    false_negative = (mask_incorrect & gt_masks_bool).sum().item() / max(num_positive, 1.0)
+
+    storage = get_event_storage()
+    storage.put_scalar("mask_rcnn/accuracy", mask_accuracy)
+    storage.put_scalar("mask_rcnn/false_positive", false_positive)
+    storage.put_scalar("mask_rcnn/false_negative", false_negative)
+    if vis_period > 0 and storage.iter % vis_period == 0:
+        pred_masks = pred_mask_logits.sigmoid()
+        vis_masks = torch.cat([pred_masks, gt_masks], axis=2)
+        name = "Left: mask prediction;   Right: mask GT"
+        for idx, vis_mask in enumerate(vis_masks):
+            vis_mask = torch.stack([vis_mask] * 3, axis=0)
+            storage.put_image(name + f" ({idx})", vis_mask)
+
+    mask_loss = F.binary_cross_entropy_with_logits(pred_mask_logits, gt_masks, reduction="mean")
+    return mask_loss
+
+
+def mask_rcnn_inference(pred_mask_logits, pred_instances):
+    """
+    Convert pred_mask_logits to estimated foreground probability masks while also
+    extracting only the masks for the predicted classes in pred_instances. For each
+    predicted box, the mask of the same class is attached to the instance by adding a
+    new "pred_masks" field to pred_instances.
+
+    Args:
+        pred_mask_logits (Tensor): A tensor of shape (B, C, Hmask, Wmask) or (B, 1, Hmask, Wmask)
+            for class-specific or class-agnostic, where B is the total number of predicted masks
+            in all images, C is the number of foreground classes, and Hmask, Wmask are the height
+            and width of the mask predictions. The values are logits.
+        pred_instances (list[Instances]): A list of N Instances, where N is the number of images
+            in the batch. Each Instances must have field "pred_classes".
+
+    Returns:
+        None. pred_instances will contain an extra "pred_masks" field storing a mask of size (Hmask,
+            Wmask) for predicted class. Note that the masks are returned as a soft (non-quantized)
+            masks the resolution predicted by the network; post-processing steps, such as resizing
+            the predicted masks to the original image resolution and/or binarizing them, is left
+            to the caller.
+    """
+    cls_agnostic_mask = pred_mask_logits.size(1) == 1
+
+    if cls_agnostic_mask:
+        mask_probs_pred = pred_mask_logits.sigmoid()
+    else:
+        # Select masks corresponding to the predicted classes
+        num_masks = pred_mask_logits.shape[0]
+        class_pred = cat([i.pred_classes for i in pred_instances])
+        indices = torch.arange(num_masks, device=class_pred.device)
+        mask_probs_pred = pred_mask_logits[indices, class_pred][:, None].sigmoid()
+    # mask_probs_pred.shape: (B, 1, Hmask, Wmask)
+
+    num_boxes_per_image = [len(i) for i in pred_instances]
+    mask_probs_pred = mask_probs_pred.split(num_boxes_per_image, dim=0)
+
+    for prob, instances in zip(mask_probs_pred, pred_instances):
+        instances.pred_masks = prob  # (1, Hmask, Wmask)
+
+
+class BaseMaskRCNNHead(nn.Module):
+    """
+    Implement the basic Mask R-CNN losses and inference logic described in :paper:`Mask R-CNN`
+    """
+
+    @configurable
+    def __init__(self, *, vis_period=0):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            vis_period (int): visualization period
+        """
+        super().__init__()
+        self.vis_period = vis_period
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        return {"vis_period": cfg.VIS_PERIOD}
+
+    def forward(self, x, instances: List[Instances]):
+        """
+        Args:
+            x: input region feature(s) provided by :class:`ROIHeads`.
+            instances (list[Instances]): contains the boxes & labels corresponding
+                to the input features.
+                Exact format is up to its caller to decide.
+                Typically, this is the foreground instances in training, with
+                "proposal_boxes" field and other gt annotations.
+                In inference, it contains boxes that are already predicted.
+
+        Returns:
+            A dict of losses in training. The predicted "instances" in inference.
+        """
+        x = self.layers(x)
+        if self.training:
+            return {"loss_mask": mask_rcnn_loss(x, instances, self.vis_period)}
+        else:
+            mask_rcnn_inference(x, instances)
+            return instances
+
+    def layers(self, x):
+        """
+        Neural network layers that makes predictions from input features.
+        """
+        raise NotImplementedError
+
+
+@ROI_MASK_HEAD_REGISTRY.register()
+class MaskRCNNConvUpsampleHead(BaseMaskRCNNHead):
+    """
+    A mask head with several conv layers, plus an upsample layer (with `ConvTranspose2d`).
+    Predictions are made with a final 1x1 conv layer.
+    """
+
+    @configurable
+    def __init__(self, input_shape: ShapeSpec, *, num_classes, conv_dims, conv_norm="", **kwargs):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            input_shape (ShapeSpec): shape of the input feature
+            num_classes (int): the number of classes. 1 if using class agnostic prediction.
+            conv_dims (list[int]): a list of N>0 integers representing the output dimensions
+                of N-1 conv layers and the last upsample layer.
+            conv_norm (str or callable): normalization for the conv layers.
+                See :func:`detectron2.layers.get_norm` for supported types.
+        """
+        super().__init__(**kwargs)
+        assert len(conv_dims) >= 1, "conv_dims have to be non-empty!"
+
+        self.conv_norm_relus = []
+
+        cur_channels = input_shape.channels
+        for k, conv_dim in enumerate(conv_dims[:-1]):
+            conv = Conv2d(
+                cur_channels,
+                conv_dim,
+                kernel_size=3,
+                stride=1,
+                padding=1,
+                bias=not conv_norm,
+                norm=get_norm(conv_norm, conv_dim),
+                activation=F.relu,
+            )
+            self.add_module("mask_fcn{}".format(k + 1), conv)
+            self.conv_norm_relus.append(conv)
+            cur_channels = conv_dim
+
+        self.deconv = ConvTranspose2d(
+            cur_channels, conv_dims[-1], kernel_size=2, stride=2, padding=0
+        )
+        cur_channels = conv_dims[-1]
+
+        self.predictor = Conv2d(cur_channels, num_classes, kernel_size=1, stride=1, padding=0)
+
+        for layer in self.conv_norm_relus + [self.deconv]:
+            weight_init.c2_msra_fill(layer)
+        # use normal distribution initialization for mask prediction layer
+        nn.init.normal_(self.predictor.weight, std=0.001)
+        if self.predictor.bias is not None:
+            nn.init.constant_(self.predictor.bias, 0)
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        ret = super().from_config(cfg, input_shape)
+        conv_dim = cfg.MODEL.ROI_MASK_HEAD.CONV_DIM
+        num_conv = cfg.MODEL.ROI_MASK_HEAD.NUM_CONV
+        ret.update(
+            conv_dims=[conv_dim] * (num_conv + 1),  # +1 for ConvTranspose
+            conv_norm=cfg.MODEL.ROI_MASK_HEAD.NORM,
+            input_shape=input_shape,
+        )
+        if cfg.MODEL.ROI_MASK_HEAD.CLS_AGNOSTIC_MASK:
+            ret["num_classes"] = 1
+        else:
+            ret["num_classes"] = cfg.MODEL.ROI_HEADS.NUM_CLASSES
+        return ret
+
+    def layers(self, x):
+        for layer in self.conv_norm_relus:
+            x = layer(x)
+        x = F.relu(self.deconv(x))
+        return self.predictor(x)
+
+
+def build_mask_head(cfg, input_shape):
+    """
+    Build a mask head defined by `cfg.MODEL.ROI_MASK_HEAD.NAME`.
+    """
+    name = cfg.MODEL.ROI_MASK_HEAD.NAME
+    return ROI_MASK_HEAD_REGISTRY.get(name)(cfg, input_shape)

detectron2/modeling/roi_heads/roi_heads.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import inspect
+import logging
+import numpy as np
+from typing import Dict, List, Optional, Tuple, Union
+import torch
+from torch import nn
+
+from detectron2.config import configurable
+from detectron2.layers import ShapeSpec
+from detectron2.structures import Boxes, ImageList, Instances, pairwise_iou
+from detectron2.utils.events import get_event_storage
+from detectron2.utils.registry import Registry
+
+from ..backbone.resnet import BottleneckBlock, make_stage
+from ..matcher import Matcher
+from ..poolers import ROIPooler
+from ..proposal_generator.proposal_utils import add_ground_truth_to_proposals
+from ..sampling import subsample_labels
+from .box_head import build_box_head
+from .fast_rcnn import FastRCNNOutputLayers
+from .keypoint_head import build_keypoint_head
+from .mask_head import build_mask_head
+
+ROI_HEADS_REGISTRY = Registry("ROI_HEADS")
+ROI_HEADS_REGISTRY.__doc__ = """
+Registry for ROI heads in a generalized R-CNN model.
+ROIHeads take feature maps and region proposals, and
+perform per-region computation.
+
+The registered object will be called with `obj(cfg, input_shape)`.
+The call is expected to return an :class:`ROIHeads`.
+"""
+
+logger = logging.getLogger(__name__)
+
+
+def build_roi_heads(cfg, input_shape):
+    """
+    Build ROIHeads defined by `cfg.MODEL.ROI_HEADS.NAME`.
+    """
+    name = cfg.MODEL.ROI_HEADS.NAME
+    return ROI_HEADS_REGISTRY.get(name)(cfg, input_shape)
+
+
+def select_foreground_proposals(
+    proposals: List[Instances], bg_label: int
+) -> Tuple[List[Instances], List[torch.Tensor]]:
+    """
+    Given a list of N Instances (for N images), each containing a `gt_classes` field,
+    return a list of Instances that contain only instances with `gt_classes != -1 &&
+    gt_classes != bg_label`.
+
+    Args:
+        proposals (list[Instances]): A list of N Instances, where N is the number of
+            images in the batch.
+        bg_label: label index of background class.
+
+    Returns:
+        list[Instances]: N Instances, each contains only the selected foreground instances.
+        list[Tensor]: N boolean vector, correspond to the selection mask of
+            each Instances object. True for selected instances.
+    """
+    assert isinstance(proposals, (list, tuple))
+    assert isinstance(proposals[0], Instances)
+    assert proposals[0].has("gt_classes")
+    fg_proposals = []
+    fg_selection_masks = []
+    for proposals_per_image in proposals:
+        gt_classes = proposals_per_image.gt_classes
+        fg_selection_mask = (gt_classes != -1) & (gt_classes != bg_label)
+        fg_idxs = fg_selection_mask.nonzero().squeeze(1)
+        fg_proposals.append(proposals_per_image[fg_idxs])
+        fg_selection_masks.append(fg_selection_mask)
+    return fg_proposals, fg_selection_masks
+
+
+def select_proposals_with_visible_keypoints(proposals: List[Instances]) -> List[Instances]:
+    """
+    Args:
+        proposals (list[Instances]): a list of N Instances, where N is the
+            number of images.
+
+    Returns:
+        proposals: only contains proposals with at least one visible keypoint.
+
+    Note that this is still slightly different from Detectron.
+    In Detectron, proposals for training keypoint head are re-sampled from
+    all the proposals with IOU>threshold & >=1 visible keypoint.
+
+    Here, the proposals are first sampled from all proposals with
+    IOU>threshold, then proposals with no visible keypoint are filtered out.
+    This strategy seems to make no difference on Detectron and is easier to implement.
+    """
+    ret = []
+    all_num_fg = []
+    for proposals_per_image in proposals:
+        # If empty/unannotated image (hard negatives), skip filtering for train
+        if len(proposals_per_image) == 0:
+            ret.append(proposals_per_image)
+            continue
+        gt_keypoints = proposals_per_image.gt_keypoints.tensor
+        # #fg x K x 3
+        vis_mask = gt_keypoints[:, :, 2] >= 1
+        xs, ys = gt_keypoints[:, :, 0], gt_keypoints[:, :, 1]
+        proposal_boxes = proposals_per_image.proposal_boxes.tensor.unsqueeze(dim=1)  # #fg x 1 x 4
+        kp_in_box = (
+            (xs >= proposal_boxes[:, :, 0])
+            & (xs <= proposal_boxes[:, :, 2])
+            & (ys >= proposal_boxes[:, :, 1])
+            & (ys <= proposal_boxes[:, :, 3])
+        )
+        selection = (kp_in_box & vis_mask).any(dim=1)
+        selection_idxs = torch.nonzero(selection, as_tuple=True)[0]
+        all_num_fg.append(selection_idxs.numel())
+        ret.append(proposals_per_image[selection_idxs])
+
+    storage = get_event_storage()
+    storage.put_scalar("keypoint_head/num_fg_samples", np.mean(all_num_fg))
+    return ret
+
+
+class ROIHeads(torch.nn.Module):
+    """
+    ROIHeads perform all per-region computation in an R-CNN.
+
+    It typically contains logic to
+    1. (in training only) match proposals with ground truth and sample them
+    2. crop the regions and extract per-region features using proposals
+    3. make per-region predictions with different heads
+
+    It can have many variants, implemented as subclasses of this class.
+    This base class contains the logic to match/sample proposals.
+    But it is not necessary to inherit this class if the sampling logic is not needed.
+    """
+
+    @configurable
+    def __init__(
+        self,
+        *,
+        num_classes,
+        batch_size_per_image,
+        positive_sample_fraction,
+        proposal_matcher,
+        proposal_append_gt=True
+    ):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            num_classes (int): number of classes. Used to label background proposals.
+            batch_size_per_image (int): number of proposals to use for training
+            positive_sample_fraction (float): fraction of positive (foreground) proposals
+                to use for training.
+            proposal_matcher (Matcher): matcher that matches proposals and ground truth
+            proposal_append_gt (bool): whether to include ground truth as proposals as well
+        """
+        super().__init__()
+        self.batch_size_per_image = batch_size_per_image
+        self.positive_sample_fraction = positive_sample_fraction
+        self.num_classes = num_classes
+        self.proposal_matcher = proposal_matcher
+        self.proposal_append_gt = proposal_append_gt
+
+    @classmethod
+    def from_config(cls, cfg):
+        return {
+            "batch_size_per_image": cfg.MODEL.ROI_HEADS.BATCH_SIZE_PER_IMAGE,
+            "positive_sample_fraction": cfg.MODEL.ROI_HEADS.POSITIVE_FRACTION,
+            "num_classes": cfg.MODEL.ROI_HEADS.NUM_CLASSES,
+            "proposal_append_gt": cfg.MODEL.ROI_HEADS.PROPOSAL_APPEND_GT,
+            # Matcher to assign box proposals to gt boxes
+            "proposal_matcher": Matcher(
+                cfg.MODEL.ROI_HEADS.IOU_THRESHOLDS,
+                cfg.MODEL.ROI_HEADS.IOU_LABELS,
+                allow_low_quality_matches=False,
+            ),
+        }
+
+    def _sample_proposals(
+        self, matched_idxs: torch.Tensor, matched_labels: torch.Tensor, gt_classes: torch.Tensor
+    ) -> Tuple[torch.Tensor, torch.Tensor]:
+        """
+        Based on the matching between N proposals and M groundtruth,
+        sample the proposals and set their classification labels.
+
+        Args:
+            matched_idxs (Tensor): a vector of length N, each is the best-matched
+                gt index in [0, M) for each proposal.
+            matched_labels (Tensor): a vector of length N, the matcher's label
+                (one of cfg.MODEL.ROI_HEADS.IOU_LABELS) for each proposal.
+            gt_classes (Tensor): a vector of length M.
+
+        Returns:
+            Tensor: a vector of indices of sampled proposals. Each is in [0, N).
+            Tensor: a vector of the same length, the classification label for
+                each sampled proposal. Each sample is labeled as either a category in
+                [0, num_classes) or the background (num_classes).
+        """
+        has_gt = gt_classes.numel() > 0
+        # Get the corresponding GT for each proposal
+        if has_gt:
+            gt_classes = gt_classes[matched_idxs]
+            # Label unmatched proposals (0 label from matcher) as background (label=num_classes)
+            gt_classes[matched_labels == 0] = self.num_classes
+            # Label ignore proposals (-1 label)
+            gt_classes[matched_labels == -1] = -1
+        else:
+            gt_classes = torch.zeros_like(matched_idxs) + self.num_classes
+
+        sampled_fg_idxs, sampled_bg_idxs = subsample_labels(
+            gt_classes, self.batch_size_per_image, self.positive_sample_fraction, self.num_classes
+        )
+
+        sampled_idxs = torch.cat([sampled_fg_idxs, sampled_bg_idxs], dim=0)
+        return sampled_idxs, gt_classes[sampled_idxs]
+
+    @torch.no_grad()
+    def label_and_sample_proposals(
+        self, proposals: List[Instances], targets: List[Instances]
+    ) -> List[Instances]:
+        """
+        Prepare some proposals to be used to train the ROI heads.
+        It performs box matching between `proposals` and `targets`, and assigns
+        training labels to the proposals.
+        It returns ``self.batch_size_per_image`` random samples from proposals and groundtruth
+        boxes, with a fraction of positives that is no larger than
+        ``self.positive_sample_fraction``.
+
+        Args:
+            See :meth:`ROIHeads.forward`
+
+        Returns:
+            list[Instances]:
+                length `N` list of `Instances`s containing the proposals
+                sampled for training. Each `Instances` has the following fields:
+
+                - proposal_boxes: the proposal boxes
+                - gt_boxes: the ground-truth box that the proposal is assigned to
+                  (this is only meaningful if the proposal has a label > 0; if label = 0
+                  then the ground-truth box is random)
+
+                Other fields such as "gt_classes", "gt_masks", that's included in `targets`.
+        """
+        gt_boxes = [x.gt_boxes for x in targets]
+        # Augment proposals with ground-truth boxes.
+        # In the case of learned proposals (e.g., RPN), when training starts
+        # the proposals will be low quality due to random initialization.
+        # It's possible that none of these initial
+        # proposals have high enough overlap with the gt objects to be used
+        # as positive examples for the second stage components (box head,
+        # cls head, mask head). Adding the gt boxes to the set of proposals
+        # ensures that the second stage components will have some positive
+        # examples from the start of training. For RPN, this augmentation improves
+        # convergence and empirically improves box AP on COCO by about 0.5
+        # points (under one tested configuration).
+        if self.proposal_append_gt:
+            proposals = add_ground_truth_to_proposals(gt_boxes, proposals)
+
+        proposals_with_gt = []
+
+        num_fg_samples = []
+        num_bg_samples = []
+        for proposals_per_image, targets_per_image in zip(proposals, targets):
+            has_gt = len(targets_per_image) > 0
+            match_quality_matrix = pairwise_iou(
+                targets_per_image.gt_boxes, proposals_per_image.proposal_boxes
+            )
+            matched_idxs, matched_labels = self.proposal_matcher(match_quality_matrix)
+            sampled_idxs, gt_classes = self._sample_proposals(
+                matched_idxs, matched_labels, targets_per_image.gt_classes
+            )
+
+            # Set target attributes of the sampled proposals:
+            proposals_per_image = proposals_per_image[sampled_idxs]
+            proposals_per_image.gt_classes = gt_classes
+
+            # We index all the attributes of targets that start with "gt_"
+            # and have not been added to proposals yet (="gt_classes").
+            if has_gt:
+                sampled_targets = matched_idxs[sampled_idxs]
+                # NOTE: here the indexing waste some compute, because heads
+                # like masks, keypoints, etc, will filter the proposals again,
+                # (by foreground/background, or number of keypoints in the image, etc)
+                # so we essentially index the data twice.
+                for (trg_name, trg_value) in targets_per_image.get_fields().items():
+                    if trg_name.startswith("gt_") and not proposals_per_image.has(trg_name):
+                        proposals_per_image.set(trg_name, trg_value[sampled_targets])
+            else:
+                gt_boxes = Boxes(
+                    targets_per_image.gt_boxes.tensor.new_zeros((len(sampled_idxs), 4))
+                )
+                proposals_per_image.gt_boxes = gt_boxes
+
+            num_bg_samples.append((gt_classes == self.num_classes).sum().item())
+            num_fg_samples.append(gt_classes.numel() - num_bg_samples[-1])
+            proposals_with_gt.append(proposals_per_image)
+
+        # Log the number of fg/bg samples that are selected for training ROI heads
+        storage = get_event_storage()
+        storage.put_scalar("roi_head/num_fg_samples", np.mean(num_fg_samples))
+        storage.put_scalar("roi_head/num_bg_samples", np.mean(num_bg_samples))
+
+        return proposals_with_gt
+
+    def forward(
+        self,
+        images: ImageList,
+        features: Dict[str, torch.Tensor],
+        proposals: List[Instances],
+        targets: Optional[List[Instances]] = None,
+    ) -> Tuple[List[Instances], Dict[str, torch.Tensor]]:
+        """
+        Args:
+            images (ImageList):
+            features (dict[str,Tensor]): input data as a mapping from feature
+                map name to tensor. Axis 0 represents the number of images `N` in
+                the input data; axes 1-3 are channels, height, and width, which may
+                vary between feature maps (e.g., if a feature pyramid is used).
+            proposals (list[Instances]): length `N` list of `Instances`. The i-th
+                `Instances` contains object proposals for the i-th input image,
+                with fields "proposal_boxes" and "objectness_logits".
+            targets (list[Instances], optional): length `N` list of `Instances`. The i-th
+                `Instances` contains the ground-truth per-instance annotations
+                for the i-th input image.  Specify `targets` during training only.
+                It may have the following fields:
+
+                - gt_boxes: the bounding box of each instance.
+                - gt_classes: the label for each instance with a category ranging in [0, #class].
+                - gt_masks: PolygonMasks or BitMasks, the ground-truth masks of each instance.
+                - gt_keypoints: NxKx3, the groud-truth keypoints for each instance.
+
+        Returns:
+            list[Instances]: length `N` list of `Instances` containing the
+            detected instances. Returned during inference only; may be [] during training.
+
+            dict[str->Tensor]:
+            mapping from a named loss to a tensor storing the loss. Used during training only.
+        """
+        raise NotImplementedError()
+
+
+@ROI_HEADS_REGISTRY.register()
+class Res5ROIHeads(ROIHeads):
+    """
+    The ROIHeads in a typical "C4" R-CNN model, where
+    the box and mask head share the cropping and
+    the per-region feature computation by a Res5 block.
+    """
+
+    def __init__(self, cfg, input_shape):
+        super().__init__(cfg)
+
+        # fmt: off
+        self.in_features  = cfg.MODEL.ROI_HEADS.IN_FEATURES
+        pooler_resolution = cfg.MODEL.ROI_BOX_HEAD.POOLER_RESOLUTION
+        pooler_type       = cfg.MODEL.ROI_BOX_HEAD.POOLER_TYPE
+        pooler_scales     = (1.0 / input_shape[self.in_features[0]].stride, )
+        sampling_ratio    = cfg.MODEL.ROI_BOX_HEAD.POOLER_SAMPLING_RATIO
+        self.mask_on      = cfg.MODEL.MASK_ON
+        # fmt: on
+        assert not cfg.MODEL.KEYPOINT_ON
+        assert len(self.in_features) == 1
+
+        self.pooler = ROIPooler(
+            output_size=pooler_resolution,
+            scales=pooler_scales,
+            sampling_ratio=sampling_ratio,
+            pooler_type=pooler_type,
+        )
+
+        self.res5, out_channels = self._build_res5_block(cfg)
+        self.box_predictor = FastRCNNOutputLayers(
+            cfg, ShapeSpec(channels=out_channels, height=1, width=1)
+        )
+
+        if self.mask_on:
+            self.mask_head = build_mask_head(
+                cfg,
+                ShapeSpec(channels=out_channels, width=pooler_resolution, height=pooler_resolution),
+            )
+
+    def _build_res5_block(self, cfg):
+        # fmt: off
+        stage_channel_factor = 2 ** 3  # res5 is 8x res2
+        num_groups           = cfg.MODEL.RESNETS.NUM_GROUPS
+        width_per_group      = cfg.MODEL.RESNETS.WIDTH_PER_GROUP
+        bottleneck_channels  = num_groups * width_per_group * stage_channel_factor
+        out_channels         = cfg.MODEL.RESNETS.RES2_OUT_CHANNELS * stage_channel_factor
+        stride_in_1x1        = cfg.MODEL.RESNETS.STRIDE_IN_1X1
+        norm                 = cfg.MODEL.RESNETS.NORM
+        assert not cfg.MODEL.RESNETS.DEFORM_ON_PER_STAGE[-1], \
+            "Deformable conv is not yet supported in res5 head."
+        # fmt: on
+
+        blocks = make_stage(
+            BottleneckBlock,
+            3,
+            first_stride=2,
+            in_channels=out_channels // 2,
+            bottleneck_channels=bottleneck_channels,
+            out_channels=out_channels,
+            num_groups=num_groups,
+            norm=norm,
+            stride_in_1x1=stride_in_1x1,
+        )
+        return nn.Sequential(*blocks), out_channels
+
+    def _shared_roi_transform(self, features, boxes):
+        x = self.pooler(features, boxes)
+        return self.res5(x)
+
+    def forward(self, images, features, proposals, targets=None):
+        """
+        See :meth:`ROIHeads.forward`.
+        """
+        del images
+
+        if self.training:
+            assert targets
+            proposals = self.label_and_sample_proposals(proposals, targets)
+        del targets
+
+        proposal_boxes = [x.proposal_boxes for x in proposals]
+        box_features = self._shared_roi_transform(
+            [features[f] for f in self.in_features], proposal_boxes
+        )
+        predictions = self.box_predictor(box_features.mean(dim=[2, 3]))
+
+        if self.training:
+            del features
+            losses = self.box_predictor.losses(predictions, proposals)
+            if self.mask_on:
+                proposals, fg_selection_masks = select_foreground_proposals(
+                    proposals, self.num_classes
+                )
+                # Since the ROI feature transform is shared between boxes and masks,
+                # we don't need to recompute features. The mask loss is only defined
+                # on foreground proposals, so we need to select out the foreground
+                # features.
+                mask_features = box_features[torch.cat(fg_selection_masks, dim=0)]
+                del box_features
+                losses.update(self.mask_head(mask_features, proposals))
+            return [], losses
+        else:
+            pred_instances, _ = self.box_predictor.inference(predictions, proposals)
+            pred_instances = self.forward_with_given_boxes(features, pred_instances)
+            return pred_instances, {}
+
+    def forward_with_given_boxes(self, features, instances):
+        """
+        Use the given boxes in `instances` to produce other (non-box) per-ROI outputs.
+
+        Args:
+            features: same as in `forward()`
+            instances (list[Instances]): instances to predict other outputs. Expect the keys
+                "pred_boxes" and "pred_classes" to exist.
+
+        Returns:
+            instances (Instances):
+                the same `Instances` object, with extra
+                fields such as `pred_masks` or `pred_keypoints`.
+        """
+        assert not self.training
+        assert instances[0].has("pred_boxes") and instances[0].has("pred_classes")
+
+        if self.mask_on:
+            features = [features[f] for f in self.in_features]
+            x = self._shared_roi_transform(features, [x.pred_boxes for x in instances])
+            return self.mask_head(x, instances)
+        else:
+            return instances
+
+
+@ROI_HEADS_REGISTRY.register()
+class StandardROIHeads(ROIHeads):
+    """
+    It's "standard" in a sense that there is no ROI transform sharing
+    or feature sharing between tasks.
+    Each head independently processes the input features by each head's
+    own pooler and head.
+
+    This class is used by most models, such as FPN and C5.
+    To implement more models, you can subclass it and implement a different
+    :meth:`forward()` or a head.
+    """
+
+    @configurable
+    def __init__(
+        self,
+        *,
+        box_in_features: List[str],
+        box_pooler: ROIPooler,
+        box_head: nn.Module,
+        box_predictor: nn.Module,
+        mask_in_features: Optional[List[str]] = None,
+        mask_pooler: Optional[ROIPooler] = None,
+        mask_head: Optional[nn.Module] = None,
+        keypoint_in_features: Optional[List[str]] = None,
+        keypoint_pooler: Optional[ROIPooler] = None,
+        keypoint_head: Optional[nn.Module] = None,
+        train_on_pred_boxes: bool = False,
+        **kwargs
+    ):
+        """
+        NOTE: this interface is experimental.
+
+        Args:
+            box_in_features (list[str]): list of feature names to use for the box head.
+            box_pooler (ROIPooler): pooler to extra region features for box head
+            box_head (nn.Module): transform features to make box predictions
+            box_predictor (nn.Module): make box predictions from the feature.
+                Should have the same interface as :class:`FastRCNNOutputLayers`.
+            mask_in_features (list[str]): list of feature names to use for the mask head.
+                None if not using mask head.
+            mask_pooler (ROIPooler): pooler to extra region features for mask head
+            mask_head (nn.Module): transform features to make mask predictions
+            keypoint_in_features, keypoint_pooler, keypoint_head: similar to ``mask*``.
+            train_on_pred_boxes (bool): whether to use proposal boxes or
+                predicted boxes from the box head to train other heads.
+        """
+        super().__init__(**kwargs)
+        # keep self.in_features for backward compatibility
+        self.in_features = self.box_in_features = box_in_features
+        self.box_pooler = box_pooler
+        self.box_head = box_head
+        self.box_predictor = box_predictor
+
+        self.mask_on = mask_in_features is not None
+        if self.mask_on:
+            self.mask_in_features = mask_in_features
+            self.mask_pooler = mask_pooler
+            self.mask_head = mask_head
+        self.keypoint_on = keypoint_in_features is not None
+        if self.keypoint_on:
+            self.keypoint_in_features = keypoint_in_features
+            self.keypoint_pooler = keypoint_pooler
+            self.keypoint_head = keypoint_head
+
+        self.train_on_pred_boxes = train_on_pred_boxes
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        ret = super().from_config(cfg)
+        ret["train_on_pred_boxes"] = cfg.MODEL.ROI_BOX_HEAD.TRAIN_ON_PRED_BOXES
+        # Subclasses that have not been updated to use from_config style construction
+        # may have overridden _init_*_head methods. In this case, those overridden methods
+        # will not be classmethods and we need to avoid trying to call them here.
+        # We test for this with ismethod which only returns True for bound methods of cls.
+        # Such subclasses will need to handle calling their overridden _init_*_head methods.
+        if inspect.ismethod(cls._init_box_head):
+            ret.update(cls._init_box_head(cfg, input_shape))
+        if inspect.ismethod(cls._init_mask_head):
+            ret.update(cls._init_mask_head(cfg, input_shape))
+        if inspect.ismethod(cls._init_keypoint_head):
+            ret.update(cls._init_keypoint_head(cfg, input_shape))
+        return ret
+
+    @classmethod
+    def _init_box_head(cls, cfg, input_shape):
+        # fmt: off
+        in_features       = cfg.MODEL.ROI_HEADS.IN_FEATURES
+        pooler_resolution = cfg.MODEL.ROI_BOX_HEAD.POOLER_RESOLUTION
+        pooler_scales     = tuple(1.0 / input_shape[k].stride for k in in_features)
+        sampling_ratio    = cfg.MODEL.ROI_BOX_HEAD.POOLER_SAMPLING_RATIO
+        pooler_type       = cfg.MODEL.ROI_BOX_HEAD.POOLER_TYPE
+        # fmt: on
+
+        # If StandardROIHeads is applied on multiple feature maps (as in FPN),
+        # then we share the same predictors and therefore the channel counts must be the same
+        in_channels = [input_shape[f].channels for f in in_features]
+        # Check all channel counts are equal
+        assert len(set(in_channels)) == 1, in_channels
+        in_channels = in_channels[0]
+
+        box_pooler = ROIPooler(
+            output_size=pooler_resolution,
+            scales=pooler_scales,
+            sampling_ratio=sampling_ratio,
+            pooler_type=pooler_type,
+        )
+        # Here we split "box head" and "box predictor", which is mainly due to historical reasons.
+        # They are used together so the "box predictor" layers should be part of the "box head".
+        # New subclasses of ROIHeads do not need "box predictor"s.
+        box_head = build_box_head(
+            cfg, ShapeSpec(channels=in_channels, height=pooler_resolution, width=pooler_resolution)
+        )
+        box_predictor = FastRCNNOutputLayers(cfg, box_head.output_shape)
+        return {
+            "box_in_features": in_features,
+            "box_pooler": box_pooler,
+            "box_head": box_head,
+            "box_predictor": box_predictor,
+        }
+
+    @classmethod
+    def _init_mask_head(cls, cfg, input_shape):
+        if not cfg.MODEL.MASK_ON:
+            return {}
+        # fmt: off
+        in_features       = cfg.MODEL.ROI_HEADS.IN_FEATURES
+        pooler_resolution = cfg.MODEL.ROI_MASK_HEAD.POOLER_RESOLUTION
+        pooler_scales     = tuple(1.0 / input_shape[k].stride for k in in_features)
+        sampling_ratio    = cfg.MODEL.ROI_MASK_HEAD.POOLER_SAMPLING_RATIO
+        pooler_type       = cfg.MODEL.ROI_MASK_HEAD.POOLER_TYPE
+        # fmt: on
+
+        in_channels = [input_shape[f].channels for f in in_features][0]
+
+        ret = {"mask_in_features": in_features}
+        ret["mask_pooler"] = ROIPooler(
+            output_size=pooler_resolution,
+            scales=pooler_scales,
+            sampling_ratio=sampling_ratio,
+            pooler_type=pooler_type,
+        )
+        ret["mask_head"] = build_mask_head(
+            cfg, ShapeSpec(channels=in_channels, width=pooler_resolution, height=pooler_resolution)
+        )
+        return ret
+
+    @classmethod
+    def _init_keypoint_head(cls, cfg, input_shape):
+        if not cfg.MODEL.KEYPOINT_ON:
+            return {}
+        # fmt: off
+        in_features       = cfg.MODEL.ROI_HEADS.IN_FEATURES
+        pooler_resolution = cfg.MODEL.ROI_KEYPOINT_HEAD.POOLER_RESOLUTION
+        pooler_scales     = tuple(1.0 / input_shape[k].stride for k in in_features)  # noqa
+        sampling_ratio    = cfg.MODEL.ROI_KEYPOINT_HEAD.POOLER_SAMPLING_RATIO
+        pooler_type       = cfg.MODEL.ROI_KEYPOINT_HEAD.POOLER_TYPE
+        # fmt: on
+
+        in_channels = [input_shape[f].channels for f in in_features][0]
+
+        ret = {"keypoint_in_features": in_features}
+        ret["keypoint_pooler"] = ROIPooler(
+            output_size=pooler_resolution,
+            scales=pooler_scales,
+            sampling_ratio=sampling_ratio,
+            pooler_type=pooler_type,
+        )
+        ret["keypoint_head"] = build_keypoint_head(
+            cfg, ShapeSpec(channels=in_channels, width=pooler_resolution, height=pooler_resolution)
+        )
+        return ret
+
+    def forward(
+        self,
+        images: ImageList,
+        features: Dict[str, torch.Tensor],
+        proposals: List[Instances],
+        targets: Optional[List[Instances]] = None,
+    ) -> Tuple[List[Instances], Dict[str, torch.Tensor]]:
+        """
+        See :class:`ROIHeads.forward`.
+        """
+        del images
+        if self.training:
+            assert targets
+            proposals = self.label_and_sample_proposals(proposals, targets)
+        del targets
+
+        if self.training:
+            losses = self._forward_box(features, proposals)
+            # Usually the original proposals used by the box head are used by the mask, keypoint
+            # heads. But when `self.train_on_pred_boxes is True`, proposals will contain boxes
+            # predicted by the box head.
+            losses.update(self._forward_mask(features, proposals))
+            losses.update(self._forward_keypoint(features, proposals))
+            return proposals, losses
+        else:
+            pred_instances = self._forward_box(features, proposals)
+            # During inference cascaded prediction is used: the mask and keypoints heads are only
+            # applied to the top scoring box detections.
+            pred_instances = self.forward_with_given_boxes(features, pred_instances)
+            return pred_instances, {}
+
+    def forward_with_given_boxes(
+        self, features: Dict[str, torch.Tensor], instances: List[Instances]
+    ) -> List[Instances]:
+        """
+        Use the given boxes in `instances` to produce other (non-box) per-ROI outputs.
+
+        This is useful for downstream tasks where a box is known, but need to obtain
+        other attributes (outputs of other heads).
+        Test-time augmentation also uses this.
+
+        Args:
+            features: same as in `forward()`
+            instances (list[Instances]): instances to predict other outputs. Expect the keys
+                "pred_boxes" and "pred_classes" to exist.
+
+        Returns:
+            instances (list[Instances]):
+                the same `Instances` objects, with extra
+                fields such as `pred_masks` or `pred_keypoints`.
+        """
+        assert not self.training
+        assert instances[0].has("pred_boxes") and instances[0].has("pred_classes")
+
+        instances = self._forward_mask(features, instances)
+        instances = self._forward_keypoint(features, instances)
+        return instances
+
+    def _forward_box(
+        self, features: Dict[str, torch.Tensor], proposals: List[Instances]
+    ) -> Union[Dict[str, torch.Tensor], List[Instances]]:
+        """
+        Forward logic of the box prediction branch. If `self.train_on_pred_boxes is True`,
+            the function puts predicted boxes in the `proposal_boxes` field of `proposals` argument.
+
+        Args:
+            features (dict[str, Tensor]): mapping from feature map names to tensor.
+                Same as in :meth:`ROIHeads.forward`.
+            proposals (list[Instances]): the per-image object proposals with
+                their matching ground truth.
+                Each has fields "proposal_boxes", and "objectness_logits",
+                "gt_classes", "gt_boxes".
+
+        Returns:
+            In training, a dict of losses.
+            In inference, a list of `Instances`, the predicted instances.
+        """
+        features = [features[f] for f in self.box_in_features]
+        box_features = self.box_pooler(features, [x.proposal_boxes for x in proposals])
+        box_features = self.box_head(box_features)
+        predictions = self.box_predictor(box_features)
+        del box_features
+
+        if self.training:
+            losses = self.box_predictor.losses(predictions, proposals)
+            # proposals is modified in-place below, so losses must be computed first.
+            if self.train_on_pred_boxes:
+                with torch.no_grad():
+                    pred_boxes = self.box_predictor.predict_boxes_for_gt_classes(
+                        predictions, proposals
+                    )
+                    for proposals_per_image, pred_boxes_per_image in zip(proposals, pred_boxes):
+                        proposals_per_image.proposal_boxes = Boxes(pred_boxes_per_image)
+            return losses
+        else:
+            pred_instances, _ = self.box_predictor.inference(predictions, proposals)
+            return pred_instances
+
+    def _forward_mask(
+        self, features: Dict[str, torch.Tensor], instances: List[Instances]
+    ) -> Union[Dict[str, torch.Tensor], List[Instances]]:
+        """
+        Forward logic of the mask prediction branch.
+
+        Args:
+            features (dict[str, Tensor]): mapping from feature map names to tensor.
+                Same as in :meth:`ROIHeads.forward`.
+            instances (list[Instances]): the per-image instances to train/predict masks.
+                In training, they can be the proposals.
+                In inference, they can be the predicted boxes.
+
+        Returns:
+            In training, a dict of losses.
+            In inference, update `instances` with new fields "pred_masks" and return it.
+        """
+        if not self.mask_on:
+            return {} if self.training else instances
+
+        features = [features[f] for f in self.mask_in_features]
+
+        if self.training:
+            # The loss is only defined on positive proposals.
+            proposals, _ = select_foreground_proposals(instances, self.num_classes)
+            proposal_boxes = [x.proposal_boxes for x in proposals]
+            mask_features = self.mask_pooler(features, proposal_boxes)
+            return self.mask_head(mask_features, proposals)
+        else:
+            pred_boxes = [x.pred_boxes for x in instances]
+            mask_features = self.mask_pooler(features, pred_boxes)
+            return self.mask_head(mask_features, instances)
+
+    def _forward_keypoint(
+        self, features: Dict[str, torch.Tensor], instances: List[Instances]
+    ) -> Union[Dict[str, torch.Tensor], List[Instances]]:
+        """
+        Forward logic of the keypoint prediction branch.
+
+        Args:
+            features (dict[str, Tensor]): mapping from feature map names to tensor.
+                Same as in :meth:`ROIHeads.forward`.
+            instances (list[Instances]): the per-image instances to train/predict keypoints.
+                In training, they can be the proposals.
+                In inference, they can be the predicted boxes.
+
+        Returns:
+            In training, a dict of losses.
+            In inference, update `instances` with new fields "pred_keypoints" and return it.
+        """
+        if not self.keypoint_on:
+            return {} if self.training else instances
+
+        features = [features[f] for f in self.keypoint_in_features]
+
+        if self.training:
+            # The loss is defined on positive proposals with >=1 visible keypoints.
+            proposals, _ = select_foreground_proposals(instances, self.num_classes)
+            proposals = select_proposals_with_visible_keypoints(proposals)
+            proposal_boxes = [x.proposal_boxes for x in proposals]
+
+            keypoint_features = self.keypoint_pooler(features, proposal_boxes)
+            return self.keypoint_head(keypoint_features, proposals)
+        else:
+            pred_boxes = [x.pred_boxes for x in instances]
+            keypoint_features = self.keypoint_pooler(features, pred_boxes)
+            return self.keypoint_head(keypoint_features, instances)

detectron2/modeling/roi_heads/rotated_fast_rcnn.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import logging
+import numpy as np
+import torch
+
+from detectron2.config import configurable
+from detectron2.layers import ShapeSpec, batched_nms_rotated
+from detectron2.structures import Instances, RotatedBoxes, pairwise_iou_rotated
+from detectron2.utils.events import get_event_storage
+
+from ..box_regression import Box2BoxTransformRotated
+from ..poolers import ROIPooler
+from ..proposal_generator.proposal_utils import add_ground_truth_to_proposals
+from .box_head import build_box_head
+from .fast_rcnn import FastRCNNOutputLayers
+from .roi_heads import ROI_HEADS_REGISTRY, StandardROIHeads
+
+logger = logging.getLogger(__name__)
+
+"""
+Shape shorthand in this module:
+
+    N: number of images in the minibatch
+    R: number of ROIs, combined over all images, in the minibatch
+    Ri: number of ROIs in image i
+    K: number of foreground classes. E.g.,there are 80 foreground classes in COCO.
+
+Naming convention:
+
+    deltas: refers to the 5-d (dx, dy, dw, dh, da) deltas that parameterize the box2box
+    transform (see :class:`box_regression.Box2BoxTransformRotated`).
+
+    pred_class_logits: predicted class scores in [-inf, +inf]; use
+        softmax(pred_class_logits) to estimate P(class).
+
+    gt_classes: ground-truth classification labels in [0, K], where [0, K) represent
+        foreground object classes and K represents the background class.
+
+    pred_proposal_deltas: predicted rotated box2box transform deltas for transforming proposals
+        to detection box predictions.
+
+    gt_proposal_deltas: ground-truth rotated box2box transform deltas
+"""
+
+
+def fast_rcnn_inference_rotated(
+    boxes, scores, image_shapes, score_thresh, nms_thresh, topk_per_image
+):
+    """
+    Call `fast_rcnn_inference_single_image_rotated` for all images.
+
+    Args:
+        boxes (list[Tensor]): A list of Tensors of predicted class-specific or class-agnostic
+            boxes for each image. Element i has shape (Ri, K * 5) if doing
+            class-specific regression, or (Ri, 5) if doing class-agnostic
+            regression, where Ri is the number of predicted objects for image i.
+            This is compatible with the output of :meth:`FastRCNNOutputs.predict_boxes`.
+        scores (list[Tensor]): A list of Tensors of predicted class scores for each image.
+            Element i has shape (Ri, K + 1), where Ri is the number of predicted objects
+            for image i. Compatible with the output of :meth:`FastRCNNOutputs.predict_probs`.
+        image_shapes (list[tuple]): A list of (width, height) tuples for each image in the batch.
+        score_thresh (float): Only return detections with a confidence score exceeding this
+            threshold.
+        nms_thresh (float):  The threshold to use for box non-maximum suppression. Value in [0, 1].
+        topk_per_image (int): The number of top scoring detections to return. Set < 0 to return
+            all detections.
+
+    Returns:
+        instances: (list[Instances]): A list of N instances, one for each image in the batch,
+            that stores the topk most confidence detections.
+        kept_indices: (list[Tensor]): A list of 1D tensor of length of N, each element indicates
+            the corresponding boxes/scores index in [0, Ri) from the input, for image i.
+    """
+    result_per_image = [
+        fast_rcnn_inference_single_image_rotated(
+            boxes_per_image, scores_per_image, image_shape, score_thresh, nms_thresh, topk_per_image
+        )
+        for scores_per_image, boxes_per_image, image_shape in zip(scores, boxes, image_shapes)
+    ]
+    return [x[0] for x in result_per_image], [x[1] for x in result_per_image]
+
+
+def fast_rcnn_inference_single_image_rotated(
+    boxes, scores, image_shape, score_thresh, nms_thresh, topk_per_image
+):
+    """
+    Single-image inference. Return rotated bounding-box detection results by thresholding
+    on scores and applying rotated non-maximum suppression (Rotated NMS).
+
+    Args:
+        Same as `fast_rcnn_inference_rotated`, but with rotated boxes, scores, and image shapes
+        per image.
+
+    Returns:
+        Same as `fast_rcnn_inference_rotated`, but for only one image.
+    """
+    valid_mask = torch.isfinite(boxes).all(dim=1) & torch.isfinite(scores).all(dim=1)
+    if not valid_mask.all():
+        boxes = boxes[valid_mask]
+        scores = scores[valid_mask]
+
+    B = 5  # box dimension
+    scores = scores[:, :-1]
+    num_bbox_reg_classes = boxes.shape[1] // B
+    # Convert to Boxes to use the `clip` function ...
+    boxes = RotatedBoxes(boxes.reshape(-1, B))
+    boxes.clip(image_shape)
+    boxes = boxes.tensor.view(-1, num_bbox_reg_classes, B)  # R x C x B
+    # Filter results based on detection scores
+    filter_mask = scores > score_thresh  # R x K
+    # R' x 2. First column contains indices of the R predictions;
+    # Second column contains indices of classes.
+    filter_inds = filter_mask.nonzero()
+    if num_bbox_reg_classes == 1:
+        boxes = boxes[filter_inds[:, 0], 0]
+    else:
+        boxes = boxes[filter_mask]
+    scores = scores[filter_mask]
+
+    # Apply per-class Rotated NMS
+    keep = batched_nms_rotated(boxes, scores, filter_inds[:, 1], nms_thresh)
+    if topk_per_image >= 0:
+        keep = keep[:topk_per_image]
+    boxes, scores, filter_inds = boxes[keep], scores[keep], filter_inds[keep]
+
+    result = Instances(image_shape)
+    result.pred_boxes = RotatedBoxes(boxes)
+    result.scores = scores
+    result.pred_classes = filter_inds[:, 1]
+
+    return result, filter_inds[:, 0]
+
+
+class RotatedFastRCNNOutputLayers(FastRCNNOutputLayers):
+    """
+    Two linear layers for predicting Rotated Fast R-CNN outputs.
+    """
+
+    @classmethod
+    def from_config(cls, cfg, input_shape):
+        args = super().from_config(cfg, input_shape)
+        args["box2box_transform"] = Box2BoxTransformRotated(
+            weights=cfg.MODEL.ROI_BOX_HEAD.BBOX_REG_WEIGHTS
+        )
+        return args
+
+    def inference(self, predictions, proposals):
+        """
+        Returns:
+            list[Instances]: same as `fast_rcnn_inference_rotated`.
+            list[Tensor]: same as `fast_rcnn_inference_rotated`.
+        """
+        boxes = self.predict_boxes(predictions, proposals)
+        scores = self.predict_probs(predictions, proposals)
+        image_shapes = [x.image_size for x in proposals]
+
+        return fast_rcnn_inference_rotated(
+            boxes,
+            scores,
+            image_shapes,
+            self.test_score_thresh,
+            self.test_nms_thresh,
+            self.test_topk_per_image,
+        )
+
+
+@ROI_HEADS_REGISTRY.register()
+class RROIHeads(StandardROIHeads):
+    """
+    This class is used by Rotated Fast R-CNN to detect rotated boxes.
+    For now, it only supports box predictions but not mask or keypoints.
+    """
+
+    @configurable
+    def __init__(self, **kwargs):
+        """
+        NOTE: this interface is experimental.
+        """
+        super().__init__(**kwargs)
+        assert (
+            not self.mask_on and not self.keypoint_on
+        ), "Mask/Keypoints not supported in Rotated ROIHeads."
+        assert not self.train_on_pred_boxes, "train_on_pred_boxes not implemented for RROIHeads!"
+
+    @classmethod
+    def _init_box_head(cls, cfg, input_shape):
+        # fmt: off
+        in_features       = cfg.MODEL.ROI_HEADS.IN_FEATURES
+        pooler_resolution = cfg.MODEL.ROI_BOX_HEAD.POOLER_RESOLUTION
+        pooler_scales     = tuple(1.0 / input_shape[k].stride for k in in_features)
+        sampling_ratio    = cfg.MODEL.ROI_BOX_HEAD.POOLER_SAMPLING_RATIO
+        pooler_type       = cfg.MODEL.ROI_BOX_HEAD.POOLER_TYPE
+        # fmt: on
+        assert pooler_type in ["ROIAlignRotated"], pooler_type
+        # assume all channel counts are equal
+        in_channels = [input_shape[f].channels for f in in_features][0]
+
+        box_pooler = ROIPooler(
+            output_size=pooler_resolution,
+            scales=pooler_scales,
+            sampling_ratio=sampling_ratio,
+            pooler_type=pooler_type,
+        )
+        box_head = build_box_head(
+            cfg, ShapeSpec(channels=in_channels, height=pooler_resolution, width=pooler_resolution)
+        )
+        # This line is the only difference v.s. StandardROIHeads
+        box_predictor = RotatedFastRCNNOutputLayers(cfg, box_head.output_shape)
+        return {
+            "box_in_features": in_features,
+            "box_pooler": box_pooler,
+            "box_head": box_head,
+            "box_predictor": box_predictor,
+        }
+
+    @torch.no_grad()
+    def label_and_sample_proposals(self, proposals, targets):
+        """
+        Prepare some proposals to be used to train the RROI heads.
+        It performs box matching between `proposals` and `targets`, and assigns
+        training labels to the proposals.
+        It returns `self.batch_size_per_image` random samples from proposals and groundtruth boxes,
+        with a fraction of positives that is no larger than `self.positive_sample_fraction.
+
+        Args:
+            See :meth:`StandardROIHeads.forward`
+
+        Returns:
+            list[Instances]: length `N` list of `Instances`s containing the proposals
+                sampled for training. Each `Instances` has the following fields:
+                - proposal_boxes: the rotated proposal boxes
+                - gt_boxes: the ground-truth rotated boxes that the proposal is assigned to
+                  (this is only meaningful if the proposal has a label > 0; if label = 0
+                   then the ground-truth box is random)
+                - gt_classes: the ground-truth classification lable for each proposal
+        """
+        gt_boxes = [x.gt_boxes for x in targets]
+        if self.proposal_append_gt:
+            proposals = add_ground_truth_to_proposals(gt_boxes, proposals)
+
+        proposals_with_gt = []
+
+        num_fg_samples = []
+        num_bg_samples = []
+        for proposals_per_image, targets_per_image in zip(proposals, targets):
+            has_gt = len(targets_per_image) > 0
+            match_quality_matrix = pairwise_iou_rotated(
+                targets_per_image.gt_boxes, proposals_per_image.proposal_boxes
+            )
+            matched_idxs, matched_labels = self.proposal_matcher(match_quality_matrix)
+            sampled_idxs, gt_classes = self._sample_proposals(
+                matched_idxs, matched_labels, targets_per_image.gt_classes
+            )
+
+            proposals_per_image = proposals_per_image[sampled_idxs]
+            proposals_per_image.gt_classes = gt_classes
+
+            if has_gt:
+                sampled_targets = matched_idxs[sampled_idxs]
+                proposals_per_image.gt_boxes = targets_per_image.gt_boxes[sampled_targets]
+            else:
+                gt_boxes = RotatedBoxes(
+                    targets_per_image.gt_boxes.tensor.new_zeros((len(sampled_idxs), 5))
+                )
+                proposals_per_image.gt_boxes = gt_boxes
+
+            num_bg_samples.append((gt_classes == self.num_classes).sum().item())
+            num_fg_samples.append(gt_classes.numel() - num_bg_samples[-1])
+            proposals_with_gt.append(proposals_per_image)
+
+        # Log the number of fg/bg samples that are selected for training ROI heads
+        storage = get_event_storage()
+        storage.put_scalar("roi_head/num_fg_samples", np.mean(num_fg_samples))
+        storage.put_scalar("roi_head/num_bg_samples", np.mean(num_bg_samples))
+
+        return proposals_with_gt

detectron2/modeling/sampling.py

+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+import torch
+
+__all__ = ["subsample_labels"]
+
+
+def subsample_labels(labels, num_samples, positive_fraction, bg_label):
+    """
+    Return `num_samples` (or fewer, if not enough found)
+    random samples from `labels` which is a mixture of positives & negatives.
+    It will try to return as many positives as possible without
+    exceeding `positive_fraction * num_samples`, and then try to
+    fill the remaining slots with negatives.
+
+    Args:
+        labels (Tensor): (N, ) label vector with values:
+            * -1: ignore
+            * bg_label: background ("negative") class
+            * otherwise: one or more foreground ("positive") classes
+        num_samples (int): The total number of labels with value >= 0 to return.
+            Values that are not sampled will be filled with -1 (ignore).
+        positive_fraction (float): The number of subsampled labels with values > 0
+            is `min(num_positives, int(positive_fraction * num_samples))`. The number
+            of negatives sampled is `min(num_negatives, num_samples - num_positives_sampled)`.
+            In order words, if there are not enough positives, the sample is filled with
+            negatives. If there are also not enough negatives, then as many elements are
+            sampled as is possible.
+        bg_label (int): label index of background ("negative") class.
+
+    Returns:
+        pos_idx, neg_idx (Tensor):
+            1D vector of indices. The total length of both is `num_samples` or fewer.
+    """
+    positive = torch.nonzero((labels != -1) & (labels != bg_label), as_tuple=True)[0]
+    negative = torch.nonzero(labels == bg_label, as_tuple=True)[0]
+
+    num_pos = int(num_samples * positive_fraction)
+    # protect against not enough positive examples
+    num_pos = min(positive.numel(), num_pos)
+    num_neg = num_samples - num_pos
+    # protect against not enough negative examples
+    num_neg = min(negative.numel(), num_neg)
+
+    # randomly select positive and negative examples
+    perm1 = torch.randperm(positive.numel(), device=positive.device)[:num_pos]
+    perm2 = torch.randperm(negative.numel(), device=negative.device)[:num_neg]
+
+    pos_idx = positive[perm1]
+    neg_idx = negative[perm2]
+    return pos_idx, neg_idx