roi_align.py

from typing import List, Union

import torch
from torch import nn, Tensor
from torch.jit.annotations import BroadcastingList2
from torch.nn.modules.utils import _pair
from torchvision.extension import _assert_has_ops

from ._utils import convert_boxes_to_roi_format, check_roi_boxes_shape


def roi_align(
    input: Tensor,
    boxes: Union[Tensor, List[Tensor]],
    output_size: BroadcastingList2[int],
    spatial_scale: float = 1.0,
    sampling_ratio: int = -1,
    aligned: bool = False,
) -> Tensor:
    """
    Performs Region of Interest (RoI) Align operator with average pooling, as described in Mask R-CNN.

    Args:
        input (Tensor[N, C, H, W]): The input tensor, i.e. a batch with ``N`` elements. Each element
            contains ``C`` feature maps of dimensions ``H x W``.
            If the tensor is quantized, we expect a batch size of ``N == 1``.
        boxes (Tensor[K, 5] or List[Tensor[L, 4]]): the box coordinates in (x1, y1, x2, y2)
            format where the regions will be taken from.
            The coordinate must satisfy ``0 <= x1 < x2`` and ``0 <= y1 < y2``.
            If a single Tensor is passed, then the first column should
            contain the index of the corresponding element in the batch, i.e. a number in ``[0, N - 1]``.
            If a list of Tensors is passed, then each Tensor will correspond to the boxes for an element i
            in the batch.
        output_size (int or Tuple[int, int]): the size of the output (in bins or pixels) after the pooling
            is performed, as (height, width).
        spatial_scale (float): a scaling factor that maps the box coordinates to
            the input coordinates. For example, if your boxes are defined on the scale
            of a 224x224 image and your input is a 112x112 feature map (resulting from a 0.5x scaling of
            the original image), you'll want to set this to 0.5. Default: 1.0
        sampling_ratio (int): number of sampling points in the interpolation grid
            used to compute the output value of each pooled output bin. If > 0,
            then exactly ``sampling_ratio x sampling_ratio`` sampling points per bin are used. If
            <= 0, then an adaptive number of grid points are used (computed as
            ``ceil(roi_width / output_width)``, and likewise for height). Default: -1
        aligned (bool): If False, use the legacy implementation.
            If True, pixel shift the box coordinates it by -0.5 for a better alignment with the two
            neighboring pixel indices. This version is used in Detectron2

    Returns:
        Tensor[K, C, output_size[0], output_size[1]]: The pooled RoIs.
    """
    _assert_has_ops()
    check_roi_boxes_shape(boxes)
    rois = boxes
    output_size = _pair(output_size)
    if not isinstance(rois, torch.Tensor):
        rois = convert_boxes_to_roi_format(rois)
    return torch.ops.torchvision.roi_align(
        input, rois, spatial_scale, output_size[0], output_size[1], sampling_ratio, aligned
    )


class RoIAlign(nn.Module):
    """
    See :func:`roi_align`.
    """

    def __init__(
        self,
        output_size: BroadcastingList2[int],
        spatial_scale: float,
        sampling_ratio: int,
        aligned: bool = False,
    ):
        super().__init__()
        self.output_size = output_size
        self.spatial_scale = spatial_scale
        self.sampling_ratio = sampling_ratio
        self.aligned = aligned

    def forward(self, input: Tensor, rois: Tensor) -> Tensor:
        return roi_align(input, rois, self.output_size, self.spatial_scale, self.sampling_ratio, self.aligned)

    def __repr__(self) -> str:
        tmpstr = self.__class__.__name__ + "("
        tmpstr += "output_size=" + str(self.output_size)
        tmpstr += ", spatial_scale=" + str(self.spatial_scale)
        tmpstr += ", sampling_ratio=" + str(self.sampling_ratio)
        tmpstr += ", aligned=" + str(self.aligned)
        tmpstr += ")"
        return tmpstr