depth_lss.py

import torch
from torch import nn
from pcdet.ops.bev_pool import bev_pool


def gen_dx_bx(xbound, ybound, zbound):
    dx = torch.Tensor([row[2] for row in [xbound, ybound, zbound]])
    bx = torch.Tensor([row[0] + row[2] / 2.0 for row in [xbound, ybound, zbound]])
    nx = torch.LongTensor(
        [(row[1] - row[0]) / row[2] for row in [xbound, ybound, zbound]]
    )
    return dx, bx, nx


class DepthLSSTransform(nn.Module):
    """
        This module implements LSS, which lists images into 3D and then splats onto bev features.
        This code is adapted from https://github.com/mit-han-lab/bevfusion/ with minimal modifications.
    """
    def __init__(self, model_cfg):
        super().__init__()
        self.model_cfg = model_cfg
        in_channel = self.model_cfg.IN_CHANNEL
        out_channel = self.model_cfg.OUT_CHANNEL
        self.image_size = self.model_cfg.IMAGE_SIZE
        self.feature_size = self.model_cfg.FEATURE_SIZE
        xbound = self.model_cfg.XBOUND
        ybound = self.model_cfg.YBOUND
        zbound = self.model_cfg.ZBOUND
        self.dbound = self.model_cfg.DBOUND
        downsample = self.model_cfg.DOWNSAMPLE

        dx, bx, nx = gen_dx_bx(xbound, ybound, zbound)
        self.dx = nn.Parameter(dx, requires_grad=False)
        self.bx = nn.Parameter(bx, requires_grad=False)
        self.nx = nn.Parameter(nx, requires_grad=False)

        self.C = out_channel
        self.frustum = self.create_frustum()
        self.D = self.frustum.shape[0]

        self.dtransform = nn.Sequential(
            nn.Conv2d(1, 8, 1),
            nn.BatchNorm2d(8),
            nn.ReLU(True),
            nn.Conv2d(8, 32, 5, stride=4, padding=2),
            nn.BatchNorm2d(32),
            nn.ReLU(True),
            nn.Conv2d(32, 64, 5, stride=2, padding=2),
            nn.BatchNorm2d(64),
            nn.ReLU(True),
        )
        self.depthnet = nn.Sequential(
            nn.Conv2d(in_channel + 64, in_channel, 3, padding=1),
            nn.BatchNorm2d(in_channel),
            nn.ReLU(True),
            nn.Conv2d(in_channel, in_channel, 3, padding=1),
            nn.BatchNorm2d(in_channel),
            nn.ReLU(True),
            nn.Conv2d(in_channel, self.D + self.C, 1),
        )
        if downsample > 1:
            assert downsample == 2, downsample
            self.downsample = nn.Sequential(
                nn.Conv2d(out_channel, out_channel, 3, padding=1, bias=False),
                nn.BatchNorm2d(out_channel),
                nn.ReLU(True),
                nn.Conv2d(out_channel, out_channel, 3, stride=downsample, padding=1, bias=False),
                nn.BatchNorm2d(out_channel),
                nn.ReLU(True),
                nn.Conv2d(out_channel, out_channel, 3, padding=1, bias=False),
                nn.BatchNorm2d(out_channel),
                nn.ReLU(True),
            )
        else:
            self.downsample = nn.Identity()
    
    def create_frustum(self):
        iH, iW = self.image_size
        fH, fW = self.feature_size

        ds = torch.arange(*self.dbound, dtype=torch.float).view(-1, 1, 1).expand(-1, fH, fW)
        D, _, _ = ds.shape
        xs = torch.linspace(0, iW - 1, fW, dtype=torch.float).view(1, 1, fW).expand(D, fH, fW)
        ys = torch.linspace(0, iH - 1, fH, dtype=torch.float).view(1, fH, 1).expand(D, fH, fW)
        frustum = torch.stack((xs, ys, ds), -1)
        
        return nn.Parameter(frustum, requires_grad=False)

    def get_geometry(self, camera2lidar_rots, camera2lidar_trans, intrins, post_rots, post_trans, **kwargs):

        camera2lidar_rots = camera2lidar_rots.to(torch.float)
        camera2lidar_trans = camera2lidar_trans.to(torch.float)
        intrins = intrins.to(torch.float)
        post_rots = post_rots.to(torch.float)
        post_trans = post_trans.to(torch.float)

        B, N, _ = camera2lidar_trans.shape

        # undo post-transformation
        # B x N x D x H x W x 3
        points = self.frustum - post_trans.view(B, N, 1, 1, 1, 3)
        points = torch.inverse(post_rots).view(B, N, 1, 1, 1, 3, 3).matmul(points.unsqueeze(-1))
        
        # cam_to_lidar
        points = torch.cat((points[:, :, :, :, :, :2] * points[:, :, :, :, :, 2:3], points[:, :, :, :, :, 2:3]), 5)
        combine = camera2lidar_rots.matmul(torch.inverse(intrins))
        points = combine.view(B, N, 1, 1, 1, 3, 3).matmul(points).squeeze(-1)
        points += camera2lidar_trans.view(B, N, 1, 1, 1, 3)

        if "extra_rots" in kwargs:
            extra_rots = kwargs["extra_rots"]
            points = extra_rots.view(B, 1, 1, 1, 1, 3, 3).repeat(1, N, 1, 1, 1, 1, 1) \
                .matmul(points.unsqueeze(-1)).squeeze(-1)
            
        if "extra_trans" in kwargs:
            extra_trans = kwargs["extra_trans"]
            points += extra_trans.view(B, 1, 1, 1, 1, 3).repeat(1, N, 1, 1, 1, 1)

        return points

    def bev_pool(self, geom_feats, x):
        geom_feats = geom_feats.to(torch.float)
        x = x.to(torch.float)

        B, N, D, H, W, C = x.shape
        Nprime = B * N * D * H * W

        # flatten x
        x = x.reshape(Nprime, C)

        # flatten indices
        geom_feats = ((geom_feats - (self.bx - self.dx / 2.0)) / self.dx).long()
        geom_feats = geom_feats.view(Nprime, 3)
        batch_ix = torch.cat([torch.full([Nprime // B, 1], ix, device=x.device, dtype=torch.long) for ix in range(B)])
        geom_feats = torch.cat((geom_feats, batch_ix), 1)

        # filter out points that are outside box
        kept = (
            (geom_feats[:, 0] >= 0)
            & (geom_feats[:, 0] < self.nx[0])
            & (geom_feats[:, 1] >= 0)
            & (geom_feats[:, 1] < self.nx[1])
            & (geom_feats[:, 2] >= 0)
            & (geom_feats[:, 2] < self.nx[2])
        )
        x = x[kept]
        geom_feats = geom_feats[kept]
        x = bev_pool(x, geom_feats, B, self.nx[2], self.nx[0], self.nx[1])

        # collapse Z
        final = torch.cat(x.unbind(dim=2), 1)

        return final

    def get_cam_feats(self, x, d):
        B, N, C, fH, fW = x.shape

        d = d.view(B * N, *d.shape[2:])
        x = x.view(B * N, C, fH, fW)

        d = self.dtransform(d)
        x = torch.cat([d, x], dim=1)
        x = self.depthnet(x)

        depth = x[:, : self.D].softmax(dim=1)
        x = depth.unsqueeze(1) * x[:, self.D : (self.D + self.C)].unsqueeze(2)

        x = x.view(B, N, self.C, self.D, fH, fW)
        x = x.permute(0, 1, 3, 4, 5, 2)
        return x

    def forward(self, batch_dict):
        """
        Args:
            batch_dict:
                image_fpn (list[tensor]): image features after image neck

        Returns:
            batch_dict:
                spatial_features_img (tensor): bev features from image modality
        """
        x = batch_dict['image_fpn'] 
        x = x[0]
        BN, C, H, W = x.size()
        img = x.view(int(BN/6), 6, C, H, W)

        camera_intrinsics = batch_dict['camera_intrinsics']
        camera2lidar = batch_dict['camera2lidar']
        img_aug_matrix = batch_dict['img_aug_matrix']
        lidar_aug_matrix = batch_dict['lidar_aug_matrix']
        lidar2image = batch_dict['lidar2image']

        intrins = camera_intrinsics[..., :3, :3]
        post_rots = img_aug_matrix[..., :3, :3]
        post_trans = img_aug_matrix[..., :3, 3]
        camera2lidar_rots = camera2lidar[..., :3, :3]
        camera2lidar_trans = camera2lidar[..., :3, 3]

        points = batch_dict['points']

        batch_size = BN // 6
        depth = torch.zeros(batch_size, img.shape[1], 1, *self.image_size).to(points[0].device)

        for b in range(batch_size):
            batch_mask = points[:,0] == b
            cur_coords = points[batch_mask][:, 1:4]
            cur_img_aug_matrix = img_aug_matrix[b]
            cur_lidar_aug_matrix = lidar_aug_matrix[b]
            cur_lidar2image = lidar2image[b]

            # inverse aug
            cur_coords -= cur_lidar_aug_matrix[:3, 3]
            cur_coords = torch.inverse(cur_lidar_aug_matrix[:3, :3]).matmul(
                cur_coords.transpose(1, 0)
            )
            # lidar2image
            cur_coords = cur_lidar2image[:, :3, :3].matmul(cur_coords)
            cur_coords += cur_lidar2image[:, :3, 3].reshape(-1, 3, 1)
            # get 2d coords
            dist = cur_coords[:, 2, :]
            cur_coords[:, 2, :] = torch.clamp(cur_coords[:, 2, :], 1e-5, 1e5)
            cur_coords[:, :2, :] /= cur_coords[:, 2:3, :]

            # do image aug
            cur_coords = cur_img_aug_matrix[:, :3, :3].matmul(cur_coords)
            cur_coords += cur_img_aug_matrix[:, :3, 3].reshape(-1, 3, 1)
            cur_coords = cur_coords[:, :2, :].transpose(1, 2)

            # normalize coords for grid sample
            cur_coords = cur_coords[..., [1, 0]]

            # filter points outside of images
            on_img = (
                (cur_coords[..., 0] < self.image_size[0])
                & (cur_coords[..., 0] >= 0)
                & (cur_coords[..., 1] < self.image_size[1])
                & (cur_coords[..., 1] >= 0)
            )
            for c in range(on_img.shape[0]):
                masked_coords = cur_coords[c, on_img[c]].long()
                masked_dist = dist[c, on_img[c]]
                depth[b, c, 0, masked_coords[:, 0], masked_coords[:, 1]] = masked_dist

        extra_rots = lidar_aug_matrix[..., :3, :3]
        extra_trans = lidar_aug_matrix[..., :3, 3]
        geom = self.get_geometry(
            camera2lidar_rots, camera2lidar_trans, intrins, post_rots, 
            post_trans, extra_rots=extra_rots, extra_trans=extra_trans,
        )
        # use points depth to assist the depth prediction in images
        x = self.get_cam_feats(img, depth)
        x = self.bev_pool(geom, x)
        x = self.downsample(x)
        # convert bev features from (b, c, x, y) to (b, c, y, x)
        x = x.permute(0, 1, 3, 2)
        batch_dict['spatial_features_img'] = x
        return batch_dict