Question about `rotation_angle` when rotating `prev_bev`

[whystopped]

BEVFormer/projects/mmdet3d_plugin/bevformer/modules/transformer.py

Lines 143 to 156 in 66b65f3

	if prev_bev is not None:
	if prev_bev.shape[1] == bev_h * bev_w:
	prev_bev = prev_bev.permute(1, 0, 2)
	if self.rotate_prev_bev:
	for i in range(bs):
	# num_prev_bev = prev_bev.size(1)
	rotation_angle = kwargs['img_metas'][i]['can_bus'][-1]
	tmp_prev_bev = prev_bev[:, i].reshape(
	bev_h, bev_w, -1).permute(2, 0, 1)
	tmp_prev_bev = rotate(tmp_prev_bev, rotation_angle,
	center=self.rotate_center)
	tmp_prev_bev = tmp_prev_bev.permute(1, 2, 0).reshape(
	bev_h * bev_w, 1, -1)
	prev_bev[:, i] = tmp_prev_bev[:, 0]

BEVFormer/projects/mmdet3d_plugin/datasets/nuscenes_dataset.py

Line 78 in 66b65f3

metas_map[i]['can_bus'][-1] -= prev_angle

According to the codes above, the rotation_angle used to rotate prev_bev is the difference between two patch angles, which is now_angle - prev_angle.

However, as shown in the following figure, if we want to align prev_bev (in yellow color) to the current frame (in black color), we are actually rotate the black rectangle (prev_bev in the previous frame coordinate) to the yellow rectangle (prev_bev in the current frame coordinate).

So my question is, the rotation_angle used to rotate prev_bev should be prev_angle - now_angle, rather than now_angle - prev_angle used in the code above, is that right?