Fix for rotating bounding box

[LeoLee7]

🐛 Bug

To Reproduce

Steps to reproduce the behavior:

1. When I tried to visualize the bounding box after rotation (90 degree), it seems not aligned with the original bounded area.
2. We can not get the original bbox when with apply a 90 degree rotation first and then apply a -90 degree rotation operation. (But the image looks good) - - We can also try the 360 degree rotation, and the bbox is changed. Ideally if we perform 360 degree rotation, the image and bbox should be identical to the original status.
3. The test case is at https://github.com/LeoLee7/AugLy/blob/main/augly/tests/image_tests/issue_of_bbox_rotation_AugLy_image.ipynb

[LeoLee7]

I think the issue might be at rotate_bbox_helper and I fixed it with two changes

angle_rad = -math.radians(degrees)
rotation_matrix = [
    round(math.cos(angle_rad), 15),
    round(math.sin(angle_rad), 15),
    0.0,
    round(math.sin(angle_rad), 15),
    round(-math.cos(angle_rad), 15),
    0.0,
]
rotation_matrix[2], rotation_matrix[5] = transform(
    -rotation_center[0], -rotation_center[1], rotation_matrix
)
rotation_matrix[2] += rotation_center[0]
rotation_matrix[5] += rotation_center[1]

# Get rotated image dimensions
src_img_corners = [(0, 0), (src_w, 0), (src_w, src_h), (0, src_h)]
(
    rotated_img_min_x,
    rotated_img_min_y,
    rotated_img_max_x,
    rotated_img_max_y,
) = get_enclosing_bbox(src_img_corners, rotation_matrix)
rotated_img_w = rotated_img_max_x - rotated_img_min_x
rotated_img_h = rotated_img_max_y - rotated_img_min_y

Potential solution:

1. rotation matrix initialization:

angle_rad = math.radians(degrees)
    rotation_matrix = [
    round(math.cos(angle_rad), 15),
    round(math.sin(angle_rad), 15),
    0.0,
    round(-math.sin(angle_rad), 15),
    round(math.cos(angle_rad), 15),
    0.0,
]

2. coordinate translation: update the transform matrix after the calculation of rotated image corners.

src_img_corners = [(0, 0), (src_w, 0), (src_w, src_h), (0, src_h)]
(
    rotated_img_min_x,
    rotated_img_min_y,
    rotated_img_max_x,
    rotated_img_max_y,
) = get_enclosing_bbox(src_img_corners, rotation_matrix)


rotation_matrix[2] += rotated_img_max_x
rotation_matrix[5] += rotated_img_max_y

[LeoLee7]

One more fixing:
3. the cropping operation: add the translation operation to the cropped-img rotate_bbox_helper

cropped_w, cropped_h = imutils.rotated_rect_with_max_area(src_w, src_h, degrees)
cropped_img_left, cropped_img_upper, cropped_img_right, cropped_img_lower = (
    (rotated_img_w - cropped_w) // 2 + rotated_img_min_x + rotated_img_max_x,
    (rotated_img_h - cropped_h) // 2 + rotated_img_min_y + rotated_img_max_y,
    (rotated_img_w + cropped_w) // 2 + rotated_img_min_x + rotated_img_max_x,
    (rotated_img_h + cropped_h) // 2 + rotated_img_min_y + rotated_img_max_y,
)

[LeoLee7]

Test case after fixing : [aligned rotating bbox]