How to rotate yolo format bounding boxes after image rotate?

1

So I am currently adding rotation augmentation using python opencv as shown below to augment yolo format bounding boxes [x, y, h, w]. I have tried various approaches online, but am currently stuck. I know that I have to compute the center of the bounding box, but how do I rotate it after to align with the rotation ? I am not sure how to transform the bounding box values to accommodate for the rotation.

rotation_angle = np.random.uniform(-45, 45)
rotation_matrix = cv.getRotationMatrix2D((width / 2, height / 2), rotation_angle, 1)
img = cv.warpAffine(img, rotation_matrix, (width, height))
x, y, w, h = bounding_box
# bounding bbox centers should simply be x, y since x, y are bounding box centers in yolo format
cx = x 
cy = y
2

calculate the corners of your box, then apply the transformation to the corners, then draw the polygon?

3

Why cant we simply mat mul the height and width of the bounding box with the rotation matrix (cos, -sin, sin, cos) ? Since we rotate around the center of the image shoudnt the center of the bounding box be also the same ?

4

but do we? where is the origin?