# Rotation center confusion

Can someone please verify my understanding or correct my misunderstanding? If I rotate an image using any one of the many online examples (mainly Stackoverflow), I get an unexpected offset in the rotated image. The examples I've found online all use center=(cols/2.0, rows/2.0) for the affine transform getRotationMatrix2D() call. I believe this is incorrect. I think this should instead be center=(cols/2.0 - 0.5, rows/2.0 - 0.5). Since the online concensus seems against me, I'm really just hoping someone here will verify that I'm not incorrect in my understanding.

Specifically, in Python, I need to do this to get what I believe is correct behavior:

def rotate_opencv(image, angle):
'''From http://stackoverflow.com/questions/9041681/opencv-python-rotate-image-by-x-degrees-around-specific-point
'''
image_center = tuple(np.array(image.shape)/2.0-0.5)
# Note: -0.5 offset; see http://answers.opencv.org/question/35111/origin-pixel-in-the-image-coordinate-system-in-opencv/
rot_mat = cv2.getRotationMatrix2D(image_center, angle, 1.0)
result = cv2.warpAffine(image, rot_mat, image.shape, flags=cv2.INTER_LINEAR)
return result


I verified this by comparing the result of rotation by 90 degrees with a direct transpose-and-flip-based rotation (numpy.rot90) function call.

I think it is related to this: http://answers.opencv.org/question/33...

If I understand that correctly, the pixel-coordinate-system and affine-transform-coordinate-system are offset by 0.5 pixels, leading to the common error.

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