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The already proposed solution will only work when having objects with circles. If not this will fail dramatically. What I suggest is, given these very simple conditions.

• Perform a segmentation of the object since the background is quite constant. My guess OTSU thresholding will work quite well on a grayscale image.
• Then do a findContours and get the contour of the object
• Apply minAreaRect on the found contour to get a rotatedRect associated to this object
• Retrieve the angle of the rotatedRect and warp it back with warpAffine