Deskew+Scale+Align

def deskew(img, SZ, bbox=16):
    m = cv2.moments(img)
    if abs(m['mu02']) < 1e-2 or m['m00'] < 1:
        return img.copy()
    skew = m['mu11']/m['mu02']
    cx, cy = m['m10']/m['m00'], m['m01']/m['m00']
    M1 = np.float32([
        [1, -skew],
        [0, 1]
    ])
    b1 = np.float32([
        [0.5*SZ*skew],
        [0]
    ])

    def deskew_pt((x, y)):
        return x-skew*y+0.5*SZ*skew, y

    _cx, _cy = deskew_pt((cx, cy))
    ctr = SZ/2
    margin = (SZ-bbox)/2
    if _cy <= img.shape[0]/2:
        # center of mass is above the center of the image
        # the bottom part is longer, so scaling will be counted from that
        scale = (bbox/2) / (img.shape[0]-1-_cy)
        vproj = deskew_pt((cx, img.shape[0]-1))
        hproj = deskew_pt((0, cy))
        src_tri = np.float32([(_cx, _cy), (_cx, vproj[1]), (hproj[0], _cy)])
        dst_tri = np.float32([(ctr, ctr), (ctr, SZ-1-margin), (ctr-(img.shape[1]/2)*scale, ctr)])
    else:
        # the top part is longer
        scale = (bbox/2) / _cy
        vproj = deskew_pt((cx, 0))
        hproj = deskew_pt((0, cy))
        src_tri = np.float32([(_cx, _cy), (_cx, vproj[1]), (hproj[0], _cy)])
        dst_tri = np.float32([(ctr, ctr), (ctr, margin), (ctr-(img.shape[1]/2)*scale, ctr)])
    MM = cv2.getAffineTransform(src_tri, dst_tri)
    M2 = MM[:, 0:2]
    b2 = MM[:, 2:]

    M = np.empty((2, 3))
    M[:, 0:2] = np.dot(M2, M1)
    M[:, 2:] = b2 + np.dot(M2, b1)

    img = cv2.warpAffine(img, M, (SZ, SZ), flags=cv2.INTER_CUBIC)
    return img