Specify an origin to warpPerspective() function in OpenCV 2.x

Question

I try to specify a different origin for the warpPerspective() function than the basic (0,0), in order to apply the transform independently of the support image size. I added

Answer 1

A much simpler and cleaner solution is to modify the perspective transformation. You can do a translation which moves the origin to the desired position, then do the perspective transformation and finally do the inverse translation.

Here is a small example program in python, which rotates an image by 45 degrees around the point(100, 100):

import cv2
import numpy as np


def translation_mat(dx, dy):
return np.array([1, 0, dx, 0, 1, dy, 0, 0,     1]).reshape((3,3))

def main():
    img = cv2.imread(r"pigeon.png", cv2.IMREAD_GRAYSCALE)

    # a simple rotation by 45 degrees
rot = np.array([np.sin(np.pi/4), -np.cos(np.pi/4), 0, np.cos(np.pi/4), np.sin(np.pi/4), 0, 0, 0, 1]).reshape((3,3))
    t1 = translation_mat(-100, -100)
    t2 = translation_mat(100, 100)
    rot_shifted = t2.dot(rot.dot(t1))
    size = (img.shape[1], img.shape[0])

    img1 = cv2.warpPerspective(img, rot, size)
    img2 = cv2.warpPerspective(img, rot_shifted, size)

    cv2.imshow("Original image", img)
    cv2.imshow("Rotated around (0,0)", img1)
    cv2.imshow("Rotated around(100, 100)", img2)
    cv2.waitKey(0)


if __name__ == '__main__':
    main()

Not that you read the order of transformations from right to left.

rot_shifted = t2.dot(rot.dot(t1))

will apply t1 first, then rot, and then t2.