Matrix from cv2.getPerspectiveTransform does not transform points appropriately-CodePudding

The

Can someone explain why the transformed corner points do not match the corners of the displayed window?

Attached below is my code:

import cv2
import numpy as np


def mouse_position(event, mouse_x, mouse_y, flags, param):
    global corners
    if event == cv2.EVENT_LBUTTONDOWN:
        print(f"Corner #{len(corners)   1}: {(mouse_x, mouse_y)}")
        corners.append((mouse_x, mouse_y))


corners = []
img = cv2.imread("Assets/Setup.jpg")
cv2.namedWindow("Select Corners of Pool Table")
cv2.setMouseCallback("Select Corners of Pool Table", mouse_position)
while len(corners) < 4:
    cv2.imshow("Select Corners of Pool Table", img)
    if cv2.waitKey(1) != -1:
        print("Exiting program.")
        cv2.destroyAllWindows()
        exit(0)
cv2.destroyAllWindows()
corners = np.array(corners, dtype="float32")
height, width = img.shape[:2]
transform_matrix = cv2.getPerspectiveTransform(corners, np.array([(0, 0), (width, 0), (0, height), (width, height)],
                                                                 dtype="float32"))
undistorted_image = cv2.warpPerspective(img, transform_matrix, (width, height))
transformed_corners = (transform_matrix @ np.hstack((corners, np.ones((corners.shape[0], 1)))).T).T.astype(int)
for corner in transformed_corners:
    cv2.circle(undistorted_image, (corner[0], corner[1]), 30, (0, 0, 255), -1)
cv2.imshow("Frame", undistorted_image)
cv2.waitKey(0)
cv2.destroyAllWindows()

CodePudding user response：

You can use cv2.perspectiveTransform to transform points.

If you want to do this manually, by multiplying (x,y,1) with the 3x3 transformation matrix, make sure that you divide the result by its z value, so it will be (x',y',1) and is a homogeneous point representation.