[FIXED] error: (-215:Assertion failed) scn + 1 == m.cols in function 'cv::perspectiveTransform'

Issue

Below is a python script that calculates the homography between two images and then map a desired point from one image to another

import cv2

import numpy as np


if __name__ == '__main__' :

  # Read source image.
  im_src = cv2.imread(r'C:/Users/kjbaili/.spyder-py3/webcam_calib/homography/khaledd 35.0 sec.jpg')

  # Five corners of the book in source image
  pts_src = np.array([[281, 238], [325, 297], [283, 330],[248, 325],[213, 321]])

  # Read destination image.
  im_dst = cv2.imread(r'C:/Users/kjbaili/.spyder-py3/webcam_calib/homography/20.jpg')

  # Five corners of the book in destination image.
  pts_dst = np.array([[377, 251],[377, 322],[316, 315],[289, 284],[263,255]])



  # Calculate Homography

  h, status = cv2.findHomography(pts_src, pts_dst)


  
  # provide a point i wish to map from image 1 to image 2
  a = np.array([[260, 228]])


  

  pointsOut = cv2.getPerspectiveTransform(a, h)

  # Display image
  cv2.imshow("treced_point_image", pointsOut)


  cv2.waitKey(0)
cv2.destroyAllWindows()

However, when i display the image that contains the mapped point it returns the following error:

error: OpenCV(4.2.0) C:\projects\opencv-python\opencv\modules\core\src\matmul.dispatch.cpp:531: 
error: (-215:Assertion failed) scn + 1 == m.cols in function 'cv::perspectiveTransform'

According to my knowledge this error means that parameter assigned to the function perspective transform is not correct or not being read. I checked the two images at the reading step and everything is fine. So anyone knows why this happens?

Thanks in advance Khaled

Solution

You are passing the wrong arguments to cv2.getPerspectiveTransform(). The function expects a set of four coordinates in the original image and the new coordinates in the transormed image. You can directly pass the pts_src and pts_dst to the funtion and you will get the transformation matrix. You can then get the transformed coordinates for point "a" by matrix multiplication like a_transformed = np.dot(matrix, a).

Answered By - chris