cv2.cornersSubPix gives only None

Question

I have no idea, why this openCV function gives me values of None, have any body some thoughts?

while True:


    retval,frame = capture.read()
    h, w = frame.shape[:2]
    gray = cv2.cvtColor(frame, cv2.cv.CV_BGR2GRAY)
    found, points = cv2.findChessboardCorners(gray, p_size)


       if found!=0:
        frame_count = frame_count + 1 
        objpoints.append(objp)
        corners2 = cv2.cornerSubPix(gray, points, (11, 11), (-1,1), criteria)
        imgpoints.append(corners2) </i>

Answer 1

If you check out the docs about cv2.cornerSubPix, you can see that it returns None. Actually it modifies the array of corners passed as the argument.

cv2.cornerSubPix(image, corners, winSize, zeroZone, criteria) → None