opencv mouse callback isn't being triggered

Question

Take a look at this function:

def showImage(im):
    def printColor(event, x, y, flag, params):
        if event == cv2.EVENT_LBUTTONDOWN:
            print(im[x,y])
            sys.exit(1)

    tag = "image"
    cv2.setMouseCallback(tag, printColor)
    cv2.imshow(tag, im)
    while True:
        if 'q' == chr(cv2.waitKey() & 255):
            cv2.destroyAllWindows()
            break

It's supposed to display an image and print the pixel at mouse position when clicked. But for some reason the callback isn't being triggered. How can I get this code working?

Answer 1

For setMouseCallback to work you will need to create window object first.

This can be done either by calling imshow before setting mouse callback, or by creating it with cv2.namedWindow()