get the numpy index for a given image location

Question

Given an image, are there any ways to let user interactively knows its location by pointing to a specific image location. For instance, when I point to the location with angle, what are the corresponding index of the numpy array used to represent this image.

Answer 1

You can set a MouseCallback function to any window generated with OpenCV. Following is a short example code derived from this blog post. An image is shown in some window, and when the left mouse button is clicked, the x, y coordinates are printed. (That's how I understood your question.)

import cv2

# Actual mouse callback function
def print_coords(event, x, y, flags, param):

    # If left mouse button is clicked, print image coordinates
    if (event == cv2.EVENT_LBUTTONDOWN):
        print([x, y])

# Read an image
image = cv2.imread('path/to/your/image.png', cv2.IMREAD_COLOR)

# Set up window and mouse callback function
cv2.namedWindow("image")
cv2.setMouseCallback("image", print_coords)

# Loop until the 'c' key is pressed
while True:

    # Display image; wait for keypress
    cv2.imshow("image", image)
    key = cv2.waitKey(1) & 0xFF

    # If 'c' key is pressed, break from loop
    if  key == ord("c"):
        break

cv2.destroyAllWindows()

Some mouse clicking inside the window then gives such outputs:

[158, 239]
[63, 6]
[9, 18]
[59, 101]
[128, 279]
[298, 249]

Now, of course, you can do whatever you want inside the mouse callback function. Pay attention, if you eventually need global variables.

Hope that helps!