Implement Mouse Callback Events on Images using OpenCV in Python

This post will be helpful in learning OpenCV using Python programming. Here I will show how to implement OpenCV functions and apply it in various aspects using some examples. Then the output will be shown with some comparisons as well.

Requirements:

OpenCV 3.4
Python 3.6+
Numpy
Image, Webcam or Video input
Documentation Source: OpenCV Official Documentation

First, you need to setup your Python Environment with OpenCV. You can easily do it by following Life2Coding’s tutorial on YouTube: Linking OpenCV 3 with Python 3

Goals:

In this tutorial, I will show you how to implement mouse callback functions using OpenCV library and Python coding.

Documentation:

Python: cv2.setMouseCallback(windowName, onMouse[, param]) → None

Sets mouse handler for the specified window

Parameters:

winname – Window name
onMouse – Name of the callback function. Whenever mouse events related to the above window occur, this callback function will be called. This function should have the signature like the following
- void FunctionName (event, x, y, flags, userdata)
  - event – Type of the mouse event. These are the entire list of mouse events
    - EVENT_MOUSEMOVE
    - EVENT_LBUTTONDOWN
    - EVENT_RBUTTONDOWN
    - EVENT_MBUTTONDOWN
    - EVENT_LBUTTONUP
    - EVENT_RBUTTONUP
    - EVENT_MBUTTONUP
    - EVENT_LBUTTONDBLCLK
    - EVENT_RBUTTONDBLCLK
    - EVENT_MBUTTONDBLCLK
  - x – x coordinate of the mouse event
  - y – y coordinate of the mouse event
  - flags – Specific condition whenever a mouse event occurs. See the next OpenCV example code for the usage of this parameter. Here is the entire list of enum values which will be possesed by “flags”
    - EVENT_FLAG_LBUTTON
    - EVENT_FLAG_RBUTTON
    - EVENT_FLAG_MBUTTON
    - EVENT_FLAG_CTRLKEY
    - EVENT_FLAG_SHIFTKEY
    - EVENT_FLAG_ALTKEY
userdata – The optional parameter passed to the callback.

Steps:

Create window using cv2.namedWindow()
Register mouse callback using cv2.setMouseCallback()
Display Image using cv2.imshow()
Wait for keyboard button press using cv2.waitKey()
Exit window and destroy all windows using cv2.destroyAllWindows()

Example Code:

import cv2
import numpy as np

# Create a black image and a window
windowName = 'MouseCallback'
img = np.zeros((512, 512, 3), np.uint8)
cv2.namedWindow(windowName)

def CallBackFunc(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        print("Left button of the mouse is clicked - position (", x, ", ",y, ")")
    elif event == cv2.EVENT_RBUTTONDOWN:
        print("Right button of the mouse is clicked - position (", x, ", ", y, ")")
    elif event == cv2.EVENT_MBUTTONDOWN:
        print("Middle button of the mouse is clicked - position (", x, ", ", y, ")")
    elif event == cv2.EVENT_MOUSEMOVE:
        print("Mouse move over the window - position (", x, ", ", y, ")")
    if flags == cv2.EVENT_FLAG_CTRLKEY + cv2.EVENT_FLAG_LBUTTON:
        print("Left mouse button is clicked while pressing CTRL key - position (", x, ", ",y, ")")
    elif flags == cv2.EVENT_FLAG_RBUTTON + cv2.EVENT_FLAG_SHIFTKEY:
        print("Right mouse button is clicked while pressing SHIFT key - position (", x, ", ", y, ")")
    elif event == cv2.EVENT_MOUSEMOVE and flags == cv2.EVENT_FLAG_ALTKEY:
        print("Mouse is moved over the window while pressing ALT key - position (", x, ", ", y, ")")

# bind the callback function to window
cv2.setMouseCallback(windowName, CallBackFunc)


def main():
    while (True):
        cv2.imshow(windowName, img)
        if cv2.waitKey(20) == 27:
            break

    cv2.destroyAllWindows()


if __name__ == "__main__":
    main()