Tutorial: Handling Mouse Events in OpenCV with Python

OpenCV provides robust functionality to handle mouse events, enabling interactive applications.

With mouse callbacks, you can respond to actions like clicks, drags, and movements, making it a powerful tool for creating GUI-based applications.

What You’ll Learn

1. Introduction to Mouse Events

OpenCV captures mouse events using the function cv2.setMouseCallback(window_name, callback_function). The callback function processes mouse events like left-click, right-click, and movement.

Mouse events include:

• cv2.EVENT_LBUTTONDOWN: Left mouse button pressed
• cv2.EVENT_RBUTTONDOWN: Right mouse button pressed
• cv2.EVENT_MOUSEMOVE: Mouse moved
• cv2.EVENT_LBUTTONUP: Left mouse button released
• cv2.EVENT_LBUTTONDBLCLK: Left mouse button double-clicked

2. Setting Up a Mouse Callback

Example: Basic Mouse Callback

import cv2

# Mouse callback function
def mouse_event(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        print(f"Left button clicked at ({x}, {y})")
    elif event == cv2.EVENT_RBUTTONDOWN:
        print(f"Right button clicked at ({x}, {y})")
    elif event == cv2.EVENT_MOUSEMOVE:
        print(f"Mouse moved to ({x}, {y})")

# Create a black image and window
image = cv2.imread("input.jpg")
cv2.namedWindow("Mouse Events")

# Set the mouse callback
cv2.setMouseCallback("Mouse Events", mouse_event)

while True:
    cv2.imshow("Mouse Events", image)
    if cv2.waitKey(1) & 0xFF == ord('q'):  # Quit on 'q'
        break

cv2.destroyAllWindows()

3. Tracking Mouse Movements

Example: Display Real-Time Mouse Coordinates

import cv2

# Mouse callback function
def show_coordinates(event, x, y, flags, param):
    if event == cv2.EVENT_MOUSEMOVE:
        print(f"Mouse moved to ({x}, {y})")

# Create a blank window
image = cv2.imread("input.jpg")
cv2.namedWindow("Mouse Tracker")

# Set the mouse callback
cv2.setMouseCallback("Mouse Tracker", show_coordinates)

while True:
    cv2.imshow("Mouse Tracker", image)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cv2.destroyAllWindows()

4. Handling Click Events

Example: Draw Points on Click

import cv2

# List to store points
points = []

# Mouse callback function
def draw_point(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        points.append((x, y))
        print(f"Point added: ({x}, {y})")

# Create a blank image
image = cv2.imread("input.jpg")
cv2.namedWindow("Draw Points")

cv2.setMouseCallback("Draw Points", draw_point)

while True:
    # Draw all points
    for point in points:
        cv2.circle(image, point, 5, (0, 255, 0), -1)

    cv2.imshow("Draw Points", image)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cv2.destroyAllWindows()

5. Drawing Shapes with Mouse

Example: Draw Rectangles

import cv2

# Variables to store the starting and ending points
start_point = None
end_point = None
drawing = False

# Mouse callback function
def draw_rectangle(event, x, y, flags, param):
    global start_point, end_point, drawing

    if event == cv2.EVENT_LBUTTONDOWN:
        start_point = (x, y)
        drawing = True
    elif event == cv2.EVENT_MOUSEMOVE and drawing:
        end_point = (x, y)
    elif event == cv2.EVENT_LBUTTONUP:
        end_point = (x, y)
        drawing = False

# Create a blank image
image = cv2.imread("input.jpg")
temp_image = image.copy()
cv2.namedWindow("Draw Rectangle")
cv2.setMouseCallback("Draw Rectangle", draw_rectangle)

while True:
    # Create a copy of the image to draw on
    temp_image = image.copy()
    if start_point and end_point:
        cv2.rectangle(temp_image, start_point, end_point, (255, 0, 0), 2)

    cv2.imshow("Draw Rectangle", temp_image)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cv2.destroyAllWindows()

Example: Draw Circles

import cv2

# Mouse callback function
def draw_circle(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        cv2.circle(image, (x, y), 20, (0, 0, 255), -1)

# Create a blank image
image = cv2.imread("input.jpg")
cv2.namedWindow("Draw Circle")
cv2.setMouseCallback("Draw Circle", draw_circle)

while True:
    cv2.imshow("Draw Circle", image)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cv2.destroyAllWindows()

6. Practical Examples

6.1 Color Picker

import cv2

# Mouse callback function
def pick_color(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        color = image[y, x]
        print(f"Color at ({x}, {y}): BGR {color}")

# Load an image
image = cv2.imread("input.jpg")
cv2.namedWindow("Color Picker")
cv2.setMouseCallback("Color Picker", pick_color)

while True:
    cv2.imshow("Color Picker", image)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cv2.destroyAllWindows()

6.2 Freehand Drawing

import cv2
import numpy as np

drawing = False

# Mouse callback function
def draw_freehand(event, x, y, flags, param):
    global drawing
    if event == cv2.EVENT_LBUTTONDOWN:
        drawing = True
    elif event == cv2.EVENT_MOUSEMOVE:
        if drawing:
            cv2.circle(image, (x, y), 3, (0, 255, 0), -1)
    elif event == cv2.EVENT_LBUTTONUP:
        drawing = False

# Create a blank canvas
image = np.zeros((500, 500, 3), dtype="uint8")
cv2.namedWindow("Freehand Drawing")
cv2.setMouseCallback("Freehand Drawing", draw_freehand)

while True:
    cv2.imshow("Freehand Drawing", image)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cv2.destroyAllWindows()

7. Summary

Key Functions

• cv2.setMouseCallback(window_name, callback_function): Sets a mouse callback for a specific window.
• Mouse Events:
  • cv2.EVENT_LBUTTONDOWN: Left button down.
  • cv2.EVENT_RBUTTONDOWN: Right button down.
  • cv2.EVENT_MOUSEMOVE: Mouse movement.
  • cv2.EVENT_LBUTTONUP: Left button up.

Best Practices

1. Use global variables to store state (e.g., start and end points).
2. Use real-time updates (imshow) to provide visual feedback.
3. Combine mouse events with OpenCV drawing functions for interactivity.