OpenCV Python - Handling Mouse Events

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OpenCV is capable of registering various mouse related events with a callback function. This is done to initiate a certain user defined action depending on the type of mouse event.

Sr.No	Mouse event & Description
1	cv.EVENT_MOUSEMOVE When the mouse pointer has moved over the window.
2	cv.EVENT_LBUTTONDOWN Indicates that the left mouse button is pressed.
3	cv.EVENT_RBUTTONDOWN Event of that the right mouse button is pressed.
4	cv.EVENT_MBUTTONDOWN Indicates that the middle mouse button is pressed.
5	cv.EVENT_LBUTTONUP When the left mouse button is released.
6	cv.EVENT_RBUTTONUP When the right mouse button is released.
7	cv.EVENT_MBUTTONUP Indicates that the middle mouse button is released.
8	cv.EVENT_LBUTTONDBLCLK This event occurs when the left mouse button is double clicked.
9	cv.EVENT_RBUTTONDBLCLK Indicates that the right mouse button is double clicked.
10	cv.EVENT_MBUTTONDBLCLK Indicates that the middle mouse button is double clicked.
11	cv.EVENT_MOUSEWHEEL Positive for forward and negative for backward scrolling.

To fire a function on a mouse event, it has to be registered with the help of setMouseCallback() function. The command for the same is as follows −

cv2.setMouseCallback(window, callbak_function)

This function passes the type and location of the event to the callback function for further processing.

Example 1

Following code draws a circle whenever left button double click event occurs on the window showing an image as background −

import numpy as np
import cv2 as cv
# mouse callback function
def drawfunction(event,x,y,flags,param):
   if event == cv.EVENT_LBUTTONDBLCLK:
      cv.circle(img,(x,y),20,(255,255,255),-1)
img = cv.imread('lena.jpg')
cv.namedWindow('image')
cv.setMouseCallback('image',drawfunction)
while(1):
   cv.imshow('image',img)
   key=cv.waitKey(1)
   if key == 27:
      break
cv.destroyAllWindows()

Output

Run the above program and double click at random locations. The similar output will appear −

Example 2

Following program interactively draws either rectangle, line or circle depending on user input (1,2 or 3) −

import numpy as np
import cv2 as cv
# mouse callback function

drawing=True
shape='r'

def draw_circle(event,x,y,flags,param):
   global x1,x2
   if event == cv.EVENT_LBUTTONDOWN:
      drawing = True
      x1,x2 = x,y
   elif event == cv.EVENT_LBUTTONUP:
      drawing = False
      if shape == 'r':
         cv.rectangle(img,(x1,x2),(x,y),(0,255,0),-1)
      if shape == 'l':
         cv.line(img,(x1,x2),(x,y),(255,255,255),3)
      if shape=='c':
         cv.circle(img,(x,y), 10, (255,255,0), -1)
img = cv.imread('lena.jpg')
cv.namedWindow('image')
cv.setMouseCallback('image',draw_circle)
while(1):
   cv.imshow('image',img)
   key=cv.waitKey(1)
   if key==ord('1'):
      shape='r'
   if key==ord('2'):
      shape='l'
   if key==ord('3'):
      shape='c'

   #print (shape)
   if key == 27:
      break
   cv.destroyAllWindows()

On the window surface, a rectangle is drawn between the coordinates of the mouse left button down and up if ‘1’ is pressed.

If user choice is 2, a line is drawn using coordinates as endpoints.

On choosing 3 for the circle, it is drawn at the coordinates of the mouse up event.

Following image will be the output after the successful execution of the above mentioned program −