Drawing with Mouse on Images using Python-OpenCV

OpenCV is a huge open-source library for computer vision, machine learning, and image processing. OpenCV supports a wide variety of programming languages like Python, C++, Java, etc. It can process images and videos to identify objects, faces, or even the handwriting of a human. In this article, we will try to draw on images with the help of the mouse. Before learning how to draw on images using a mouse, we need to understand what is a callback.<

Callback

A callback in programming means to call this function (the callback) when a process completes. The same applies to event-oriented programming in general. When a mouse button is clicked (an event), call a function. We don’t know when the button will be clicked. All we can do is tell the button to “call me back” or call this function when the mouse button is clicked.

Mouse Callbacks

A callback can happen when a user performs an operation using the mouse; this operation is usually known as an event. Only one callback is present for a mouse, which is setMouseCallback(), all mouse operation will call this function only.

We can have conditional blocks to execute something based on the event/operation performed using the mouse. The mouse events/operations could be:

• EVENT_MOUSEMOVE
• EVENT_LBUTTONDOWN
• EVENT_RBUTTONDOWN
• EVENT_LBUTTONUP
• EVENT_RBUTTONUP

When should this callback occur :

We want to have this call back only when we use the mouse on the pop-up window, which has the title as "Title of Popup Window."

cv2.namedWindow("Title of Popup Window")

Example 1: Draw Circle when we left-click on a popup with OpenCV : Python3

import cv2


img = cv2.imread("flower.jpg")

def draw_circle(event, x, y, flags, param):
    
    if event == cv2.EVENT_LBUTTONDOWN:
        print("hello")
        cv2.circle(img, (x, y), 100, (0, 255, 0), -1)
        
cv2.namedWindow(winname = "Title of Popup Window")
cv2.setMouseCallback("Title of Popup Window", draw_circle)

while True:
    cv2.imshow("Title of Popup Window", img)
    
    if cv2.waitKey(10) & 0xFF == 27:
        break
        
cv2.destroyAllWindows()

Output: Example 2: Drawing a Rectangle by dragging on Images with OpenCV Python3

import cv2


img = cv2.imread("flower.jpg")

# variables
ix = -1
iy = -1
drawing = False

def draw_rectangle_with_drag(event, x, y, flags, param):
    
    global ix, iy, drawing, img
    
    if event == cv2.EVENT_LBUTTONDOWN:
        drawing = True
        ix = x
        iy = y            
            
    elif event == cv2.EVENT_MOUSEMOVE:
        if drawing == True:
            cv2.rectangle(img, pt1 =(ix, iy),
                          pt2 =(x, y),
                          color =(0, 255, 255),
                          thickness =-1)
    
    elif event == cv2.EVENT_LBUTTONUP:
        drawing = False
        cv2.rectangle(img, pt1 =(ix, iy),
                      pt2 =(x, y),
                      color =(0, 255, 255),
                      thickness =-1)
        
cv2.namedWindow(winname = "Title of Popup Window")
cv2.setMouseCallback("Title of Popup Window", 
                     draw_rectangle_with_drag)

while True:
    cv2.imshow("Title of Popup Window", img)
    
    if cv2.waitKey(10) == 27:
        break

cv2.destroyAllWindows()

Output:

What does (cv2.waitKey(10) & 0xFF == 27) do ?

cv2.waitKey() returns a 32 Bit integer value (might be dependent on the platform). The key input is in ASCII which is an 8 Bit integer value. So you only care about these 8 bits and want all other bits to be 0. This you can achieve with: cv2.waitKey(10) & 0xFF == 27