OpenCV Python ŌĆō How to draw curves using Mouse Events?

OpenCV is a popular computer vision library that includes a wide variety of image processing tasks. One important feature of OpenCV is the ability to use mouse events to interact with images. In this article, we’ll learn how to use mouse events to draw curves in OpenCV Python.

Required Libraries

Before we get started, we need to make sure we have the necessary libraries installed. In order to run OpenCV in Python, we need the following libraries:

pip install opencv-python 
pip install opencv-python-headless 

We also need the following libraries for handling mouse events:

pip install Pillow 
pip install numpy

Creating a blank canvas

The first step in drawing curves is to create a blank canvas. To do this, we import the necessary libraries and create a blank image:

import cv2 
import numpy as np 

# Create a blank canvas 
canvas = np.ones((500, 500, 3), dtype="uint8") * 255 

We create a canvas with a height and width of 500 pixels each and a depth of 3 (for RGB values). We set the initial pixel values to white (255).

Next, we define a function that will be called every time the mouse is clicked or dragged:

def draw_curve(event, x, y, flags, params): 
    # Check if the left mouse button was pressed 
    if event == cv2.EVENT_LBUTTONDOWN:
        # Store the starting points of the curve 
        params.start_points = [(x, y)]

    # Check if the mouse is being dragged 
    if event == cv2.EVENT_MOUSEMOVE and flags == cv2.EVENT_FLAG_LBUTTON:
        # Draw the curve as the mouse is being dragged 
        cv2.line(params.canvas, params.start_points[-1], (x, y), (0, 0, 0), thickness=3)
        # Update the starting points of the curve 
        params.start_points.append((x, y))

    # Check if the left mouse button was released 
    if event == cv2.EVENT_LBUTTONUP:
        # Draw the final curve 
        cv2.line(params.canvas, params.start_points[-1], (x, y), (0, 0, 0), thickness=3) 

This function takes five arguments:

• event: the type of mouse event (e.g. click, drag, release)
• x and y: the coordinates of the mouse
• flags: any additional flags that are sent with the mouse event
• params: additional parameters we want to pass to the function

The function first checks if the left mouse button was pressed. If it was, we store the starting point of the curve.

If the mouse is being dragged (i.e. the left mouse button is down and moving), we draw a line on the canvas as the mouse is being dragged. We also update the starting point of the curve.

If the left mouse button is released, we draw the final curve on the canvas.

Running the program

Now that we have our canvas and mouse event function, we can create a window and set the mouse event callback function:

# Store the starting points of the curve  
params = cv2.SimpleNamespace(start_points=(), canvas=canvas)

# Create a window for the canvas 
cv2.namedWindow("Canvas")

# Set the mouse callback function 
cv2.setMouseCallback("Canvas", draw_curve, param=params)

# Keep the window open 
while True:
    cv2.imshow("Canvas", params.canvas)
    if cv2.waitKey(1) == ord("q"):
        break 

cv2.destroyAllWindows() 

We create a namespace object to store the starting points of the curve and the canvas. We then create a window and set the mouse event callback function to our draw_curve function.

Finally, we display the canvas and wait for a keyboard event. When the “q” key is pressed, we break out of the loop and close the window.

Conclusion

In this article, we learned how to use mouse events to draw curves in OpenCV Python. By creating a blank canvas and setting a mouse event callback function, we were able to interact with the canvas and draw curves using the left mouse button. This technique can be applied to a wide variety of image processing tasks, including object detection, segmentation, and tracking.