Drawing a Rectangle Using OpenCV

Introduction

OpenCV (Open Source Computer Vision Library) is an open-source computer vision and machine learning software library. It contains several hundred computer vision algorithms and is widely used for real-time computer vision applications. One of the basic tasks you can perform with OpenCV is drawing shapes on images, such as rectangles.

Syntax for Drawing a Rectangle

The function used to draw a rectangle in OpenCV is cv2.rectangle(). The syntax is as follows:

• image: The image on which the rectangle is to be drawn.
• start_point: The top-left corner of the rectangle (x, y).
• end_point: The bottom-right corner of the rectangle (x, y).
• color: The color of the rectangle in BGR format.
• thickness: The thickness of the rectangle border. If -1 is passed, the rectangle is filled.

Example

Here is a simple example to demonstrate how to draw a rectangle on an image using OpenCV:

import cv2
import numpy as np


# Create a blank image (black background)
image = np.zeros((500, 500, 3), dtype="uint8")


# Define the top-left and bottom-right corners of the rectangle
top_left_corner = (100, 100)
bottom_right_corner = (400, 400)


# Define the color (BGR - Blue, Green, Red)
color = (255, 0, 0)  # Blue color


# Define the thickness of the rectangle border
thickness = 2  # 2 pixels


# Draw the rectangle
cv2.rectangle(image, top_left_corner, bottom_right_corner, color, thickness)


# Display the image
cv2.imshow("Rectangle", image)


# Wait for a key press and close the image window
cv2.waitKey(0)
cv2.destroyAllWindows()

In this example:

• We create a 500×500 pixel black image using NumPy.
• We define the coordinates for the top-left and bottom-right corners of the rectangle.
• We set the rectangle color to blue and the thickness to 2 pixels.
• We use the cv2.rectangle() function to draw the rectangle.
• Finally, we display the image using cv2.imshow() and wait for a key press to close the window.

This simple script effectively demonstrates how to draw a rectangle using OpenCV.