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Add Rectangles with OpenCV

This section explains how to draw rectangles on an image using OpenCV in Python.

Drawing rectangles is commonly used for: - Highlighting regions of interest (ROI) - Visualizing detection results - Debugging and annotation in computer vision pipelines

1. Implementation Principle​

OpenCV provides the cv2.rectangle() function to draw a rectangle directly on an image.

cv2.rectangle(image, top_left, bottom_right, color, thickness)

Where: - image is the input image - top_left is the (x, y) coordinate of the top-left corner - bottom_right is the (x, y) coordinate of the bottom-right corner - color is the rectangle color in BGR format - thickness defines the line thickness (-1 fills the rectangle)

2. Implementation Effect​

Navigate to the OpenCV working directory:

cd ~/opencv

Run the rectangle drawing script:

python3 12.image_draw_rectangle.py
note

Select the image window and press q to exit the program.

Rectangle Drawing Result

3. Implementation Code​

import cv2

def draw_rectangle(input_path, output_path, top_left, bottom_right, color, thickness):
    image = cv2.imread(input_path)

    if image is None:
        print("Error: Unable to open image file.")
        return

    cv2.rectangle(image, top_left, bottom_right, color, thickness)

    if cv2.imwrite(output_path, image):
        print(f"Image saved to {output_path}")
        cv2.imshow('Image Preview', cv2.imread(output_path))
        cv2.waitKey(0)
        cv2.destroyAllWindows()
    else:
        print("Error: Unable to save image file.")

draw_rectangle(
    '/home/jetson/opencv/images/hemihex_logo.png',
    '/home/jetson/opencv/images/hemihex_logo_rectangle.png',
    (25, 25),
    (750, 150),
    (0, 255, 0),
    5
)

4. Code Explanation​

  • cv2.imread() loads the source image
  • cv2.rectangle() draws the rectangle
  • cv2.imwrite() saves the annotated image
  • Display functions preview the result

Maintained by HemiHex for OpenCV-based image processing workflows.