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10-Image Edge Detection with OpenCV

This section explains how to detect edges in an image using OpenCV's Canny edge detection algorithm.

Edge detection is commonly used for: - Feature extraction - Shape detection - Object boundary analysis - Preprocessing for vision pipelines

1. Implementation Principle​

OpenCV provides the cv2.Canny() function for edge detection.

cv2.Canny(image, threshold1, threshold2)

Where:

  • image is a grayscale image
  • threshold1 is the lower hysteresis threshold
  • threshold2 is the upper hysteresis threshold

The output is a binary image highlighting strong edges.

2. Implementation Effect​

Navigate to the OpenCV working directory:

cd ~/opencv

Run the edge detection script:

python3 10.image_edge.py
note

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

Image Edge Detection Result

3. Implementation Code​

import cv2

def edge_detection(input_path, output_path, threshold1, threshold2):
    image = cv2.imread(input_path, cv2.IMREAD_GRAYSCALE)

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

    edges = cv2.Canny(image, threshold1, threshold2)

    if cv2.imwrite(output_path, edges):
        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.")

edge_detection(
    '/home/jetson/opencv/images/hemihex_logo.png',
    '/home/jetson/opencv/images/hemihex_logo_edge.png',
    100,
    200
)

4. Code Explanation​

  • cv2.imread(..., cv2.IMREAD_GRAYSCALE) Loads the image in grayscale mode (required for Canny).

  • cv2.Canny() Detects edges based on gradient intensity.

  • cv2.imwrite() Saves the edge-detected image to disk.

  • cv2.imshow() / cv2.waitKey() / cv2.destroyAllWindows() Displays the result and closes the window.

Summary​

  • Edge detection highlights object boundaries
  • Canny is one of the most widely used edge detectors
  • Requires grayscale input
  • Threshold values control sensitivity

Maintained by HemiHex for OpenCV-based image processing workflows.