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Add Ellipses with OpenCV

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

Ellipses are commonly used for: - Highlighting rounded regions of interest - Visual annotations - Debugging and visualization in computer vision workflows

1. Implementation Principle​

Use the cv2.ellipse() function to draw an ellipse on an image.

cv2.ellipse(image, center, axes, angle, start_angle, end_angle, color, thickness)

Where: - image is the input image - center is the (x, y) coordinate of the ellipse center - axes defines the length of the major and minor axes - angle is the rotation angle of the ellipse - start_angle and end_angle define the arc range - color is specified in BGR format - thickness controls line thickness (-1 fills the ellipse)

2. Implementation Effect​

Navigate to the OpenCV working directory:

cd ~/opencv

Run the ellipse drawing script:

python3 14.image_draw_ellipse.py
note

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

Ellipse Drawing Result

3. Implementation Code​

import cv2

def draw_ellipse(input_path, output_path, center, axes, angle, start_angle, end_angle, color, thickness):
    image = cv2.imread(input_path)

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

    cv2.ellipse(image, center, axes, angle, start_angle, end_angle, 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_ellipse(
    '/home/jetson/opencv/images/hemihex_logo.png',
    '/home/jetson/opencv/images/hemihex_logo_ellipse.png',
    (400, 85),
    (375, 75),
    0,
    0,
    360,
    (0, 255, 0),
    2
)

Summary​

  • Ellipses are drawn using cv2.ellipse()
  • Supports rotation and partial arcs
  • Useful for rounded annotations and visual overlays

Maintained by HemiHex for OpenCV-based image processing workflows.