Python OpenCV cv2.circle() Guide

OpenCV is a powerful library for image processing. One of its key functions is cv2.circle(). This function allows you to draw circles on images. It is useful for highlighting areas of interest.

What is cv2.circle()?

The cv2.circle() function is used to draw circles on images. You can specify the center, radius, color, and thickness of the circle. This function is often used in image annotation and object detection.

Syntax of cv2.circle()

The syntax for cv2.circle() is straightforward. It requires an image, center coordinates, radius, color, and thickness. Here is the basic syntax:

cv2.circle(image, center, radius, color, thickness)

Parameters:

• image: The image on which the circle is drawn.
• center: The center coordinates of the circle (x, y).
• radius: The radius of the circle.
• color: The color of the circle in BGR format.
• thickness: The thickness of the circle outline. Use -1 to fill the circle.

Example: Drawing a Circle on an Image

Let's see how to use cv2.circle() to draw a circle on an image. First, we need to load an image using cv2.imread(). Then, we can draw the circle and display the result.

import cv2

# Load an image
image = cv2.imread('image.jpg')

# Define the center and radius of the circle
center = (100, 100)
radius = 50

# Define the color (BGR format) and thickness
color = (0, 255, 0)  # Green
thickness = 2

# Draw the circle on the image
cv2.circle(image, center, radius, color, thickness)

# Display the image
cv2.imshow('Circle on Image', image)
cv2.waitKey(0)
cv2.destroyAllWindows()

In this example, a green circle with a radius of 50 pixels is drawn at the center (100, 100) on the image. The circle has a thickness of 2 pixels.

Filling a Circle

To fill a circle, set the thickness parameter to -1. This will create a solid circle instead of an outline.

import cv2

# Load an image
image = cv2.imread('image.jpg')

# Define the center and radius of the circle
center = (100, 100)
radius = 50

# Define the color (BGR format) and thickness
color = (0, 0, 255)  # Red
thickness = -1  # Fill the circle

# Draw the filled circle on the image
cv2.circle(image, center, radius, color, thickness)

# Display the image
cv2.imshow('Filled Circle on Image', image)
cv2.waitKey(0)
cv2.destroyAllWindows()

This code will draw a red filled circle at the specified center and radius.

Combining cv2.circle() with Other Functions

cv2.circle() can be combined with other OpenCV functions for more advanced image processing. For example, you can use cv2.rectangle() to draw rectangles alongside circles. Check out our Python OpenCV cv2.rectangle() Guide for more details.

You can also use cv2.circle() with cv2.findContours() to highlight detected objects. Learn more in our Python OpenCV cv2.findContours() Guide.

Conclusion

The cv2.circle() function is a simple yet powerful tool in OpenCV. It allows you to draw circles on images with ease. Whether you're annotating images or highlighting objects, this function is essential for image processing tasks.

For more OpenCV tutorials, check out our guides on Canny Edge Detection and Gaussian Blur.