OpenCV-Based Circle Detection in Python
OpenCV's HoughCircles() Function for Circle Detection
OpenCV, a popular open-source computer vision library, offers a powerful tool for circle detection: the HoughCircles() function. This function utilises a specialised form of the Hough Transform called HOUGH_GRADIENT, which leverages edge gradients for faster and more accurate circle detection.
The HoughCircles() function takes an input grayscale image and uses the Canny edge detector gradients to identify potential circle centers and radii efficiently. The function signature and parameters are as follows:
- : The input grayscale image.
- : Detection method; typically .
- : The inverse ratio of the accumulator resolution to the image resolution.
- : Minimum distance between the centers of detected circles to avoid multiple detections of the same circle.
- : The higher threshold for the Canny edge detector used internally.
- : Accumulator threshold for the circle centers at the detection stage.
- : Minimum circle radius to detect.
- : Maximum circle radius to detect.
The detection process involves detecting edges via Canny, then mapping edge points into Gaussian accumulators for circle parameters, and finally deciding circle centers and radii based on accumulator peaks. Tuning the parameters, especially , , , and radius limits, is crucial for robust detection and varies according to the application and image conditions.
OpenCV's HoughCircles() function returns a list of circles, each containing the center (x, y) coordinates and the radius of the detected circles. This function is significant in various fields, including biomedicine, robotics, quality inspection, and object tracking.
In an example of detecting circles in an eye image, the image is duplicated to draw on without changing the original, converted to grayscale, median blurred with a kernel size of 5 to remove noise, circles are detected using HOUGH_GRADIENT, and the first circle's center and radius are extracted.
It's worth noting that OpenCV provides an optimized approach called HOUGH_GRADIENT for faster circle detection, which is different from the classical Hough Transform for Circles. The latter involves pre-processing the image, applying blurring, converting to grayscale, detecting edges, creating an accumulator matrix, allowing edge pixels to "vote" for potential circle centers and radii, and picking the best candidates based on the highest votes.
In conclusion, OpenCV's HoughCircles() function is a valuable tool for circle detection in images, offering a faster and more accurate method compared to the classical Hough Transform for Circles. By understanding the function's parameters and tuning them according to the application and image conditions, users can achieve robust circle detection results.
References: 1. Detailed breakdown from GeeksforGeeks 2. Recommendations on parameter adjustments for practical use 3. Application examples in research contexts
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