Use OpenCV to identiy hollow and filled circles


I’m using OpenCV houghcircles to identify all the circles (both hollow and filled). Follow is my code:

import numpy as np
import cv2

img = cv2.imread('images/32x32.png')

gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

bilateral = cv2.bilateralFilter(gray,10,50,50)

minDist = 30
param1 = 30
param2 = 50
minRadius = 5
maxRadius = 100

circles = cv2.HoughCircles(bilateral, cv2.HOUGH_GRADIENT, 1, minDist, param1=param1, param2=param2, minRadius=minRadius, maxRadius=maxRadius)

if circles is not None:
    circles = np.uint16(np.around(circles))
    for i in circles[0,:]:, (i[0], i[1]), i[2], (0, 0, 255), 2)

# Show result for testing:
cv2.imshow('img', img)

Test input image 1:
enter image description here

Test output image1:
enter image description here

As you can see I’m able identity most of the circles except for few. What am I missing here? I’ve tried varying the parameters but this is the best i could get.

Also, if I use even more compact circles the script does not identify any circles whatsoever.

enter image description here


An alternative idea is to use find contour method and chek whether the contour is a circle using appox as below.

import cv2

img = cv2.imread('32x32.png')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

inputImageCopy = img.copy()

# Find the circle blobs on the binary mask:
contours, hierarchy = cv2.findContours(gray, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

# Use a list to store the center and radius of the target circles:
detectedCircles = []

# Look for the outer contours:
for i, c in enumerate(contours):

    # Approximate the contour to a circle:
    (x, y), radius = cv2.minEnclosingCircle(c)

    peri = cv2.arcLength(c, True)
    approx = cv2.approxPolyDP(c, 0.02 * peri, True)
    if len(approx)>5: # check if the contour is circle
        # Compute the center and radius:
        center = (int(x), int(y))
        radius = int(radius)

        # Draw the circles:, center, radius, (0, 0, 255), 2)

        # Store the center and radius:
        detectedCircles.append([center, radius])

cv2.imshow("Circles", inputImageCopy)

enter image description here

enter image description here

Answered By – cyborg

Answer Checked By – Gilberto Lyons (AngularFixing Admin)

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