Opencv HoughCircles missing obvious circle

Question

I have tried some variations in the parameters, read a detailed description of their meaning, but I can't seem to detect a simple circle in an image. This is a simplified function I have tried:

def get_a_circles(image):
    gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
    circles = cv2.HoughCircles(gray, # input grayscale image
                              cv2.HOUGH_GRADIENT, 
                              2, 
                              minDist=5, 
                              param1=10, param2=200,
                              minRadius=0, 
                               maxRadius=0) 
    return circles

which, when run on this image:

img = cv2.imread("step2.jpg")
get_a_circle(img.copy())

returns none. It does however detect the circles in this image:

image	circles found & highlighted

I tried to add some blur to the image that fails, with either gray = cv2.medianBlur(gray, 5) or gray = cv2.GaussianBlur(gray,(5,5),0) but it does not improve the results.

Any suggestions on what to try with that image? (It would seem it's an obvious circle)

Answer 1

You need to lower param2 in HoughCircles in Python/OpenCV.

Input:

import cv2
import numpy as np

# Read image
img = cv2.imread('dot.jpg')
hh, ww = img.shape[:2]

# Convert to grayscale
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

# median filter
gray = cv2.medianBlur(gray, 5)

# get Hough circles
min_dist = int(ww/20)
circles = cv2.HoughCircles(gray, cv2.HOUGH_GRADIENT, 1, minDist=min_dist, param1=150, param2=10, minRadius=0, maxRadius=0)
print(circles)

# draw circles
result = img.copy()
for circle in circles[0]:
    # draw the circle in the output image, then draw a rectangle
    # corresponding to the center of the circle
    (x,y,r) = circle
    x = int(x)
    y = int(y)
    cv2.circle(result, (x, y), r, (0, 0, 255), 1)

# save results
cv2.imwrite('dot_circle.jpg', result)

# show images
cv2.imshow('result', result)
cv2.waitKey(0)
cv2.destroyAllWindows()

Result: