findCirclesGrid can't find sub-pattern with hollow insides

asked 2020-12-15 02:29:38 -0500

sas gravatar image

I'm trying to find a unique 2x2 hollow center sub-pattern inside a circle grid pattern. Using "detect", generates keypoints that indicate that the hollow circles are detected correctly, but the 2x2 pattern is not detected. Looking for a sub-pattern with full circles works.

The code could be boiled down to:

import numpy as np
import cv2
import matplotlib.pyplot as plt

orientation_grid_size = (2, 2)

params_orientaion = cv2.SimpleBlobDetector_Params()
params_orientaion.minArea = 200
params_orientaion.maxArea = 800

params_orientaion.minRepeatability = 1
params_orientaion.filterByColor = True
params_orientaion.blobColor = 255  # Detect only white centers

detector_orientation = cv2.SimpleBlobDetector_create(params_orientaion)

color_image = cv2.imread("1607961786.3423488.jpg")
color_image = cv2.cvtColor(color_image, cv2.COLOR_BGR2RGB)

status_orientation, grid_points_orientation = cv2.findCirclesGrid(color_image,

keypoints = detector_orientation.detect(color_image)
im_with_keypoints = cv2.drawKeypoints(color_image, keypoints, np.array([]), (255, 0, 0),

The original image: image description

The image with the keypoints: image description

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