Get the maximum/largest inscribed circle inside mask/polygon/contours based on cv2. Support non-convex/hollow shape.

get_max_inscribed_circle.py

import cv2
import numpy as np
def get_test_mask():
    # Create an image
    r = 100
    mask = np.zeros((4 * r, 4 * r), dtype=np.uint8)

    # Create a sequence of points to make a contour
    vert = [None] * 6
    vert[0] = (3 * r // 2, int(1.34 * r))
    vert[1] = (1 * r, 2 * r)
    vert[2] = (3 * r // 2, int(2.866 * r))
    vert[3] = (5 * r // 2, int(2.866 * r))
    vert[4] = (3 * r, 2 * r)
    vert[5] = (5 * r // 2, int(1.34 * r))
    # Draw it in mask
    for i in range(6):
        cv2.line(mask, vert[i], vert[(i + 1) % 6], (255), 63)
    return mask


mask = get_test_mask()

"""
Get the maximum/largest inscribed circle inside mask/polygon/contours.
Support non-convex/hollow shape
"""
dist_map = cv2.distanceTransform(mask, cv2.DIST_L2, cv2.DIST_MASK_PRECISE)
_, radius, _, center = cv2.minMaxLoc(dist_map)

result = cv2.cvtColor(mask, cv2.COLOR_GRAY2BGR)
cv2.circle(result, tuple(center), int(radius), (0, 0, 255), 2, cv2.LINE_8, 0)

# minEnclosingCircle directly by cv2
contours, _ = cv2.findContours(mask, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)[-2:]
center2, radius2 = cv2.minEnclosingCircle(np.concatenate(contours, 0))
cv2.circle(result, (int(center2[0]), int(center2[1])), int(radius2), (0, 255, 0,), 2)

cv2.imshow("mask", mask)
cv2.imshow("result", result)
cv2.waitKey(0)

Red circle is max inscribed circle