无法在OpenCV中确定对象的凹度

import cv2
import numpy as np

# 读取图像
image = cv2.imread('path_to_image.jpg')
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)

# 使用Canny边缘检测获取边界
edges = cv2.Canny(gray, 50, 150)

# 查找轮廓
contours, _ = cv2.findContours(edges, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

for contour in contours:
    # 计算凸包
    hull = cv2.convexHull(contour, returnPoints=False)
    
    # 计算凹点
    defects = cv2.convexityDefects(contour, hull)
    
    if defects is not None:
        for i in range(defects.shape[0]):
            s, e, f, d = defects[i, 0]
            start = tuple(contour[s][0])
            end = tuple(contour[e][0])
            far = tuple(contour[f][0])
            
            # 绘制凹点
            cv2.circle(image, far, 5, [0, 0, 255], -1)
            cv2.line(image, start, end, [0, 255, 0], 2)

# 显示结果
cv2.imshow('Image', image)
cv2.waitKey(0)
cv2.destroyAllWindows()