Python opencv概率霍夫线变换-TypeError：'NoneType'类型的对象没有len（）

Question

我想对棋盘图像执行概率霍夫线变换，但遇到了错误。这个错误的一个奇怪之处在于 它只在我为 OpenCV 函数 HoughLinesP() 的参数指定关键字时发生。

首先，查看我的代码行和收到的错误消息：

#-*- coding:utf-8-*-
import matplotlib.pyplot as plt
import cv2
from skimage import io
import numpy as np

# Load image file
fpath = 'D:/Users/'
image = io.imread(fpath + 'checkerboard.JPG')
image_original = image.copy()

edges = cv2.Canny(image, 50, 200, apertureSize=3)
gray = cv2.cvtColor(edges, cv2.COLOR_GRAY2BGR)

minLineLength = 100
maxLineGap = 0
# Perform the probabilistic Hough transform
lines = cv2.HoughLinesP(image=edges, rho=1, theta=np.pi/180, threshold=100, minLineLength=minLineLength, maxLineGap=maxLineGap)

for i in range(len(lines)):
    for x1, y1, x2, y2 in lines[i]:
        cv2.line(image, (x1, y1), (x2, y2), (0, 0, 255), 3)

plt.figure(figsize=(10, 5), dpi=150)
plt.subplot(1, 2, 1)
plt.imshow(image_original, cmap='gray', vmin=0, vmax=255)

plt.subplot(1, 2, 2)
plt.imshow(image, cmap='gray', vmin=0, vmax=255)

plt.show()

Traceback (most recent call last):
  File "D:\Users\sihohan\Anaconda3\lib\site-packages\IPython\core\interactiveshell.py", line 3326, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-2-c2c4eb0ac4e8>", line 1, in <module>
    runfile('D:/Users/2_prob_hough.py', wdir='D:/Users')
  File "C:\Program Files\JetBrains\PyCharm 2019.2.2\helpers\pydev\_pydev_bundle\pydev_umd.py", line 197, in runfile
    pydev_imports.execfile(filename, global_vars, local_vars)  # execute the script
  File "C:\Program Files\JetBrains\PyCharm 2019.2.2\helpers\pydev\_pydev_imps\_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "D:/Users/2_prob_hough.py", line 20, in <module>
    for i in range(len(lines)):
TypeError: object of type 'NoneType' has no len()

如上所述，如果我不为 HoughLinesP() 的参数指定关键字，我的代码可以正常工作：

# Perform the probabilistic Hough transform
lines = cv2.HoughLinesP(edges, 1, np.pi/180, 100, minLineLength, maxLineGap)

这可能是什么原因？ 谢谢。

（以下是 maxLineGap = 0、10 和 20 的输出图像）

• maxLineGap = 0： checkerboard_maxLineGap_0
• maxLineGap = 10： checkerboard_maxLineGap_10
• maxLineGap = 20： checkerboard_maxLineGap_20

Answer 1

OpenCV houghLinesP parameters 声明cv2.HoughLinesP 包含冗余参数lines。所以你的 minLineLength 在

# Perform the probabilistic Hough transform
lines = cv2.HoughLinesP(edges, 1, np.pi/180, 100, minLineLength, maxLineGap)

用作lines 参数。因此，maxLineGap 被用作 minLineLength，因为它被设置为零，所以会产生更多的结果。