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pythonpython-3.xopencvhough-transform

Python opencv probabilistic Hough line transform - TypeError: object of type 'NoneType' has no len()

I wanted to perform a probabilistic Hough line transform on a checkerboard image, but ran into an error. A weird thing about this error is that it only occurs when I specify keywords for the arguments of the OpenCV function HoughLinesP().

First, check out my code lines and the error message I got:

#-*- 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()

As mentioned above, my code works fine if I don't specify the keywords for the arguments of HoughLinesP():

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

What could be the reason for this? Thank you.

(Below are the output images for maxLineGap = 0, 10, and 20)

Solution

  • OpenCV houghLinesP parameters states that cv2.HoughLinesP contains the redundant parameter lines. So your minLineLength in

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

    is used as lines parameter. Therefore maxLineGap is used as minLineLength yielding more results given that it is set to zero.