python TypeError：'NoneType'对象没有属性'__getitem__'

Question

这次我正在尝试Solem's blog的另一个例子。它是一个通过使用霍夫变换检测图像中的线条和圆圈的模块。 这是代码（houghlines.py）：

import numpy as np
import cv2

"""
Script using OpenCV's Hough transforms for reading images of 
simple dials.
"""

# load grayscale image
im = cv2.imread("house2.jpg")
gray_im = cv2.cvtColor(im, cv2.COLOR_RGB2GRAY)

# create version to draw on and blurred version
draw_im = cv2.cvtColor(gray_im, cv2.COLOR_GRAY2BGR)
blur = cv2.GaussianBlur(gray_im, (0,0), 5)

m,n = gray_im.shape

# Hough transform for circles
circles = cv2.HoughCircles(gray_im, cv2.cv.CV_HOUGH_GRADIENT, 2, 10, np.array([]), 20, 60, m/10)[0]

# Hough transform for lines (regular and probabilistic)
edges = cv2.Canny(blur, 20, 60)
lines = cv2.HoughLines(edges, 2, np.pi/90, 40)[0]
plines = cv2.HoughLinesP(edges, 1, np.pi/180, 20, np.array([]), 10)[0]

# draw 
for c in circles[:3]:
 # green for circles (only draw the 3 strongest)
 cv2.circle(draw_im, (c[0],c[1]), c[2], (0,255,0), 2) 

for (rho, theta) in lines[:5]:
 # blue for infinite lines (only draw the 5 strongest)
 x0 = np.cos(theta)*rho 
 y0 = np.sin(theta)*rho
 pt1 = ( int(x0 + (m+n)*(-np.sin(theta))), int(y0 + (m+n)*np.cos(theta)) )
 pt2 = ( int(x0 - (m+n)*(-np.sin(theta))), int(y0 - (m+n)*np.cos(theta)) )
 cv2.line(draw_im, pt1, pt2, (255,0,0), 2) 

for l in plines:
 # red for line segments
 cv2.line(draw_im, (l[0],l[1]), (l[2],l[3]), (0,0,255), 2)

cv2.imshow("circles",draw_im)
cv2.waitKey()

# save the resulting image
cv2.imwrite("res.jpg",draw_im)

当我在python中执行文件时：

execfile('houghlines.py')

出现以下错误：

File "houghlines.py", line 24, in <module>
    lines = cv2.HoughLines(edges, 2, np.pi/90, 40)[0]
TypeError: 'NoneType' object has no attribute '__getitem__'

你们有任何想法如何解决它？
提前谢谢。

Answer 1

有时HoughLines和HoughLinesP没有返回。我认为这意味着＆＃34;没有线＆＃34;。我真的很惊讶文档中的示例并不能解释它。也许这是一个错误。

在任何情况下，您都可以使用简单的if result != None:停止代码失败，或者使用默认列表(HoughLinesP(... args ...) or [[]])替换无。这不会解决您的线路未被检测到的事实，但它允许您在这种情况下执行某些操作而不是失败。

Answer 2

这是因为lines = cv2.HoughLines(edges, 2, np.pi/90, 40)[0]中的阈值，是您传递的最后一个参数。它表示最小长度被视为一条线，在您的情况下为40像素。尝试减少它。

Answer 3

我也在处理这个错误。当函数cv2.HoughCircles未检测到任何圆时，它将返回None。所以我有两种方法可以解决它：

• 使用尝试/例外来包围代码块

 try:
    ...
 except Exception as e:
    print 'There is no circles to be detected!'

• 使用 if / else 以避免操作无对象

 if circles is not None:
   ...
 else:
   print 'There is no circles to be detected!'

Answer 4

我已经使用以下代码克服了这一点：

    lines = []
    linesDetected = False
    while not linesDetected:
        lines = cv.HoughLinesP(edges, 1, np.pi/180, 30, minLineLength = 60, maxLineGap=300)
        try:
            if len(lines) !=0:
                linesDetected = True
        except:
            linesDetected = False

Answer 5

更改阈值将起作用

canny=cv2.Canny(gray,50,150,apertureSize=3)

lines=cv2.HoughLines(canny,1,numpy.pi/180,40)

打印（行）