opencv troubleshooting when using Canny

[rickyw2777]

I was trying to use a tracebar to change the value of shresholds for Canny. Here is my code:

#HoughLinesP 
import cv2
import numpy as np
filename  = input("Enter name of file to process: ")
original  = cv2.imread(filename)
height, width = original.shape[:2]
Max=1000
Min=1000
def blank(x):       # null function for trackbar
   pass;
MaxVal = (f'MaxVal 0: {Max-1}')
MinVal = (f'MinVall 0: {Min-1}')
cv2.namedWindow('window', cv2.WINDOW_NORMAL)
cv2.resizeWindow('window', width, height)
cv2.createTrackbar(MaxVal, 'window', 0, Max-1, blank)
cv2.createTrackbar(MinVal, 'window', 0, Min-1,  blank)

while True:
	maxv = cv2.getTrackbarPos(MaxVal, 'window')
	minv = cv2.getTrackbarPos(MinVal, 'window')
	edge=cv2.Canny(original,minv,maxv)
	lines=cv2.HoughLinesP(
		edge,  
		1, 
		np.pi/180, 
		threshold =100, 
		# lines        
		minLineLength=20, 
		maxLineGap =10)   
	for i in range(len(lines)):
		for x1, y1, x2, y2 in lines[i]:
			cv2.line(original, (x1, y1), (x2, y2), (0, 255, 0), 2)
	cv2.imshow("houghline",original)
	# cv2.waitKey()
	# cv2.destroyAllWindows()
	if cv2.waitKey(1) == 32:     # stop when space bar hit
		cv2.destroyAllWindows()
	

When I run, I got this error:

Enter name of file to process: sidewalk.jpg
QSocketNotifier: Can only be used with threads started with QThread
QStandardPaths: wrong permissions on runtime directory /run/user/1000, 0770 instead of 0700
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
Traceback (most recent call last):
File "/home/ricky/Desktop/module练习/module5.py", line 227, in <module>
for i in range(len(lines)):
^^^^^^^^^^
TypeError: object of type 'NoneType' has no len()

Please help

[Pedroski55]

lines is your problem there. I got:

Traceback (most recent call last):
File "<pyshell#21>", line 13, in <module>
for i in range(len(lines)):
TypeError: object of type 'NoneType' has no len()

If I change the for loop to:

for i in range(len(lines)-1):
        for x1, y1, x2, y2 in lines[i]:
            cv2.line(original, (x1, y1), (x2, y2), (0, 255, 0), 2)

It runs without a problem.

Not sure exactly what you want to do though!

This is info on trackbars

This is info on Hough lines

[rickyw2777]

Thanks for replying. But I was trying to use a window with tracebar that can change the lower threshold and upper threshold for Canny Edge Detection. I tried the method. But After I changed the value for Canny, it still gives me this error:
And Problem solved thanks to this: https://forums.raspberrypi.com/viewtopic...0#p2293400