why using cv2.calcHist always has an errer "returned NULL without setting an error"

Question

I am using Opencv with python, and come across a question.
When I run the following code:

img = cv2.imread('test.jpg',0)
hist = cv2.calcHist([img],[0],None,256,[0,256])

An error occurs: SystemError: <built-in function calcHist> returned NULL without setting an error
I am confused and can not find the same error in networks, so what's wrong?
Thanks.

PS: I run the same code in both Windows and Ubuntu,and get the same error,so it may not the reason of system ?

Answer 1

The error message is confusing indeed, but the actual error is simple. Even though you're just using 1 channel, you still need to give a list of histogram sizes and ranges. You've done that for the range, but not for the size. This should work:

img = cv2.imread('test.jpg',0)
hist = cv2.calcHist([img],[0],None,[256],[0,256])

Wo-Ki anticipates your next problem, as it's unusual to make a histogram of just the blue channel, and you probably want a histogram of the intensity instead. In that case, use the intermediate conversion step as Wo-Ki suggested.

Answer 2

I had the same problem. Apparently the ranges [0, 256] have to be same type as img. Try

hist = cv2.calcHist([img.astype('uint8')],[0],None,[256],[0, 256]) 

instead. It should work fine.