How to calculate 3D histogram in python using open CV

Question

I want to calculate 3D histogram of my Cielab image in python. I am using openCV to calculate my histogram. I want to compare images using compareHist function of openCV, thats why I am using openCV to compute 3D histogram of my image.

I tried with the following variables:

i_lab = image.copy()
i_lab = i_lab.astype(np.uint8)
Range_hist = [[0, 100], [-100, 100], [-100, 100]]    
hist_1 = cv2.calcHist([i_lab], [[0], [1], [2]], None, [[20], [20], [20]], Range_hist)

But it gives error SystemError: error return without exception set Please tell me what am I doing wrong and if it is possible to compute 3D histogram using openCV in python

Answer 1

I came across this question while trying to make a 3D histogram of an HSV image, and encountered the same error. It turns out that the OpenCV documentation is leading us astray here. The docs are written for the C++ API and as such can only be used as a vague guide to the Python cv2 API (although I have found that the docs are misleading for C++ as well at times).

The function signature is as follows:

cv2.calcHist(images, channels, mask, histSize, ranges[, hist[, accumulate]]) -> hist

The key point is that the channels, histSize and ranges parameters should be flat lists, not nested lists as in your example. Try the following, assuming i_lab is a three-channel image:

range_hist = [0, 100, -100, 100, -100, 100]
hist_1 = cv2.calcHist([i_lab], [0, 1, 2], None, [20, 20, 20], range_hist)

For a more complete example, try this code listing from the opencvpython blog.