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The simplest solution is to quantize first the image into 8 levels simply by dividing it by 32 (integer division). Then, every channel will range between 0..7.

Rq=R//32


To get the Hrgb,combine the three channels into one:

Himg=Rq+Gq*8+Bq*64


Himg will contain values between 0...511, corresponding to the 512 bins of Hrgb. So you only need to build the histogram for Himg to get Hrgb (using cv2.hist() or numpy.histogram())

Hrgb = cv2.calcHist([Himg],[0],None,[512],[0,512])