about Histogram Function of OpenCV?
 

I am a new commer to the OpenCV.

I want to compute the color histogram of only the skin region in an image.

When computing the color histogram of the total pixels,
"cvCalcHist(image, histogram, 0, 0)" can be used.

But when it just needs to consider the skin pixels, such as the histogram of hand-face region, how can I set the parameter of cvCalcHistogram,
especially " const CvArr* mask=0".

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cvCalcHist(IplImage** img, CvHistogram* hist,
int doNotClear=0, const CvArr* mask=0)"
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I do not know much about the construct of the "mask".

Thanks for your advice!