I have a doubt regarding the **calcHist** function:
From [here](http://docs.opencv.org/3.0-beta/modules/imgproc/doc/histograms.html) I read:
"Optional mask. If the matrix is not empty, it must be an 8-bit array of the same size as images[i] . The non-zero mask elements mark the array elements counted in the histogram."
Is not clear to me how can be used the mask.
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