MATLAB: # of counts in whole image vs #counts in masked image +mask? shoul be same or not??

masknumber of countsroi

Dear all,
I did histograms of :
1. whole image
2. image masked 
3. image with inverted masked
I would expect sum of counts in 2+3 shall be same as in 1. But it is not. So is this because of the mask line width (I do masking with imfreehand).. and double assigning of some pixels to masked and reversed masked region?, or is my procedure somehow wrong. Scheme is as follows
masked_image=whole_image.*mask;
inverse_masked_image=whole_image.*(~mask)
the numbers differ like 1=65536, and(2+3)=78086..
thanks!

Best Answer

All images are the same size (unless you cropped them), so all images should have the same number of counts . The fact that some pixels have been set to 0 or 255 (masked) does not affect the number of pixels they have and so the count from imhist() or hist() or histc() will be the same, which is numel(grayImage).
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