Histogram is a statistic of the image that gives information about the number of pixels of a particular intensity. It is irreversible, in the sense that with a histogram, you cannot recreate the image, and hence cannot predict the pixel value at a particular point. Having said this, it is possible to calculate the percentage of a particular gray level. This is equivalent to just normalizing the histogram on a unit scale. For example,
I = imread('rice.png');
hist = imhist(I);
%percentage of pixels that have the value 105, is
hist(105)/sum(hist) %This is the fraction.
and as a check, sum(hist) should always be equal to the number of pixels in the image.
Hope this helps!
