code: public static double variance(int[] histogram, double mean , int N ) {
  		return variance(histogram, mean, count(histogram,N), N);
  	}
docs: Computes the variance of pixel intensity values for a GrayU8 image represented by the given histogram.
  
  @param histogram Histogram with N bins
  @param mean Mean of the image.
  @param N number of bins in the histogram.
  @return variance