code: public static double variance( GrayF64 img , double mean ) {
  
  		if( BoofConcurrency.USE_CONCURRENT ) {
  			return ImplImageStatistics_MT.variance(img,mean);
  		} else {
  			return ImplImageStatistics.variance(img,mean);
  		}
  	}
docs: Computes the variance of pixel intensity values inside the image.
  
  @param img Input image. Not modified.
  @param mean Mean pixel intensity value.
  @return Pixel variance