code: protected double computeVariance(int x0, int y0, int x1, int y1) {
  		// can use unsafe operations here since x0 > 0 and y0 > 0
  		double square = GIntegralImageOps.block_unsafe(integralSq, x0 - 1, y0 - 1, x1 - 1, y1 - 1);
  
  		double area = (x1-x0)*(y1-y0);
  		double mean = GIntegralImageOps.block_unsafe(integral, x0 - 1, y0 - 1, x1 - 1, y1 - 1)/area;
  
  		return square/area - mean*mean;
  	}
docs: Computes the variance inside the specified rectangle.  x0 and y0 must be &gt; 0.
  
  @return variance