docs: public void computeNccDescriptor( NccFeature f , float x0 , float y0 , float x1 , float y1 ) {
  		double mean = 0;
  		float widthStep = (x1-x0)/15.0f;
  		float heightStep = (y1-y0)/15.0f;
  
  		// compute the mean value
  		int index = 0;
  		for( int y = 0; y < 15; y++ ) {
  			float sampleY = y0 + y*heightStep;
  			for( int x = 0; x < 15; x++ ) {
  				mean += f.value[index++] = interpolate.get_fast(x0 + x * widthStep, sampleY);
  			}
  		}
  		mean /= 15*15;
  
  		// compute the variance and save the difference from the mean
  		double variance = 0;
  		index = 0;
  		for( int y = 0; y < 15; y++ ) {
  			for( int x = 0; x < 15; x++ ) {
  				double v = f.value[index++] -= mean;
  				variance += v*v;
  			}
  		}
  		variance /= 15*15;
  		f.mean = mean;
  		f.sigma = Math.sqrt(variance);
  	}