code: public static double ncc(NccFeature a, NccFeature b) {
  		double top = 0;
  
  		final int N = a.value.length;
  		for( int i = 0; i < N; i++ ) {
  			top += a.value[i]*b.value[i];
  		}
  
  		return top/(N*a.sigma * b.sigma);
  	}
docs: <p>
  Normalized cross correlation (NCC) computed using a faster technique.<br>
  <br>
  NCC = sum(a[i]*b[i]) / (N*sigma_a * sigma_b)<br>
  where a[i] = I[i]-mean(a), I[i] is the image pixel intensity around the feature, and N is the number of
  elements.
  </p>
  
  @param a First descriptor
  @param b Second descriptor
  @return NCC score