docs: public static <I extends ImageGray<I>, O extends ImageGray<O>>
  	void gradient(ImagePyramid<I> input, ImageGradient<I, O> gradient, O[] derivX, O[] derivY )
  	{
  		for( int i = 0; i < input.getNumLayers(); i++ ) {
  			I imageIn = input.getLayer(i);
  			gradient.process(imageIn,derivX[i],derivY[i]);
  		}
  	}