code: public static void process(GrayU8 orig,
  							   GrayS16 derivX,
  							   GrayS16 derivY) {
  		final byte[] data = orig.data;
  		final short[] imgX = derivX.data;
  		final short[] imgY = derivY.data;
  
  		final int width = orig.getWidth();
  		final int height = orig.getHeight() - 1;
  
  		//CONCURRENT_BELOW BoofConcurrency.loopFor(1,height,y->{
  		for (int y = 1; y < height; y++) {
  			int endX = width * y + width - 1;
  
  			for (int index = width * y + 1; index < endX; index++) {
  				int v = (data[index + width + 1] & 0xFF) - (data[index - width - 1] & 0xFF);
  				int w = (data[index + width - 1] & 0xFF) - (data[index - width + 1] & 0xFF);
  
  				imgY[index] = (short) (((data[index + width] & 0xFF) - (data[index - width] & 0xFF)) * 2 + v + w);
  
  				imgX[index] = (short) (((data[index + 1] & 0xFF) - (data[index - 1] & 0xFF)) * 2 + v - w);
  			}
  		}
  		//CONCURRENT_ABOVE });
  	}
docs: Computes derivative of GrayU8.  None of the images can be sub-images.