docs: public static <I extends ImageGray<I>, D extends ImageGray<D>>
  	ImageGradient<Planar<I>,Planar<D>>
  	gradientPL(DerivativeType type , int numBands , Class<I> inputType , Class<D> derivType )
  	{
  		ImageGradient<I,D> g = gradientSB(type,inputType,derivType);
  		return new ImageGradient_PL<>(g, numBands);
  	}