code: 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);
  	}
docs: Filters for computing the gradient of {@link Planar} images.
  
  @param type Which gradient to compute
  @param numBands Number of bands in the image
  @param inputType Type of data on input
  @param derivType Type of data on output (null for default)
  @param <I> Image type
  @param <D> Derivative type
  @return the filter