code: public static Planar<GrayU8> nv21ToPlanarYuv_U8(byte[] data , int width , int height ,
  													Planar<GrayU8> output ) {
  		if( output == null ) {
  			output = new Planar<>(GrayU8.class,width,height,3);
  		} else {
  			output.reshape(width, height, 3);
  		}
  
  		if(BoofConcurrency.USE_CONCURRENT ) {
  			ImplConvertNV21_MT.nv21ToPlanarYuv_U8(data, output);
  		} else {
  			ImplConvertNV21.nv21ToPlanarYuv_U8(data, output);
  		}
  		return output;
  	}
docs: Converts an NV21 image into a {@link Planar} YUV image with U8 bands.
  
  @param data Input: NV21 image data
  @param width Input: NV21 image width
  @param height Input: NV21 image height
  @param output Output: Optional storage for output image.  Can be null.