code: public static GrayU8 nv21ToGray(byte[] data , int width , int height , GrayU8 output ) {
  		if( output != null ) {
  			output.reshape(width,height);
  		} else {
  			output = new GrayU8(width,height);
  		}
  
  		if(BoofConcurrency.USE_CONCURRENT ) {
  			ImplConvertNV21_MT.nv21ToGray(data, output);
  		} else {
  			ImplConvertNV21.nv21ToGray(data, output);
  		}
  
  		return output;
  	}
docs: Converts an NV21 image into a gray scale U8 image.
  
  @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.
  @return Gray scale image