code: public static
  	void gradient_noborder(GrayF32 ii , double tl_x , double tl_y , double samplePeriod ,
  						   int regionSize , double kernelWidth ,
  						   float[] derivX , float[] derivY )
  	{
  		ImplSurfDescribeOps.gradientInner(ii,tl_x,tl_y,samplePeriod,regionSize, kernelWidth, derivX,derivY);
  	}
docs: Faster version of {@link #gradient} which assumes the region is entirely contained inside the
  of the image.  This includes the convolution kernel's radius.