public span_image_resample_gray(source_type src, 
                          interpolator_type inter,
                          image_filter_lut filter) :
     base(src, inter, filter)
 {}
 public span_image_resample_gray(source_type src,
                                 interpolator_type inter,
                                 image_filter_lut filter) :
     base(src, inter, filter)
 {
 }
 public span_image_filter_gray_2x2(source_type src, 
                            interpolator_type inter,
                            image_filter_lut filter) :
     base(src, inter, &filter) 
 {}
 public span_image_filter_gray_2x2(source_type src,
                                   interpolator_type inter,
                                   image_filter_lut filter) :
     base(src, inter, &filter)
 {
 }