eduction, low-pass rank-order filtering can be
   done in advance.  Isotropic integer expansion is done by pixel replication.

   We also provide 2x, 3x, 4x, 6x, 8x, and 16x scale-to-gray reduction
   on binary images, to produce high quality reduced grayscale images.
   These are integrated into a scale-to-gray function with arbitrary
   reduction.

   Conversely, we have special 2x and 4x scale-to-binary expansion
   on grayscale images, using linear interpolation on grayscale
   raster line buffers follo