The SiS 5598 is not capable of supporting overlays but does support hardware gamma color correction in all color depths.
Similar overlays are sold by Cerium Visual Technologies, the company which markets the intuitive colorimeter under license from the UK Medical Research Council.
A system of transparent overlays was also developed to permit even more accurate up dating to be made.
With the disk drive, loading speed was substantially increased and it became possible to use program overlays.
As you look at the different stencils available for purchase, pay close attention to the number of overlays that a given stencil has.