This comes in 5 modes which is accessible in the Display Options menu on both GZDoom and Raze. (The 5th being simply "off")
Note that this requires a devbuild of either GZDoom or Raze in order to function properly.
"Classic" - which is basically the palette tonemap:
Spoiler:"Checkerboard" - error diffusion dither to a single level:
Spoiler:"Square" - 2x2 error diffusion dither to 3 levels:
Spoiler:"Shaded" - The image is downmixed to a palette but it is blended and the brightness is readjusted according to the original pixel (it gives a very "advanced retro" feel):
License: 3-clause BSD for the code, CC-BY for the image (allows derivative works, only attribution required)
Spoiler: Old versions