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A cut-down port to Zandronum of torridgristle's old RetroShader he made back in 2016 for GZDoom.
Zandronum's hardware renderer has no post-processing stage, so all of this applies just to the game viewport, not the HUD or anything else. This means things like pixelating the screen like the original aren't really possible, but posterization (which is the small color reduction the shader applies) and PS1-style vertex warping are still here. Both of those things can be enabled/disabled and also tweaked by going into the pk3 > shaders > glsl > main.fp/vp, and modifying the defines at the top.
Some screenshots taken with light mode set to software and light scale set to 2x. I could put more screenshots here but the defaults aren't very noticeable in still images, just try it out, the mod itself is smaller than any one of these screenshots lol.
Spoiler: Screenshots (Open)
Spoiler: Credits (Open)