Adapted to work in GZDoom, from https://www.shadertoy.com/view/4sSGDK
Shader originaly by Mattias Gustavsson
If you don't need GZDoom's various additions, and want to play vanillia Doom with a CRT shader, you should probably check out Mattias' Doom-CRT instead!
Download this repository as a .zip file, then load the .zip file as a mod with gzdoom via the -file command line flag.