Sometime around 1998/1999 (Q3A-era) I recall John Carmack commenting, probably either in a .plan entry or at Quakecon, that you could calculate arbitrary Render

Sometime around 1998/1999 (Q3A-era) I recall John Carmack commenting, probably either in a .plan entry or at Quakecon, that you could calculate arbitrary RenderMan shaders using ye old 1.1-ish fixed-function OpenGL given enough rendering passes, long before GLSL or even ARB_fragment_program was a thing. So the answer is probably longer ago than you'd think.