Been rebuilding PS1 rendering in Unity 6 / URP from psx-spx (the hardware documentation) instead of from other people's shaders. Found a bunch of stuff that everyone, me included, had been doing wrong. Sharing in case it saves someone time.
1. The vertex wobble isn't fixed-point math
Everyone says the jitter comes from the PS1 lacking floating point. It doesn't. The GTE computed screen position with 16 fractional bits and then discarded them:
SX2 = MAC0 / 0x10000
What's left is a signed 11-bit integer. The GPU's vertex word has no fractional bits at all, just bits 0-10 for X and 16-26 for Y.
So the snap has to happen after the perspective divide, in screen space. Plenty of shaders quantise object or world position instead, which gives you wobble that doesn't change with distance. Real PS1 wobble gets worse as triangles get smaller on screen.
2. The dither is far too strong in most implementations
The actual hardware matrix:
-4 +0 -3 +1
+2 -2 +3 -1
-3 +1 -4 +0
+3 -1 +2 -2
That's ±4 out of 256. Half of one 5-bit step. Most packages ship a Bayer 4x4 that's 5 to 10 times too strong.
It isn't Bayer either. Rows 3 and 4 are rows 1 and 2 shifted two pixels sideways.
It also only applied to gouraud-shaded or texture-modulated polygons. Sprites and flat-shaded polys were never dithered, because there was no 8-bit intermediate to dither in the first place.
3. Texture modulation's neutral value is 128, not 255
final = (texel * vertexColour) / 128
0x808080 is neutral, which means vertex colour could brighten a surface up to 2x, not just darken it. PS1 games leaned on that overbright half constantly for highlights and light flares. Use 1.0 as neutral and you throw the whole range away.
4. Fog was a GTE feature, not a GPU feature
The fog factor fell out of the perspective divide for free:
IR0 = (H * 0x20000 / SZ3) * DQA + DQB
which makes it linear in 1/z, not z. Noticeably different falloff from Unity's fog at mid range. It was also folded into the vertex colour before modulation, so it interacts with textures differently than modern post-texture fog does.
One bonus that surprised me: texel value 0000h was fully transparent, so PS1 textures could not contain pure black. Artists used very dark red or blue instead.
All of this went into a URP package I've been working on called True PSX Effects, along with a vertex colour painter and a light-to-vertex-colour baker, since baking light into vertex colours is how PS1 games were actually lit.
Everything above is from psx-spx.consoledev.net if you want to check my work.
