From ea785355cd980765b60169621634d903440f6473 Mon Sep 17 00:00:00 2001
From: Brandon Thomas <echelon@gmail.com>
Date: Fri, 9 Oct 2026 10:22:01 -0400
Subject: [PATCH] GPU: note why Posterize's division correction is an fma (#405
 follow-up)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 crates/gpu/src/shaders/fx_stylize.wgsl | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/crates/gpu/src/shaders/fx_stylize.wgsl b/crates/gpu/src/shaders/fx_stylize.wgsl
index b98c7474..3db44275 100644
--- a/crates/gpu/src/shaders/fx_stylize.wgsl
+++ b/crates/gpu/src/shaders/fx_stylize.wgsl
@@ -40,6 +40,8 @@ fn fxs_smoothstep(e0: f32, e1: f32, x: f32) -> f32 {
 fn fxs_qdiv(x: vec3<f32>, a: f32) -> vec3<f32> {
     let q = x / a;
     let r = fma(-q, vec3<f32>(a), x);
+    // `q + r / a`, written as an fma so drivers (NVIDIA Vulkan) can't contract it another way
+    // and round differently from the CPU.
     return fma(r / a, vec3<f32>(1.0), q);
 }