From 4eee95c0f91bd01f119ce8977472bf0a986d0921 Mon Sep 17 00:00:00 2001
From: Lukas Stockner <lukas@lukasstockner.de>
Date: Mon, 15 Sep 2025 18:20:33 +0200
Subject: [PATCH] Fix: Cycles: Incorrect thin film IOR for backfacing Glass
 with OSL

Thanks to @main-menu-theme for noticing this!
---
 intern/cycles/kernel/osl/shaders/node_glass_bsdf.osl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/intern/cycles/kernel/osl/shaders/node_glass_bsdf.osl b/intern/cycles/kernel/osl/shaders/node_glass_bsdf.osl
index 280b67edca3..a6806cec25a 100644
--- a/intern/cycles/kernel/osl/shaders/node_glass_bsdf.osl
+++ b/intern/cycles/kernel/osl/shaders/node_glass_bsdf.osl
@@ -18,8 +18,8 @@ shader node_glass_bsdf(color Color = 0.8,
   float r2 = clamp(Roughness, 0.0, 1.0);
   r2 = r2 * r2;
   float eta = max(IOR, 1e-5);
-  eta = backfacing() ? 1.0 / eta : eta;
   float thinfilm_ior = backfacing() ? ThinFilmIOR / eta : ThinFilmIOR;
+  eta = backfacing() ? 1.0 / eta : eta;
   color F0 = F0_from_ior(eta);
   color F90 = color(1.0);