From bc8737d707595aebd7cc11d6d5a5d65ede750f59 Mon Sep 17 00:00:00 2001
From: James Canete <SmileTheory@gmail.com>
Date: Tue, 20 Jul 2021 13:51:19 -0700
Subject: [PATCH] OpenGL2: Flip normals for backfacing triangles.
 https://github.com/ioquake/ioq3/issues/513

---
 code/renderergl2/glsl/lightall_fp.glsl | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/code/renderergl2/glsl/lightall_fp.glsl b/code/renderergl2/glsl/lightall_fp.glsl
index ff7fc125..28b59eb7 100644
--- a/code/renderergl2/glsl/lightall_fp.glsl
+++ b/code/renderergl2/glsl/lightall_fp.glsl
@@ -264,7 +264,8 @@ void main()
 	float NL, NH, NE, EH, attenuation;
 
 #if defined(USE_LIGHT) && !defined(USE_FAST_LIGHT)
-	mat3 tangentToWorld = mat3(var_Tangent.xyz, var_Bitangent.xyz, var_Normal.xyz);
+	vec3 surfNormal = (!gl_FrontFacing ? var_Normal : -var_Normal).xyz;
+	mat3 tangentToWorld = mat3(var_Tangent.xyz, var_Bitangent.xyz, surfNormal);
 	viewDir = vec3(var_Normal.w, var_Tangent.w, var_Bitangent.w);
 	E = normalize(viewDir);
 #endif
@@ -335,7 +336,7 @@ void main()
 	N.z = sqrt(clamp((0.25 - N.x * N.x) - N.y * N.y, 0.0, 1.0));
 	N = tangentToWorld * N;
   #else
-	N = var_Normal.xyz;
+	N = surfNormal;
   #endif
 
 	N = normalize(N);
@@ -361,7 +362,7 @@ void main()
 
   #if !defined(USE_LIGHT_VECTOR)
 	ambientColor = lightColor;
-	float surfNL = clamp(dot(var_Normal.xyz, L), 0.0, 1.0);
+	float surfNL = clamp(dot(surfNormal, L), 0.0, 1.0);
 
 	// reserve 25% ambient to avoid black areas on normalmaps
 	lightColor *= 0.75;