From 556e4db9fbfbcb9ca32f58d134e291a2a199d850 Mon Sep 17 00:00:00 2001
From: wheatleymf <20972271+wheatleymf@users.noreply.github.com>
Date: Sat, 29 Nov 2025 18:28:28 +0300
Subject: [PATCH] Transform tangent normal along seamless UV angle

Previous implementation of seamless UV was sampling tangent normal map 'as is', without transforming vectors to be inline with the UV rotation angle - which resulted into NdotL shading errors. This is now corrected and tangent vector accounts for UV rotation.
---
 game/addons/base/Assets/shaders/terrain.shader | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/game/addons/base/Assets/shaders/terrain.shader b/game/addons/base/Assets/shaders/terrain.shader
index 593ebe5b..012c181f 100644
--- a/game/addons/base/Assets/shaders/terrain.shader
+++ b/game/addons/base/Assets/shaders/terrain.shader
@@ -173,7 +173,9 @@ PS
         for ( int i = 0; i < 4; i++ )
         {
             float2 layerUV = texUV * g_TerrainMaterials[ i ].uvscale;
-            float2 seamlessUV = Terrain_SampleSeamlessUV( layerUV );
+
+            float2x2 uvRotation;
+            float2 seamlessUV = Terrain_SampleSeamlessUV( layerUV, uvRotation );
 
             Texture2D tBcr = Bindless::GetTexture2D( g_TerrainMaterials[ i ].bcr_texid );
             Texture2D tNho = Bindless::GetTexture2D( g_TerrainMaterials[ i ].nho_texid );
@@ -183,6 +185,10 @@ PS
 
             float3 normal = ComputeNormalFromRGTexture( nho.rg );
             normal.xz *= g_TerrainMaterials[ i ].normalstrength;
+            
+            // Rotate normal map to match seamless UV angle and avoid shading errors 
+            normal.xy = mul( uvRotation, normal.xy );
+
             normal = normalize( normal );
 
             albedos[i] = SrgbGammaToLinear( bcr.rgb );