From ca3660589dbb71c9d55ce8b7c2f4c4bcca6cff89 Mon Sep 17 00:00:00 2001
From: Charlton Rodda <charlton.rodda@collabora.com>
Date: Tue, 13 Jun 2023 22:45:08 +0100
Subject: [PATCH] Fix handedness mismatch with generated tangents

---
 shared/gltf/GltfHelper.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/shared/gltf/GltfHelper.cpp b/shared/gltf/GltfHelper.cpp
index c23e9be..126e391 100644
--- a/shared/gltf/GltfHelper.cpp
+++ b/shared/gltf/GltfHelper.cpp
@@ -55,7 +55,10 @@ namespace
             primitive->Vertices[vertexIndex].Tangent.x = fvTangent[0];
             primitive->Vertices[vertexIndex].Tangent.y = fvTangent[1];
             primitive->Vertices[vertexIndex].Tangent.z = fvTangent[2];
-            primitive->Vertices[vertexIndex].Tangent.w = fSign;
+            // handedness difference, see:
+            // https://github.com/KhronosGroup/glTF-Sample-Models/issues/174
+            // https://github.com/KhronosGroup/glTF/issues/2056
+            primitive->Vertices[vertexIndex].Tangent.w = -fSign;
         };
 
         // Run the MikkTSpace algorithm.