From c14215dcc69f7d40d983899d86dc63f33a5fe21c Mon Sep 17 00:00:00 2001
From: Florian Amsallem <florian.amsallem@epita.fr>
Date: Sun, 5 Nov 2023 00:17:52 +0100
Subject: [PATCH] fix tangent normalization

---
 src/lib.rs                |   5 ++++-
 src/scene/model/mod.rs    |   3 ++-
 src/scene/model/vertex.rs |   1 +
 tests/cube.glb            | Bin 4288 -> 4908 bytes
 4 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 6242f39..ec4f940 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -180,12 +180,15 @@ mod tests {
         let model = &scene.models[0];
         assert!(model.has_normals());
         assert!(model.has_tex_coords());
-        assert!(!model.has_tangents());
+        assert!(model.has_tangents());
         for t in model.triangles().unwrap().iter().flatten() {
             let pos = t.position;
             assert!(pos.x > -0.01 && pos.x < 1.01);
             assert!(pos.y > -0.01 && pos.y < 1.01);
             assert!(pos.z > -0.01 && pos.z < 1.01);
+
+            // Check that the tangent w component is 1 or -1
+            assert_eq!(t.tangent.w.abs(), 1.);
         }
     }
 
diff --git a/src/scene/model/mod.rs b/src/scene/model/mod.rs
index b1d1c4b..d59579e 100644
--- a/src/scene/model/mod.rs
+++ b/src/scene/model/mod.rs
@@ -312,7 +312,8 @@ impl Model {
         // Fill tangents
         let has_tangents = if let Some(tangents) = reader.read_tangents() {
             for (i, tangent) in tangents.enumerate() {
-                vertices[i].tangent = Self::apply_transform_tangent(tangent, transform).normalize();
+                let tangent = Self::apply_transform_tangent(tangent, transform);
+                vertices[i].tangent = tangent.truncate().normalize().extend(tangent.w);
             }
             true
         } else {
diff --git a/src/scene/model/vertex.rs b/src/scene/model/vertex.rs
index 1c85a96..9df025d 100644
--- a/src/scene/model/vertex.rs
+++ b/src/scene/model/vertex.rs
@@ -15,6 +15,7 @@ pub struct Vertex {
     /// Normalized normal
     pub normal: Vector3<f32>,
     /// Tangent normal
+    /// The w component is the handedness of the tangent basis (can be -1 or 1)
     pub tangent: Vector4<f32>,
     /// Texture coordinates
     pub tex_coords: Vector2<f32>,
diff --git a/tests/cube.glb b/tests/cube.glb
index 20b937777af3b6eaa8deb9314c20958fc90aac72..01d07f44cbef5ce2cd3f1524c6d65cefdcd781ae 100644
GIT binary patch
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