fix tangent normalization

Author: flomonster

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.);
         }
     }
 

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 {

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>,