From edba54adac6c14d6b6f8199bb87c3dcef85a7989 Mon Sep 17 00:00:00 2001
From: aloucks <aloucks@cofront.net>
Date: Fri, 7 Mar 2025 12:39:42 -0500
Subject: [PATCH] Fix mesh tangent attribute matching in mesh transform
 operations (#17992)

Fixes #17170

# Objective

Tangents are not currently transformed as described in #17170. I came
across this while working on #17989 and it seemed like low hanging
fruit.
---
 crates/bevy_mesh/src/mesh.rs | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/crates/bevy_mesh/src/mesh.rs b/crates/bevy_mesh/src/mesh.rs
index 26363d50ed..eec67d6664 100644
--- a/crates/bevy_mesh/src/mesh.rs
+++ b/crates/bevy_mesh/src/mesh.rs
@@ -909,13 +909,16 @@ impl Mesh {
             });
         }
 
-        if let Some(VertexAttributeValues::Float32x3(tangents)) =
+        if let Some(VertexAttributeValues::Float32x4(tangents)) =
             self.attribute_mut(Mesh::ATTRIBUTE_TANGENT)
         {
             // Transform tangents, taking into account non-uniform scaling and rotation
             tangents.iter_mut().for_each(|tangent| {
+                let handedness = tangent[3];
                 let scaled_tangent = Vec3::from_slice(tangent) * transform.scale;
-                *tangent = (transform.rotation * scaled_tangent.normalize_or_zero()).to_array();
+                *tangent = (transform.rotation * scaled_tangent.normalize_or_zero())
+                    .extend(handedness)
+                    .to_array();
             });
         }
     }
@@ -981,12 +984,15 @@ impl Mesh {
             });
         }
 
-        if let Some(VertexAttributeValues::Float32x3(tangents)) =
+        if let Some(VertexAttributeValues::Float32x4(tangents)) =
             self.attribute_mut(Mesh::ATTRIBUTE_TANGENT)
         {
             // Transform tangents
             tangents.iter_mut().for_each(|tangent| {
-                *tangent = (rotation * Vec3::from_slice(tangent).normalize_or_zero()).to_array();
+                let handedness = tangent[3];
+                *tangent = (rotation * Vec3::from_slice(tangent).normalize_or_zero())
+                    .extend(handedness)
+                    .to_array();
             });
         }
     }
@@ -1033,13 +1039,17 @@ impl Mesh {
             });
         }
 
-        if let Some(VertexAttributeValues::Float32x3(tangents)) =
+        if let Some(VertexAttributeValues::Float32x4(tangents)) =
             self.attribute_mut(Mesh::ATTRIBUTE_TANGENT)
         {
             // Transform tangents, taking into account non-uniform scaling
             tangents.iter_mut().for_each(|tangent| {
+                let handedness = tangent[3];
                 let scaled_tangent = Vec3::from_slice(tangent) * scale;
-                *tangent = scaled_tangent.normalize_or_zero().to_array();
+                *tangent = scaled_tangent
+                    .normalize_or_zero()
+                    .extend(handedness)
+                    .to_array();
             });
         }
     }