Support calculating normals for indexed meshes (#11654)

# Objective

- Finish #3987

## Solution

- Rebase and fix typo.

Co-authored-by: Robert Bragg <robert@sixbynine.org>

crates/bevy_gltf/src/loader.rs

@@ -476,18 +476,10 @@ async fn load_gltf<'a, 'b, 'c>(
             if mesh.attribute(Mesh::ATTRIBUTE_NORMAL).is_none()
                 && matches!(mesh.primitive_topology(), PrimitiveTopology::TriangleList)
             {
-                let vertex_count_before = mesh.count_vertices();
-                mesh.duplicate_vertices();
-                mesh.compute_flat_normals();
-                let vertex_count_after = mesh.count_vertices();
-
-                if vertex_count_before != vertex_count_after {
-                    bevy_utils::tracing::debug!("Missing vertex normals in indexed geometry, computing them as flat. Vertex count increased from {} to {}", vertex_count_before, vertex_count_after);
-                } else {
                 bevy_utils::tracing::debug!(
-                        "Missing vertex normals in indexed geometry, computing them as flat."
+                    "Automatically calculating missing vertex normals for geometry."
                 );
-                }
+                mesh.compute_flat_normals();
             }
 
             if let Some(vertex_attribute) = reader

crates/bevy_render/src/mesh/mesh/mod.rs

@@ -544,12 +544,13 @@ impl Mesh {
     /// Calculates the [`Mesh::ATTRIBUTE_NORMAL`] of a mesh.
     ///
     /// # Panics
-    /// Panics if [`Indices`] are set or [`Mesh::ATTRIBUTE_POSITION`] is not of type `float3` or
-    /// if the mesh has any other topology than [`PrimitiveTopology::TriangleList`].
-    /// Consider calling [`Mesh::duplicate_vertices`] or export your mesh with normal attributes.
+    /// Panics if [`Mesh::ATTRIBUTE_POSITION`] is not of type `float3`.
+    /// Panics if the mesh has any other topology than [`PrimitiveTopology::TriangleList`].
+    ///
+    /// FIXME: The should handle more cases since this is called as a part of gltf
+    /// mesh loading where we can't really blame users for loading meshes that might
+    /// not conform to the limitations here!
     pub fn compute_flat_normals(&mut self) {
-        assert!(self.indices().is_none(), "`compute_flat_normals` can't work on indexed geometry. Consider calling `Mesh::duplicate_vertices`.");
-
         assert!(
             matches!(self.primitive_topology, PrimitiveTopology::TriangleList),
             "`compute_flat_normals` can only work on `TriangleList`s"
@@ -561,6 +562,42 @@ impl Mesh {
             .as_float3()
             .expect("`Mesh::ATTRIBUTE_POSITION` vertex attributes should be of type `float3`");
 
+        match self.indices() {
+            Some(indices) => {
+                let mut count: usize = 0;
+                let mut corners = [0_usize; 3];
+                let mut normals = vec![[0.0f32; 3]; positions.len()];
+                let mut adjacency_counts = vec![0_usize; positions.len()];
+
+                for i in indices.iter() {
+                    corners[count % 3] = i;
+                    count += 1;
+                    if count % 3 == 0 {
+                        let normal = face_normal(
+                            positions[corners[0]],
+                            positions[corners[1]],
+                            positions[corners[2]],
+                        );
+                        for corner in corners {
+                            normals[corner] =
+                                (Vec3::from(normal) + Vec3::from(normals[corner])).into();
+                            adjacency_counts[corner] += 1;
+                        }
+                    }
+                }
+
+                // average (smooth) normals for shared vertices...
+                // TODO: support different methods of weighting the average
+                for i in 0..normals.len() {
+                    let count = adjacency_counts[i];
+                    if count > 0 {
+                        normals[i] = (Vec3::from(normals[i]) / (count as f32)).normalize().into();
+                    }
+                }
+
+                self.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
+            }
+            None => {
                 let normals: Vec<_> = positions
                     .chunks_exact(3)
                     .map(|p| face_normal(p[0], p[1], p[2]))
@@ -569,10 +606,12 @@ impl Mesh {
 
                 self.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
             }
+        }
+    }
 
     /// Consumes the mesh and returns a mesh with calculated [`Mesh::ATTRIBUTE_NORMAL`].
     ///
-    /// (Alternatively, you can use [`Mesh::compute_flat_normals`] to mutate an existing mesh in-place)
+    /// (Alternatively, you can use [`Mesh::with_computed_flat_normals`] to mutate an existing mesh in-place)
     ///
     /// # Panics
     /// Panics if [`Indices`] are set or [`Mesh::ATTRIBUTE_POSITION`] is not of type `float3` or