Revert "Support calculating normals for indexed meshes" (#12716) and add support for calculating smooth normals (#13333)

# Objective

- Refactor the changes merged in #11654 to compute flat normals for
indexed meshes instead of smooth normals.
- Fixes #12716 

## Solution

- Partially revert the changes in #11654 to compute flat normals for
both indexed and unindexed meshes in `compute_flat_normals`
- Create a new method, `compute_smooth_normals`, that computes smooth
normals for indexed meshes
- Create a new method, `compute_normals`, that computes smooth normals
for indexed meshes and flat normals for unindexed meshes by default. Use
this new method instead of `compute_flat_normals`.

## Testing

- Run the example with and without the changes to ensure that the
results are identical.

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

@@ -544,15 +544,44 @@ impl Mesh {
     }
 
     /// Calculates the [`Mesh::ATTRIBUTE_NORMAL`] of a mesh.
+    /// If the mesh is indexed, this defaults to smooth normals. Otherwise, it defaults to flat
+    /// normals.
     ///
     /// # Panics
     /// 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
+    /// FIXME: This 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_normals(&mut self) {
+        assert!(
+            matches!(self.primitive_topology, PrimitiveTopology::TriangleList),
+            "`compute_normals` can only work on `TriangleList`s"
+        );
+        if self.indices().is_none() {
+            self.compute_flat_normals();
+        } else {
+            self.compute_smooth_normals();
+        }
+    }
+
+    /// Calculates the [`Mesh::ATTRIBUTE_NORMAL`] of a mesh.
+    ///
+    /// # Panics
+    /// Panics if [`Indices`] are set or [`Mesh::ATTRIBUTE_POSITION`] is not of type `float3`.
+    /// Panics if the mesh has any other topology than [`PrimitiveTopology::TriangleList`].
+    /// Consider calling [`Mesh::duplicate_vertices`] or exporting your mesh with normal
+    /// attributes.
+    ///
+    /// FIXME: This 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 either `Mesh::compute_smooth_normals` or `Mesh::duplicate_vertices` followed by `Mesh::compute_flat_normals`."
+        );
         assert!(
             matches!(self.primitive_topology, PrimitiveTopology::TriangleList),
             "`compute_flat_normals` can only work on `TriangleList`s"
@@ -564,42 +593,6 @@ 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]))
@@ -608,23 +601,106 @@ impl Mesh {
 
         self.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
     }
+
+    /// Calculates the [`Mesh::ATTRIBUTE_NORMAL`] of an indexed mesh, smoothing normals for shared
+    /// vertices.
+    ///
+    /// # Panics
+    /// Panics if [`Mesh::ATTRIBUTE_POSITION`] is not of type `float3`.
+    /// Panics if the mesh has any other topology than [`PrimitiveTopology::TriangleList`].
+    /// Panics if the mesh does not have indices defined.
+    ///
+    /// FIXME: This 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_smooth_normals(&mut self) {
+        assert!(
+            matches!(self.primitive_topology, PrimitiveTopology::TriangleList),
+            "`compute_smooth_normals` can only work on `TriangleList`s"
+        );
+        assert!(
+            self.indices().is_some(),
+            "`compute_smooth_normals` can only work on indexed meshes"
+        );
+
+        let positions = self
+            .attribute(Mesh::ATTRIBUTE_POSITION)
+            .unwrap()
+            .as_float3()
+            .expect("`Mesh::ATTRIBUTE_POSITION` vertex attributes should be of type `float3`");
+
+        let mut normals = vec![Vec3::ZERO; positions.len()];
+        let mut adjacency_counts = vec![0_usize; positions.len()];
+
+        self.indices()
+            .unwrap()
+            .iter()
+            .collect::<Vec<usize>>()
+            .chunks_exact(3)
+            .for_each(|face| {
+                let [a, b, c] = [face[0], face[1], face[2]];
+                let normal = Vec3::from(face_normal(positions[a], positions[b], positions[c]));
+                [a, b, c].iter().for_each(|pos| {
+                    normals[*pos] += normal;
+                    adjacency_counts[*pos] += 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] = (normals[i] / (count as f32)).normalize();
             }
         }
 
+        self.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
+    }
+
+    /// Consumes the mesh and returns a mesh with calculated [`Mesh::ATTRIBUTE_NORMAL`].
+    /// If the mesh is indexed, this defaults to smooth normals. Otherwise, it defaults to flat
+    /// normals.
+    ///
+    /// (Alternatively, you can use [`Mesh::compute_normals`] to mutate an existing mesh in-place)
+    ///
+    /// # Panics
+    /// Panics if [`Mesh::ATTRIBUTE_POSITION`] is not of type `float3`.
+    /// Panics if the mesh has any other topology than [`PrimitiveTopology::TriangleList`].
+    #[must_use]
+    pub fn with_computed_normals(mut self) -> Self {
+        self.compute_normals();
+        self
+    }
+
     /// Consumes the mesh and returns a mesh with calculated [`Mesh::ATTRIBUTE_NORMAL`].
     ///
-    /// (Alternatively, you can use [`Mesh::with_computed_flat_normals`] to mutate an existing mesh in-place)
+    /// (Alternatively, you can use [`Mesh::compute_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
+    /// Panics if [`Mesh::ATTRIBUTE_POSITION`] is not of type `float3`.
-    /// if the mesh has any other topology than [`PrimitiveTopology::TriangleList`].
+    /// Panics if the mesh has any other topology than [`PrimitiveTopology::TriangleList`].
-    /// Consider calling [`Mesh::with_duplicated_vertices`] or export your mesh with normal attributes.
+    /// Panics if the mesh has indices defined
     #[must_use]
     pub fn with_computed_flat_normals(mut self) -> Self {
         self.compute_flat_normals();
         self
     }
 
+    /// Consumes the mesh and returns a mesh with calculated [`Mesh::ATTRIBUTE_NORMAL`].
+    ///
+    /// (Alternatively, you can use [`Mesh::compute_smooth_normals`] to mutate an existing mesh in-place)
+    ///
+    /// # Panics
+    /// Panics if [`Mesh::ATTRIBUTE_POSITION`] is not of type `float3`.
+    /// Panics if the mesh has any other topology than [`PrimitiveTopology::TriangleList`].
+    /// Panics if the mesh does not have indices defined.
+    #[must_use]
+    pub fn with_computed_smooth_normals(mut self) -> Self {
+        self.compute_smooth_normals();
+        self
+    }
+
     /// Generate tangents for the mesh using the `mikktspace` algorithm.
     ///
     /// Sets the [`Mesh::ATTRIBUTE_TANGENT`] attribute if successful.