calculate flat normals for mesh if missing (bevyengine#1808)

If the gltf loader encounters a mesh without normal attributes, it will duplicate the vertex attributes and compute flat normals, as defined by https://github.com/KhronosGroup/glTF/blob/master/specification/2.0/README.md#meshes:

> **Implementation Note**: When normals are not specified, client implementations should calculate flat normals.

![image](https://user-images.githubusercontent.com/22177966/113483243-bb204880-94a2-11eb-8fa1-c4828a4882c5.png)

Helps with bevyengine#1802 

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_gltf/Cargo.toml

@@ -24,6 +24,7 @@ bevy_render = { path = "../bevy_render", version = "0.5.0" }
 bevy_transform = { path = "../bevy_transform", version = "0.5.0" }
 bevy_math = { path = "../bevy_math", version = "0.5.0" }
 bevy_scene = { path = "../bevy_scene", version = "0.5.0" }
+bevy_log = { path = "../bevy_log", version = "0.5.0" }
 
 # other
 gltf = { version = "0.15.2", default-features = false, features = ["utils", "names", "KHR_materials_unlit"] }

crates/bevy_gltf/src/loader.rs

@@ -153,6 +153,21 @@ async fn load_gltf<'a, 'b>(
                 mesh.set_indices(Some(Indices::U32(indices.into_u32().collect())));
             };
 
+            if mesh.attribute(Mesh::ATTRIBUTE_NORMAL).is_none() {
+                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_log::debug!("Missing vertex normals in indexed geometry, computing them as flat. Vertex count increased from {} to {}", vertex_count_before, vertex_count_after);
+                } else {
+                    bevy_log::debug!(
+                        "Missing vertex normals in indexed geometry, computing them as flat."
+                    );
+                }
+            }
+
             let mesh = load_context.set_labeled_asset(&primitive_label, LoadedAsset::new(mesh));
             primitives.push(super::GltfPrimitive {
                 mesh,

crates/bevy_render/src/mesh/mesh.rs

@@ -95,6 +95,13 @@ impl VertexAttributeValues {
         self.len() == 0
     }
 
+    fn as_float3(&self) -> Option<&[[f32; 3]]> {
+        match self {
+            VertexAttributeValues::Float3(values) => Some(values),
+            _ => None,
+        }
+    }
+
     // TODO: add vertex format as parameter here and perform type conversions
     /// Flattens the VertexAttributeArray into a sequence of bytes. This is
     /// useful for serialization and sending to the GPU.
@@ -254,6 +261,29 @@ pub enum Indices {
     U32(Vec<u32>),
 }
 
+impl Indices {
+    fn iter(&self) -> impl Iterator<Item = usize> + '_ {
+        match self {
+            Indices::U16(vec) => IndicesIter::U16(vec.iter()),
+            Indices::U32(vec) => IndicesIter::U32(vec.iter()),
+        }
+    }
+}
+enum IndicesIter<'a> {
+    U16(std::slice::Iter<'a, u16>),
+    U32(std::slice::Iter<'a, u32>),
+}
+impl Iterator for IndicesIter<'_> {
+    type Item = usize;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self {
+            IndicesIter::U16(iter) => iter.next().map(|val| *val as usize),
+            IndicesIter::U32(iter) => iter.next().map(|val| *val as usize),
+        }
+    }
+}
+
 impl From<&Indices> for IndexFormat {
     fn from(indices: &Indices) -> Self {
         match indices {
@@ -431,6 +461,88 @@ impl Mesh {
 
         attributes_interleaved_buffer
     }
+
+    /// Duplicates the vertex attributes so that no vertices are shared.
+    ///
+    /// This can dramatically increase the vertex count, so make sure this is what you want.
+    /// Does nothing if no [Indices] are set.
+    pub fn duplicate_vertices(&mut self) {
+        fn duplicate<T: Copy>(values: &[T], indices: impl Iterator<Item = usize>) -> Vec<T> {
+            indices.map(|i| values[i]).collect()
+        }
+
+        assert!(
+            matches!(self.primitive_topology, PrimitiveTopology::TriangleList),
+            "can only duplicate vertices for `TriangleList`s"
+        );
+
+        let indices = match self.indices.take() {
+            Some(indices) => indices,
+            None => return,
+        };
+        for (_, attributes) in self.attributes.iter_mut() {
+            let indices = indices.iter();
+            match attributes {
+                VertexAttributeValues::Float(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Int(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Uint(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Float2(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Int2(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Uint2(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Float3(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Int3(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Uint3(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Int4(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Uint4(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Float4(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Short2(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Short2Norm(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Ushort2(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Ushort2Norm(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Short4(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Short4Norm(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Ushort4(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Ushort4Norm(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Char2(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Char2Norm(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Uchar2(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Uchar2Norm(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Char4(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Char4Norm(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Uchar4(vec) => *vec = duplicate(&vec, indices),
+                VertexAttributeValues::Uchar4Norm(vec) => *vec = duplicate(&vec, indices),
+            }
+        }
+    }
+
+    /// Calculates the [`Mesh::ATTRIBUTE_NORMAL`] of a mesh.
+    ///
+    /// Panics if [`Indices`] are set.
+    /// Consider calling [Mesh::duplicate_vertices] or export your mesh with normal attributes.
+    pub fn compute_flat_normals(&mut self) {
+        if self.indices().is_some() {
+            panic!("`compute_flat_normals` can't work on indexed geometry. Consider calling `Mesh::duplicate_vertices`.");
+        }
+
+        let positions = self
+            .attribute(Mesh::ATTRIBUTE_POSITION)
+            .unwrap()
+            .as_float3()
+            .expect("`Mesh::ATTRIBUTE_POSITION` vertex attributes should be of type `float3`");
+
+        let normals: Vec<_> = positions
+            .chunks_exact(3)
+            .map(|p| face_normal(p[0], p[1], p[2]))
+            .flat_map(|normal| std::array::IntoIter::new([normal, normal, normal]))
+            .collect();
+
+        self.set_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
+    }
+}
+
+fn face_normal(a: [f32; 3], b: [f32; 3], c: [f32; 3]) -> [f32; 3] {
+    let (a, b, c) = (Vec3::from(a), Vec3::from(b), Vec3::from(c));
+    (b - a).cross(c - a).normalize().into()
 }
 
 fn remove_resource_save(