feat(gltf): add name component to gltf mesh primitive (#13912)

# Objective

- fixes https://github.com/bevyengine/bevy/issues/13473

## Solution

- When a single mesh is assigned multiple materials, it is divided into
several primitive nodes, with each primitive assigned a unique material.
Presently, these primitives are named using the format Mesh.index, which
complicates querying. To improve this, we can assign a specific name to
each primitive based on the material’s name, since each primitive
corresponds to one material exclusively.

## Testing

- I have included a simple example which shows how to query a material
and mesh part based on the new name component.

## Changelog
- adds `GltfMaterialName` component to the mesh entity of the gltf
primitive node.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

Cargo.toml

@@ -910,6 +910,17 @@ description = "Loads and renders a glTF file as a scene, including the gltf extr
 category = "3D Rendering"
 wasm = true
 
+[[example]]
+name = "query_gltf_primitives"
+path = "examples/3d/query_gltf_primitives.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.query_gltf_primitives]
+name = "Query glTF primitives"
+description = "Query primitives in a glTF scene"
+category = "3D Rendering"
+wasm = true
+
 [[example]]
 name = "motion_blur"
 path = "examples/3d/motion_blur.rs"

crates/bevy_gltf/src/lib.rs

@@ -153,6 +153,7 @@ impl Plugin for GltfPlugin {
             .register_type::<GltfSceneExtras>()
             .register_type::<GltfMeshExtras>()
             .register_type::<GltfMaterialExtras>()
+            .register_type::<GltfMaterialName>()
             .init_asset::<Gltf>()
             .init_asset::<GltfNode>()
             .init_asset::<GltfPrimitive>()
@@ -460,6 +461,13 @@ pub struct GltfMaterialExtras {
     pub value: String,
 }
 
+/// The material name of a glTF primitive.
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-material).
+#[derive(Clone, Debug, Reflect, Default, Component)]
+#[reflect(Component)]
+pub struct GltfMaterialName(pub String);
+
 /// Labels that can be used to load part of a glTF
 ///
 /// You can use [`GltfAssetLabel::from_asset`] to add it to an asset path

crates/bevy_gltf/src/loader.rs

@@ -1,6 +1,6 @@
 use crate::{
     vertex_attributes::convert_attribute, Gltf, GltfAssetLabel, GltfExtras, GltfMaterialExtras,
-    GltfMeshExtras, GltfNode, GltfSceneExtras, GltfSkin,
+    GltfMaterialName, GltfMeshExtras, GltfNode, GltfSceneExtras, GltfSkin,
 };
 
 use alloc::collections::VecDeque;
@@ -1380,6 +1380,10 @@ fn load_node(
                         });
                     }
 
+                    if let Some(name) = material.name() {
+                        mesh_entity.insert(GltfMaterialName(String::from(name)));
+                    }
+
                     mesh_entity.insert(Name::new(primitive_name(&mesh, &primitive)));
                     // Mark for adding skinned mesh
                     if let Some(skin) = gltf_node.skin() {

examples/3d/query_gltf_primitives.rs

@@ -0,0 +1,96 @@
+//! This example demonstrates how to query a [`StandardMaterial`] within a glTF scene.
+//! It is particularly useful for glTF scenes with a mesh that consists of multiple primitives.
+
+use std::f32::consts::PI;
+
+use bevy::{
+    gltf::GltfMaterialName,
+    pbr::{CascadeShadowConfigBuilder, DirectionalLightShadowMap},
+    prelude::*,
+    render::mesh::VertexAttributeValues,
+};
+
+fn main() {
+    App::new()
+        .insert_resource(DirectionalLightShadowMap { size: 4096 })
+        .add_plugins(DefaultPlugins)
+        .add_systems(Startup, setup)
+        .add_systems(Update, find_top_material_and_mesh)
+        .run();
+}
+
+fn find_top_material_and_mesh(
+    mut materials: ResMut<Assets<StandardMaterial>>,
+    mut meshes: ResMut<Assets<Mesh>>,
+    time: Res<Time>,
+    mat_query: Query<(&Handle<StandardMaterial>, &Handle<Mesh>, &GltfMaterialName)>,
+) {
+    for (mat_handle, mesh_handle, name) in mat_query.iter() {
+        // locate a material by material name
+        if name.0 == "Top" {
+            if let Some(material) = materials.get_mut(mat_handle) {
+                if let Color::Hsla(ref mut hsla) = material.base_color {
+                    *hsla = hsla.rotate_hue(time.delta_seconds() * 100.0);
+                } else {
+                    material.base_color = Color::from(Hsla::hsl(0.0, 0.8, 0.5));
+                }
+            }
+
+            if let Some(mesh) = meshes.get_mut(mesh_handle) {
+                if let Some(VertexAttributeValues::Float32x3(positions)) =
+                    mesh.attribute_mut(Mesh::ATTRIBUTE_POSITION)
+                {
+                    positions[0] = (
+                        ops::sin(2.0 * PI * time.elapsed_seconds()),
+                        positions[0][1],
+                        positions[0][2],
+                    )
+                        .into();
+                }
+            }
+        }
+    }
+}
+
+fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
+    commands.spawn((
+        Camera3dBundle {
+            transform: Transform::from_xyz(0.6, 1.6, 11.3)
+                .looking_at(Vec3::new(0.0, 0.0, 3.0), Vec3::Y),
+            ..default()
+        },
+        EnvironmentMapLight {
+            diffuse_map: asset_server.load("environment_maps/pisa_diffuse_rgb9e5_zstd.ktx2"),
+            specular_map: asset_server.load("environment_maps/pisa_specular_rgb9e5_zstd.ktx2"),
+            intensity: 500.0,
+            rotation: Quat::IDENTITY,
+        },
+    ));
+
+    commands.spawn(DirectionalLightBundle {
+        directional_light: DirectionalLight {
+            shadows_enabled: true,
+            ..default()
+        },
+        // This is a relatively small scene, so use tighter shadow
+        // cascade bounds than the default for better quality.
+        // We also adjusted the shadow map to be larger since we're
+        // only using a single cascade.
+        cascade_shadow_config: CascadeShadowConfigBuilder {
+            num_cascades: 1,
+            maximum_distance: 1.6,
+            ..default()
+        }
+        .into(),
+        ..default()
+    });
+    commands.spawn(SceneBundle {
+        scene: asset_server
+            .load(GltfAssetLabel::Scene(0).from_asset("models/GltfPrimitives/gltf_primitives.glb")),
+        transform: Transform {
+            rotation: Quat::from_rotation_y(-90.0 / 180.0 * PI),
+            ..default()
+        },
+        ..default()
+    });
+}

examples/README.md

@@ -159,6 +159,7 @@ Example | Description
 [Parenting](../examples/3d/parenting.rs) | Demonstrates parent->child relationships and relative transformations
 [Percentage-closer soft shadows](../examples/3d/pcss.rs) | Demonstrates percentage-closer soft shadows (PCSS)
 [Physically Based Rendering](../examples/3d/pbr.rs) | Demonstrates use of Physically Based Rendering (PBR) properties
+[Query glTF primitives](../examples/3d/query_gltf_primitives.rs) | Query primitives in a glTF scene
 [Reflection Probes](../examples/3d/reflection_probes.rs) | Demonstrates reflection probes
 [Render to Texture](../examples/3d/render_to_texture.rs) | Shows how to render to a texture, useful for mirrors, UI, or exporting images
 [Rotate Environment Map](../examples/3d/rotate_environment_map.rs) | Demonstrates how to rotate the skybox and the environment map simultaneously