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//! A shader that reads a mesh's custom vertex attribute.

use bevy::{
    pbr::{MaterialPipeline, MaterialPipelineKey},
    prelude::*,
    reflect::TypePath,
    render::{
        mesh::{MeshVertexAttribute, MeshVertexBufferLayoutRef},
        render_resource::{
            AsBindGroup, RenderPipelineDescriptor, ShaderRef, SpecializedMeshPipelineError,
            VertexFormat,
        },
    },
};

fn main() {
    App::new()
        .add_plugins((DefaultPlugins, MaterialPlugin::<CustomMaterial>::default()))
        .add_systems(Startup, setup)
        .run();
}

// A "high" random id should be used for custom attributes to ensure consistent sorting and avoid collisions with other attributes.
// See the MeshVertexAttribute docs for more info.
const ATTRIBUTE_BLEND_COLOR: MeshVertexAttribute =
    MeshVertexAttribute::new("BlendColor", 988540917, VertexFormat::Float32x4);

/// set up a simple 3D scene
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<CustomMaterial>>,
) {
    let mesh = Mesh::from(Cuboid::default())
        // Sets the custom attribute
        .with_inserted_attribute(
            ATTRIBUTE_BLEND_COLOR,
            // The cube mesh has 24 vertices (6 faces, 4 vertices per face), so we insert one BlendColor for each
            vec![[1.0, 0.0, 0.0, 1.0]; 24],
        );

    // cube
    commands.spawn(MaterialMeshBundle {
        mesh: meshes.add(mesh),
        transform: Transform::from_xyz(0.0, 0.5, 0.0),
        material: materials.add(CustomMaterial {
            color: LinearRgba::WHITE,
        }),
        ..default()
    });

    // camera
    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..default()
    });
}

// This is the struct that will be passed to your shader
#[derive(Asset, TypePath, AsBindGroup, Debug, Clone)]
struct CustomMaterial {
    #[uniform(0)]
    color: LinearRgba,
}

impl Material for CustomMaterial {
    fn vertex_shader() -> ShaderRef {
        "shaders/custom_vertex_attribute.wgsl".into()
    }
    fn fragment_shader() -> ShaderRef {
        "shaders/custom_vertex_attribute.wgsl".into()
    }

    fn specialize(
        _pipeline: &MaterialPipeline<Self>,
        descriptor: &mut RenderPipelineDescriptor,
        layout: &MeshVertexBufferLayoutRef,
        _key: MaterialPipelineKey<Self>,
    ) -> Result<(), SpecializedMeshPipelineError> {
        let vertex_layout = layout.0.get_layout(&[
            Mesh::ATTRIBUTE_POSITION.at_shader_location(0),
            ATTRIBUTE_BLEND_COLOR.at_shader_location(1),
        ])?;
        descriptor.vertex.buffers = vec![vertex_layout];
        Ok(())
    }
}