Shaders / Shader Defs

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//! A shader that uses "shaders defs", which selectively toggle parts of a shader.

use bevy::{
    pbr::{MaterialPipeline, MaterialPipelineKey},
            AsBindGroup, RenderPipelineDescriptor, ShaderRef, SpecializedMeshPipelineError,

fn main() {
        .add_systems(Startup, setup)

/// set up a simple 3D scene
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<CustomMaterial>>,
) {
    // blue cube
    commands.spawn(MaterialMeshBundle {
        mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
        transform: Transform::from_xyz(-1.0, 0.5, 0.0),
        material: materials.add(CustomMaterial {
            color: Color::BLUE,
            is_red: false,

    // red cube (with green color overridden by the IS_RED "shader def")
    commands.spawn(MaterialMeshBundle {
        mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
        transform: Transform::from_xyz(1.0, 0.5, 0.0),
        material: materials.add(CustomMaterial {
            color: Color::GREEN,
            is_red: true,

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

impl Material for CustomMaterial {
    fn fragment_shader() -> ShaderRef {

    fn specialize(
        _pipeline: &MaterialPipeline<Self>,
        descriptor: &mut RenderPipelineDescriptor,
        _layout: &MeshVertexBufferLayout,
        key: MaterialPipelineKey<Self>,
    ) -> Result<(), SpecializedMeshPipelineError> {
        if key.bind_group_data.is_red {
            let fragment = descriptor.fragment.as_mut().unwrap();

// This is the struct that will be passed to your shader
#[derive(AsBindGroup, TypeUuid, Debug, Clone)]
#[uuid = "f690fdae-d598-45ab-8225-97e2a3f056e0"]
pub struct CustomMaterial {
    color: Color,
    is_red: bool,

// This key is used to identify a specific permutation of this material pipeline.
// In this case, we specialize on whether or not to configure the "IS_RED" shader def.
// Specialization keys should be kept as small / cheap to hash as possible,
// as they will be used to look up the pipeline for each drawn entity with this material type.
#[derive(Eq, PartialEq, Hash, Clone)]
pub struct CustomMaterialKey {
    is_red: bool,

impl From<&CustomMaterial> for CustomMaterialKey {
    fn from(material: &CustomMaterial) -> Self {
        Self {
            is_red: material.is_red,