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Add example showing how to use SpecializedMeshPipeline (#14370)

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# Objective

- A lot of mid-level rendering apis are hard to figure out because they
don't have any examples
- SpecializedMeshPipeline can be really useful in some cases when you
want more flexibility than a Material without having to go to low level
apis.

## Solution

- Add an example showing how to make a custom `SpecializedMeshPipeline`.

## Testing

- Did you test these changes? If so, how?
- Are there any parts that need more testing?
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?

---

## Showcase

The examples just spawns 3 triangles in a triangle pattern.


![image](https://github.com/user-attachments/assets/c3098758-94c4-4775-95e5-1d7c7fb9eb86)

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
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11
Cargo.toml
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@ -2394,6 +2394,17 @@ description = "A shader that shows how to bind and sample multiple textures as a
category = "Shaders"
wasm = false
[[example]]
name = "specialized_mesh_pipeline"
path = "examples/shader/specialized_mesh_pipeline.rs"
doc-scrape-examples = true
[package.metadata.example.specialized_mesh_pipeline]
name = "Specialized Mesh Pipeline"
description = "Demonstrates how to write a specialized mesh pipeline"
category = "Shaders"
wasm = true
# Stress tests
[[package.metadata.example_category]]
name = "Stress Tests"

48
assets/shaders/specialized_mesh_pipeline.wgsl Normal file
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@ -0,0 +1,48 @@
//! Very simple shader used to demonstrate how to get the world position and pass data
//! between the vertex and fragment shader. Also shows the custom vertex layout.
// First we import everything we need from bevy_pbr
// A 2d shader would be vevry similar but import from bevy_sprite instead
#import bevy_pbr::{
mesh_functions,
view_transformations::position_world_to_clip
}
struct Vertex {
// This is needed if you are using batching and/or gpu preprocessing
// It's a built in so you don't need to define it in the vertex layout
@builtin(instance_index) instance_index: u32,
// Like we defined for the vertex layout
// position is at location 0
@location(0) position: vec3<f32>,
// and color at location 1
@location(1) color: vec4<f32>,
};
// This is the output of the vertex shader and we also use it as the input for the fragment shader
struct VertexOutput {
@builtin(position) clip_position: vec4<f32>,
@location(0) world_position: vec4<f32>,
@location(1) color: vec3<f32>,
};
@vertex
fn vertex(vertex: Vertex) -> VertexOutput {
var out: VertexOutput;
// This is how bevy computes the world position
// The vertex.instance_index is very important. Esepecially if you are using batching and gpu preprocessing
var world_from_local = mesh_functions::get_world_from_local(vertex.instance_index);
out.world_position = mesh_functions::mesh_position_local_to_world(world_from_local, vec4(vertex.position, 1.0));
out.clip_position = position_world_to_clip(out.world_position.xyz);
// We just use the raw vertex color
out.color = vertex.color.rgb;
return out;
}
@fragment
fn fragment(in: VertexOutput) -> @location(0) vec4<f32> {
// output the color directly
return vec4(in.color, 1.0);
}

1
examples/README.md
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@ -392,6 +392,7 @@ Example | Description
[Material Prepass](../examples/shader/shader_prepass.rs) | A shader that uses the various textures generated by the prepass
[Post Processing - Custom Render Pass](../examples/shader/custom_post_processing.rs) | A custom post processing effect, using a custom render pass that runs after the main pass
[Shader Defs](../examples/shader/shader_defs.rs) | A shader that uses "shaders defs" (a bevy tool to selectively toggle parts of a shader)
[Specialized Mesh Pipeline](../examples/shader/specialized_mesh_pipeline.rs) | Demonstrates how to write a specialized mesh pipeline
[Texture Binding Array (Bindless Textures)](../examples/shader/texture_binding_array.rs) | A shader that shows how to bind and sample multiple textures as a binding array (a.k.a. bindless textures).
## State

358
examples/shader/specialized_mesh_pipeline.rs Normal file
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@ -0,0 +1,358 @@
//! Demonstrates how to define and use specialized mesh pipeline
//!
//! This example shows how to use the built-in [`SpecializedMeshPipeline`]
//! functionality with a custom [`RenderCommand`] to allow custom mesh rendering with
//! more flexibility than the material api.
//!
//! [`SpecializedMeshPipeline`] let's you customize the entire pipeline used when rendering a mesh.
use bevy::{
core_pipeline::core_3d::{Opaque3d, Opaque3dBinKey, CORE_3D_DEPTH_FORMAT},
math::{vec3, vec4},
pbr::{
DrawMesh, MeshPipeline, MeshPipelineKey, MeshPipelineViewLayoutKey, RenderMeshInstances,
SetMeshBindGroup, SetMeshViewBindGroup,
},
prelude::*,
render::{
extract_component::{ExtractComponent, ExtractComponentPlugin},
mesh::{Indices, MeshVertexBufferLayoutRef, PrimitiveTopology, RenderMesh},
render_asset::{RenderAssetUsages, RenderAssets},
render_phase::{
AddRenderCommand, BinnedRenderPhaseType, DrawFunctions, SetItemPipeline,
ViewBinnedRenderPhases,
},
render_resource::{
ColorTargetState, ColorWrites, CompareFunction, DepthStencilState, Face, FragmentState,
FrontFace, MultisampleState, PipelineCache, PolygonMode, PrimitiveState,
RenderPipelineDescriptor, SpecializedMeshPipeline, SpecializedMeshPipelineError,
SpecializedMeshPipelines, TextureFormat, VertexState,
},
texture::BevyDefault as _,
view::{self, ExtractedView, ViewTarget, VisibilitySystems, VisibleEntities},
Render, RenderApp, RenderSet,
},
};
const SHADER_ASSET_PATH: &str = "shaders/specialized_mesh_pipeline.wgsl";
fn main() {
App::new()
.add_plugins(DefaultPlugins)
.add_plugins(CustomRenderedMeshPipelinePlugin)
.add_systems(Startup, setup)
.run();
}
/// Spawns the objects in the scene.
fn setup(mut commands: Commands, mut meshes: ResMut<Assets<Mesh>>) {
// Build a custom triangle mesh with colors
// We define a custom mesh because the examples only uses a limited
// set of vertex attributes for simplicity
let mesh = Mesh::new(
PrimitiveTopology::TriangleList,
RenderAssetUsages::default(),
)
.with_inserted_indices(Indices::U32(vec![0, 1, 2, 0, 2, 3]))
.with_inserted_attribute(
Mesh::ATTRIBUTE_POSITION,
vec![
vec3(-0.5, -0.5, 0.0),
vec3(0.5, -0.5, 0.0),
vec3(0.0, 0.25, 0.0),
],
)
.with_inserted_attribute(
Mesh::ATTRIBUTE_COLOR,
vec![
vec4(1.0, 0.0, 0.0, 1.0),
vec4(0.0, 1.0, 0.0, 1.0),
vec4(0.0, 0.0, 1.0, 1.0),
],
);
// spawn 3 triangles to show that batching works
for (x, y) in [-0.5, 0.0, 0.5].into_iter().zip([-0.25, 0.5, -0.25]) {
// Spawn an entity with all the required components for it to be rendered with our custom pipeline
commands.spawn((
// We use a marker component to identify the mesh that will be rendered
// with our specialized pipeline
CustomRenderedEntity,
// We need to add the mesh handle to the entity
meshes.add(mesh.clone()),
// This bundle's components are needed for something to be rendered
SpatialBundle {
transform: Transform::from_xyz(x, y, 0.0),
..SpatialBundle::INHERITED_IDENTITY
},
));
}
// Spawn the camera.
commands.spawn(Camera3dBundle {
// Move the camera back a bit to see all the triangles
transform: Transform::from_xyz(0.0, 0.0, 3.0).looking_at(Vec3::ZERO, Vec3::Y),
..default()
});
}
// When writing custom rendering code it's generally recommended to use a plugin.
// The main reason for this is that it gives you access to the finish() hook
// which is called after rendering resources are initialized.
struct CustomRenderedMeshPipelinePlugin;
impl Plugin for CustomRenderedMeshPipelinePlugin {
fn build(&self, app: &mut App) {
app.add_plugins(ExtractComponentPlugin::<CustomRenderedEntity>::default())
.add_systems(
PostUpdate,
// Make sure to tell Bevy to check our entity for visibility. Bevy won't
// do this by default, for efficiency reasons.
// This will do things like frustum culling and hierarchy visibility
view::check_visibility::<WithCustomRenderedEntity>
.in_set(VisibilitySystems::CheckVisibility),
);
// We make sure to add these to the render app, not the main app.
let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
return;
};
render_app
// This is needed to tell bevy about your custom pipeline
.init_resource::<SpecializedMeshPipelines<CustomMeshPipeline>>()
// We need to use a custom draw command so we need to register it
.add_render_command::<Opaque3d, DrawSpecializedPipelineCommands>()
.add_systems(Render, queue_custom_mesh_pipeline.in_set(RenderSet::Queue));
}
fn finish(&self, app: &mut App) {
let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
return;
};
// Creating this pipeline needs the RenderDevice and RenderQueue
// which are only available once rendering plugins are initialized.
render_app.init_resource::<CustomMeshPipeline>();
}
}
/// A marker component that represents an entity that is to be rendered using
/// our specialized pipeline.
///
/// Note the [`ExtractComponent`] trait implementation. This is necessary to
/// tell Bevy that this object should be pulled into the render world.
#[derive(Clone, Component, ExtractComponent)]
struct CustomRenderedEntity;
/// The custom draw commands that Bevy executes for each entity we enqueue into
/// the render phase.
type DrawSpecializedPipelineCommands = (
// Set the pipeline
SetItemPipeline,
// Set the view uniform at bind group 0
SetMeshViewBindGroup<0>,
// Set the mesh uniform at bind group 1
SetMeshBindGroup<1>,
// Draw the mesh
DrawMesh,
);
/// A query filter that tells [`view::check_visibility`] about our custom
/// rendered entity.
type WithCustomRenderedEntity = With<CustomRenderedEntity>;
// This contains the state needed to speciazlize a mesh pipeline
#[derive(Resource)]
struct CustomMeshPipeline {
/// The base mesh pipeline defined by bevy
///
/// This isn't required, but if you want to use a bevy `Mesh` it's easier when you
/// have access to the base `MeshPipeline` that bevy already defines
mesh_pipeline: MeshPipeline,
/// Stores the shader used for this pipeline directly on the pipeline.
/// This isn't required, it's only done like this for simplicity.
shader_handle: Handle<Shader>,
}
impl FromWorld for CustomMeshPipeline {
fn from_world(world: &mut World) -> Self {
// Load the shader
let shader_handle: Handle<Shader> = world.resource::<AssetServer>().load(SHADER_ASSET_PATH);
Self {
mesh_pipeline: MeshPipeline::from_world(world),
shader_handle,
}
}
}
impl SpecializedMeshPipeline for CustomMeshPipeline {
/// Pipeline use keys to determine how to specialize it.
/// The key is also used by the pipeline cache to determine if
/// it needs to create a new pipeline or not
///
/// In this example we just use the base `MeshPipelineKey` defined by bevy, but this could be anything.
/// For example, if you want to make a pipeline with a procedural shader you could add the Handle<Shader> to the key.
type Key = MeshPipelineKey;
fn specialize(
&self,
mesh_key: Self::Key,
layout: &MeshVertexBufferLayoutRef,
) -> Result<RenderPipelineDescriptor, SpecializedMeshPipelineError> {
// Define the vertex attributes based on a standard bevy [`Mesh`]
let mut vertex_attributes = Vec::new();
if layout.0.contains(Mesh::ATTRIBUTE_POSITION) {
// Make sure this matches the shader location
vertex_attributes.push(Mesh::ATTRIBUTE_POSITION.at_shader_location(0));
}
if layout.0.contains(Mesh::ATTRIBUTE_COLOR) {
// Make sure this matches the shader location
vertex_attributes.push(Mesh::ATTRIBUTE_COLOR.at_shader_location(1));
}
// This will automatically generate the correct `VertexBufferLayout` based on the vertex attributes
let vertex_buffer_layout = layout.0.get_layout(&vertex_attributes)?;
Ok(RenderPipelineDescriptor {
label: Some("Specialized Mesh Pipeline".into()),
layout: vec![
// Bind group 0 is the view uniform
self.mesh_pipeline
.get_view_layout(MeshPipelineViewLayoutKey::from(mesh_key))
.clone(),
// Bind group 1 is the mesh uniform
self.mesh_pipeline.mesh_layouts.model_only.clone(),
],
push_constant_ranges: vec![],
vertex: VertexState {
shader: self.shader_handle.clone(),
shader_defs: vec![],
entry_point: "vertex".into(),
// Customize how to store the meshes' vertex attributes in the vertex buffer
buffers: vec![vertex_buffer_layout],
},
fragment: Some(FragmentState {
shader: self.shader_handle.clone(),
shader_defs: vec![],
entry_point: "fragment".into(),
targets: vec![Some(ColorTargetState {
// This isn't required, but bevy supports HDR and non-HDR rendering
// so it's generally recommended to specialize the pipeline for that
format: if mesh_key.contains(MeshPipelineKey::HDR) {
ViewTarget::TEXTURE_FORMAT_HDR
} else {
TextureFormat::bevy_default()
},
// For this example we only use opaque meshes,
// but if you wanted to use alpha blending you would need to set it here
blend: None,
write_mask: ColorWrites::ALL,
})],
}),
primitive: PrimitiveState {
topology: mesh_key.primitive_topology(),
front_face: FrontFace::Ccw,
cull_mode: Some(Face::Back),
polygon_mode: PolygonMode::Fill,
..default()
},
// Note that if your view has no depth buffer this will need to be
// changed.
depth_stencil: Some(DepthStencilState {
format: CORE_3D_DEPTH_FORMAT,
depth_write_enabled: true,
depth_compare: CompareFunction::GreaterEqual,
stencil: default(),
bias: default(),
}),
// It's generally recommended to specialize your pipeline for MSAA,
// but it's not always possible
multisample: MultisampleState {
count: mesh_key.msaa_samples(),
..MultisampleState::default()
},
})
}
}
/// A render-world system that enqueues the entity with custom rendering into
/// the opaque render phases of each view.
#[allow(clippy::too_many_arguments)]
fn queue_custom_mesh_pipeline(
pipeline_cache: Res<PipelineCache>,
custom_mesh_pipeline: Res<CustomMeshPipeline>,
mut opaque_render_phases: ResMut<ViewBinnedRenderPhases<Opaque3d>>,
opaque_draw_functions: Res<DrawFunctions<Opaque3d>>,
mut specialized_mesh_pipelines: ResMut<SpecializedMeshPipelines<CustomMeshPipeline>>,
views: Query<(Entity, &VisibleEntities, &ExtractedView, &Msaa), With<ExtractedView>>,
render_meshes: Res<RenderAssets<RenderMesh>>,
render_mesh_instances: Res<RenderMeshInstances>,
) {
// Get the id for our custom draw function
let draw_function_id = opaque_draw_functions
.read()
.id::<DrawSpecializedPipelineCommands>();
// Render phases are per-view, so we need to iterate over all views so that
// the entity appears in them. (In this example, we have only one view, but
// it's good practice to loop over all views anyway.)
for (view_entity, view_visible_entities, view, msaa) in views.iter() {
let Some(opaque_phase) = opaque_render_phases.get_mut(&view_entity) else {
continue;
};
// Create the key based on the view. In this case we only care about MSAA and HDR
let view_key = MeshPipelineKey::from_msaa_samples(msaa.samples())
| MeshPipelineKey::from_hdr(view.hdr);
// Find all the custom rendered entities that are visible from this
// view.
for &visible_entity in view_visible_entities
.get::<WithCustomRenderedEntity>()
.iter()
{
// Get the mesh instance
let Some(mesh_instance) = render_mesh_instances.render_mesh_queue_data(visible_entity)
else {
continue;
};
// Get the mesh data
let Some(mesh) = render_meshes.get(mesh_instance.mesh_asset_id) else {
continue;
};
// Specialize the key for the current mesh entity
// For this example we only specialize based on the mesh topology
// but you could have more complex keys and that's where you'd need to create those keys
let mut mesh_key = view_key;
mesh_key |= MeshPipelineKey::from_primitive_topology(mesh.primitive_topology());
// Finally, we can specialize the pipeline based on the key
let pipeline_id = specialized_mesh_pipelines
.specialize(
&pipeline_cache,
&custom_mesh_pipeline,
mesh_key,
&mesh.layout,
)
// This should never with this example, but if your pipeline specialization
// can fail you need to handle the error here
.expect("Failed to specialize mesh pipeline");
// Add the mesh with our specialized pipeline
opaque_phase.add(
Opaque3dBinKey {
draw_function: draw_function_id,
pipeline: pipeline_id,
// The asset ID is arbitrary; we simply use [`AssetId::invalid`],
// but you can use anything you like. Note that the asset ID need
// not be the ID of a [`Mesh`].
asset_id: AssetId::<Mesh>::invalid().untyped(),
material_bind_group_id: None,
lightmap_image: None,
},
visible_entity,
// This example supports batching, but if your pipeline doesn't
// support it you can use `BinnedRenderPhaseType::UnbatchableMesh`
BinnedRenderPhaseType::BatchableMesh,
);
}
}
}