add a post-processing example (bevyengine#4797)

# Objective

- Add an example showing a custom post processing effect, done after the first rendering pass.

## Solution

- A simple post processing "chromatic aberration" effect. I mixed together examples `3d/render_to_texture`, and `shader/shader_material_screenspace_texture`
- Reading a bit how bevyengine#3430 was done gave me pointers to apply the main pass to the 2d render rather than using a 3d quad.

This work might be or not be relevant to bevyengine#2724

<details>

<summary> ⚠️ Click for a video of the render ⚠️ I’ve been told it might hurt the eyes 👀 , maybe we should choose another effect just in case ?</summary>

https://user-images.githubusercontent.com/2290685/169138830-a6dc8a9f-8798-44b9-8d9e-449e60614916.mp4

</details>

# Request for feedbacks

- [ ] Is chromatic aberration effect ok ? (Correct term, not a danger for the eyes ?) I'm open to suggestion to make something different.
- [ ] Is the code idiomatic ? I preferred a "main camera -> **new camera with post processing applied to a quad**" approach to emulate minimum modification to existing code wanting to add global post processing.

---

## Changelog

- Add a full screen post processing shader example

Cargo.toml

@@ -512,6 +512,10 @@ path = "examples/scene/scene.rs"
 name = "custom_vertex_attribute"
 path = "examples/shader/custom_vertex_attribute.rs"
 
+[[example]]
+name = "post_processing"
+path = "examples/shader/post_processing.rs"
+
 [[example]]
 name = "shader_defs"
 path = "examples/shader/shader_defs.rs"

assets/shaders/custom_material_chromatic_aberration.wgsl

@@ -0,0 +1,25 @@
+#import bevy_pbr::mesh_view_bindings
+
+[[group(1), binding(0)]]
+var texture: texture_2d<f32>;
+
+[[group(1), binding(1)]]
+var our_sampler: sampler;
+
+
+[[stage(fragment)]]
+fn fragment([[builtin(position)]] position: vec4<f32>) -> [[location(0)]] vec4<f32> {
+    // Get screen position with coordinates from 0 to 1
+    let uv = position.xy / vec2<f32>(view.width, view.height);
+    let offset_strength = 0.02;
+
+    // Sample each color channel with an arbitrary shift
+    var output_color = vec4<f32>(
+        textureSample(texture, our_sampler, uv + vec2<f32>(offset_strength, -offset_strength)).r,
+        textureSample(texture, our_sampler, uv + vec2<f32>(-offset_strength, 0.0)).g,
+        textureSample(texture, our_sampler, uv + vec2<f32>(0.0, offset_strength)).b,
+        1.0
+        );
+
+    return output_color;
+}

examples/README.md

@@ -253,6 +253,7 @@ Example | File | Description
 `animate_shader` | [`shader/animate_shader.rs`](./shader/animate_shader.rs) | A shader that uses dynamic data like the time since startup.
 `compute_shader_game_of_life` | [`shader/compute_shader_game_of_life.rs`](./shader/compute_shader_game_of_life.rs) | A compute shader that simulates Conway's Game of Life.
 `custom_vertex_attribute` | [`shader/custom_vertex_attribute.rs`](./shader/custom_vertex_attribute.rs) | A shader that reads a mesh's custom vertex attribute.
+`post_processing` | [`shader/post_processing.rs`](./shader/post_processing.rs) | A custom post processing effect, using two cameras, with one reusing the render texture of the first one.
 `shader_defs` | [`shader/shader_defs.rs`](./shader/shader_defs.rs) | A shader that uses "shaders defs" (a bevy tool to selectively toggle parts of a shader).
 `shader_instancing` | [`shader/shader_instancing.rs`](./shader/shader_instancing.rs) | A shader that renders a mesh multiple times in one draw call.
 `shader_material` | [`shader/shader_material.rs`](./shader/shader_material.rs) | A shader and a material that uses it.

examples/shader/post_processing.rs

@@ -0,0 +1,256 @@
+//! A custom post processing effect, using two cameras, with one reusing the render texture of the first one.
+//! Here a chromatic aberration is applied to a 3d scene containting a rotating cube.
+//! This example is useful to implement your own post-processing effect such as
+//! edge detection, blur, pixelization, vignette... and countless others.
+
+use bevy::{
+    core_pipeline::clear_color::ClearColorConfig,
+    ecs::system::{lifetimeless::SRes, SystemParamItem},
+    prelude::*,
+    reflect::TypeUuid,
+    render::{
+        camera::{Camera, RenderTarget},
+        render_asset::{PrepareAssetError, RenderAsset, RenderAssets},
+        render_resource::{
+            BindGroup, BindGroupDescriptor, BindGroupEntry, BindGroupLayout,
+            BindGroupLayoutDescriptor, BindGroupLayoutEntry, BindingResource, BindingType,
+            Extent3d, SamplerBindingType, ShaderStages, TextureDescriptor, TextureDimension,
+            TextureFormat, TextureSampleType, TextureUsages, TextureViewDimension,
+        },
+        renderer::RenderDevice,
+        view::RenderLayers,
+    },
+    sprite::{Material2d, Material2dPipeline, Material2dPlugin, MaterialMesh2dBundle},
+};
+
+fn main() {
+    let mut app = App::new();
+    app.add_plugins(DefaultPlugins)
+        .add_plugin(Material2dPlugin::<PostProcessingMaterial>::default())
+        .add_startup_system(setup)
+        .add_system(main_camera_cube_rotator_system);
+
+    app.run();
+}
+
+/// Marks the first camera cube (rendered to a texture.)
+#[derive(Component)]
+struct MainCube;
+
+fn setup(
+    mut commands: Commands,
+    mut windows: ResMut<Windows>,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut post_processing_materials: ResMut<Assets<PostProcessingMaterial>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+    mut images: ResMut<Assets<Image>>,
+) {
+    let window = windows.get_primary_mut().unwrap();
+    let size = Extent3d {
+        width: window.physical_width(),
+        height: window.physical_height(),
+        ..default()
+    };
+
+    // This is the texture that will be rendered to.
+    let mut image = Image {
+        texture_descriptor: TextureDescriptor {
+            label: None,
+            size,
+            dimension: TextureDimension::D2,
+            format: TextureFormat::Bgra8UnormSrgb,
+            mip_level_count: 1,
+            sample_count: 1,
+            usage: TextureUsages::TEXTURE_BINDING
+                | TextureUsages::COPY_DST
+                | TextureUsages::RENDER_ATTACHMENT,
+        },
+        ..default()
+    };
+
+    // fill image.data with zeroes
+    image.resize(size);
+
+    let image_handle = images.add(image);
+
+    let cube_handle = meshes.add(Mesh::from(shape::Cube { size: 4.0 }));
+    let cube_material_handle = materials.add(StandardMaterial {
+        base_color: Color::rgb(0.8, 0.7, 0.6),
+        reflectance: 0.02,
+        unlit: false,
+        ..default()
+    });
+
+    // The cube that will be rendered to the texture.
+    commands
+        .spawn_bundle(PbrBundle {
+            mesh: cube_handle,
+            material: cube_material_handle,
+            transform: Transform::from_translation(Vec3::new(0.0, 0.0, 1.0)),
+            ..default()
+        })
+        .insert(MainCube);
+
+    // Light
+    // NOTE: Currently lights are ignoring render layers - see https://github.com/bevyengine/bevy/issues/3462
+    commands.spawn_bundle(PointLightBundle {
+        transform: Transform::from_translation(Vec3::new(0.0, 0.0, 10.0)),
+        ..default()
+    });
+
+    // Main camera, first to render
+    commands.spawn_bundle(Camera3dBundle {
+        camera_3d: Camera3d {
+            clear_color: ClearColorConfig::Custom(Color::WHITE),
+        },
+        camera: Camera {
+            target: RenderTarget::Image(image_handle.clone()),
+            ..default()
+        },
+        transform: Transform::from_translation(Vec3::new(0.0, 0.0, 15.0))
+            .looking_at(Vec3::default(), Vec3::Y),
+        ..default()
+    });
+
+    // This specifies the layer used for the post processing camera, which will be attached to the post processing camera and 2d quad.
+    let post_processing_pass_layer = RenderLayers::layer((RenderLayers::TOTAL_LAYERS - 1) as u8);
+
+    let quad_handle = meshes.add(Mesh::from(shape::Quad::new(Vec2::new(
+        size.width as f32,
+        size.height as f32,
+    ))));
+
+    // This material has the texture that has been rendered.
+    let material_handle = post_processing_materials.add(PostProcessingMaterial {
+        source_image: image_handle,
+    });
+
+    // Post processing 2d quad, with material using the render texture done by the main camera, with a custom shader.
+    commands
+        .spawn_bundle(MaterialMesh2dBundle {
+            mesh: quad_handle.into(),
+            material: material_handle,
+            transform: Transform {
+                translation: Vec3::new(0.0, 0.0, 1.5),
+                ..default()
+            },
+            ..default()
+        })
+        .insert(post_processing_pass_layer);
+
+    // The post-processing pass camera.
+    commands
+        .spawn_bundle(Camera2dBundle {
+            camera: Camera {
+                // renders after the first main camera which has default value: 0.
+                priority: 1,
+                ..default()
+            },
+            ..Camera2dBundle::default()
+        })
+        .insert(post_processing_pass_layer);
+}
+
+/// Rotates the cube rendered by the main camera
+fn main_camera_cube_rotator_system(
+    time: Res<Time>,
+    mut query: Query<&mut Transform, With<MainCube>>,
+) {
+    for mut transform in query.iter_mut() {
+        transform.rotation *= Quat::from_rotation_x(0.55 * time.delta_seconds());
+        transform.rotation *= Quat::from_rotation_z(0.15 * time.delta_seconds());
+    }
+}
+
+// Region below declares of the custom material handling post processing effect
+
+/// Our custom post processing material
+#[derive(TypeUuid, Clone)]
+#[uuid = "bc2f08eb-a0fb-43f1-a908-54871ea597d5"]
+struct PostProcessingMaterial {
+    /// In this example, this image will be the result of the main camera.
+    source_image: Handle<Image>,
+}
+
+struct PostProcessingMaterialGPU {
+    bind_group: BindGroup,
+}
+
+impl Material2d for PostProcessingMaterial {
+    fn bind_group(material: &PostProcessingMaterialGPU) -> &BindGroup {
+        &material.bind_group
+    }
+
+    fn bind_group_layout(render_device: &RenderDevice) -> BindGroupLayout {
+        render_device.create_bind_group_layout(&BindGroupLayoutDescriptor {
+            label: None,
+            entries: &[
+                BindGroupLayoutEntry {
+                    binding: 0,
+                    visibility: ShaderStages::FRAGMENT,
+                    ty: BindingType::Texture {
+                        multisampled: false,
+                        view_dimension: TextureViewDimension::D2,
+                        sample_type: TextureSampleType::Float { filterable: true },
+                    },
+                    count: None,
+                },
+                BindGroupLayoutEntry {
+                    binding: 1,
+                    visibility: ShaderStages::FRAGMENT,
+                    ty: BindingType::Sampler(SamplerBindingType::Filtering),
+                    count: None,
+                },
+            ],
+        })
+    }
+
+    fn fragment_shader(asset_server: &AssetServer) -> Option<Handle<Shader>> {
+        asset_server.watch_for_changes().unwrap();
+        Some(asset_server.load("shaders/custom_material_chromatic_aberration.wgsl"))
+    }
+}
+
+impl RenderAsset for PostProcessingMaterial {
+    type ExtractedAsset = PostProcessingMaterial;
+    type PreparedAsset = PostProcessingMaterialGPU;
+    type Param = (
+        SRes<RenderDevice>,
+        SRes<Material2dPipeline<PostProcessingMaterial>>,
+        SRes<RenderAssets<Image>>,
+    );
+
+    fn prepare_asset(
+        extracted_asset: PostProcessingMaterial,
+        (render_device, pipeline, images): &mut SystemParamItem<Self::Param>,
+    ) -> Result<PostProcessingMaterialGPU, PrepareAssetError<PostProcessingMaterial>> {
+        let (view, sampler) = if let Some(result) = pipeline
+            .mesh2d_pipeline
+            .get_image_texture(images, &Some(extracted_asset.source_image.clone()))
+        {
+            result
+        } else {
+            return Err(PrepareAssetError::RetryNextUpdate(extracted_asset));
+        };
+
+        let bind_group = render_device.create_bind_group(&BindGroupDescriptor {
+            label: None,
+            layout: &pipeline.material2d_layout,
+            entries: &[
+                BindGroupEntry {
+                    binding: 0,
+                    resource: BindingResource::TextureView(view),
+                },
+                BindGroupEntry {
+                    binding: 1,
+                    resource: BindingResource::Sampler(sampler),
+                },
+            ],
+        });
+        Ok(PostProcessingMaterialGPU { bind_group })
+    }
+
+    fn extract_asset(&self) -> PostProcessingMaterial {
+        self.clone()
+    }
+}