Reuse texture when resolving multiple passes (#3552)

# Objective

Fixes https://github.com/bevyengine/bevy/issues/3499

## Solution

Uses a `HashMap` from `RenderTarget` to sampled textures when preparing `ViewTarget`s to ensure that two passes with the same render target get sampled to the same texture.

This builds on and depends on https://github.com/bevyengine/bevy/pull/3412, so this will be a draft PR until #3412 is merged. All changes for this PR are in the last commit.

Cargo.toml

@@ -216,6 +216,10 @@ path = "examples/3d/texture.rs"
 name = "render_to_texture"
 path = "examples/3d/render_to_texture.rs"
 
+[[example]]
+name = "two_passes"
+path = "examples/3d/two_passes.rs"
+
 [[example]]
 name = "update_gltf_scene"
 path = "examples/3d/update_gltf_scene.rs"

crates/bevy_core_pipeline/src/lib.rs

@@ -23,7 +23,7 @@ use bevy_app::{App, Plugin};
 use bevy_core::FloatOrd;
 use bevy_ecs::prelude::*;
 use bevy_render::{
-    camera::{ActiveCamera, Camera2d, Camera3d, RenderTarget},
+    camera::{ActiveCamera, Camera2d, Camera3d, ExtractedCamera, RenderTarget},
     color::Color,
     render_graph::{EmptyNode, RenderGraph, SlotInfo, SlotType},
     render_phase::{
@@ -390,7 +390,7 @@ pub fn prepare_core_views_system(
     msaa: Res<Msaa>,
     render_device: Res<RenderDevice>,
     views_3d: Query<
-        (Entity, &ExtractedView),
+        (Entity, &ExtractedView, Option<&ExtractedCamera>),
         (
             With<RenderPhase<Opaque3d>>,
             With<RenderPhase<AlphaMask3d>>,
@@ -398,8 +398,10 @@ pub fn prepare_core_views_system(
         ),
     >,
 ) {
-    for (entity, view) in views_3d.iter() {
+    let mut textures = HashMap::default();
-        let cached_texture = texture_cache.get(
+    for (entity, view, camera) in views_3d.iter() {
+        let mut get_cached_texture = || {
+            texture_cache.get(
                 &render_device,
                 TextureDescriptor {
                     label: Some("view_depth_texture"),
@@ -415,7 +417,16 @@ pub fn prepare_core_views_system(
                                                           * bit depth for better performance */
                     usage: TextureUsages::RENDER_ATTACHMENT,
                 },
-        );
+            )
+        };
+        let cached_texture = if let Some(camera) = camera {
+            textures
+                .entry(camera.target.clone())
+                .or_insert_with(get_cached_texture)
+                .clone()
+        } else {
+            get_cached_texture()
+        };
         commands.entity(entity).insert(ViewDepthTexture {
             texture: cached_texture.texture,
             view: cached_texture.default_view,

crates/bevy_core_pipeline/src/main_pass_3d.rs

@@ -142,12 +142,15 @@ impl Node for MainPass3dNode {
                 })],
                 depth_stencil_attachment: Some(RenderPassDepthStencilAttachment {
                     view: &depth.view,
-                    // NOTE: For the transparent pass we load the depth buffer but do not write to it.
+                    // NOTE: For the transparent pass we load the depth buffer. There should be no
+                    // need to write to it, but store is set to `true` as a workaround for issue #3776,
+                    // https://github.com/bevyengine/bevy/issues/3776
+                    // so that wgpu does not clear the depth buffer.
                     // As the opaque and alpha mask passes run first, opaque meshes can occlude
                     // transparent ones.
                     depth_ops: Some(Operations {
                         load: LoadOp::Load,
-                        store: false,
+                        store: true,
                     }),
                     stencil_ops: None,
                 }),

crates/bevy_render/src/texture/texture_cache.rs

@@ -18,6 +18,7 @@ struct CachedTextureMeta {
 /// A cached GPU [`Texture`] with corresponding [`TextureView`].
 /// This is useful for textures that are created repeatedly (each frame) in the rendering process
 /// to reduce the amount of GPU memory allocations.
+#[derive(Clone)]
 pub struct CachedTexture {
     pub texture: Texture,
     pub default_view: TextureView,

crates/bevy_render/src/view/mod.rs

@@ -21,6 +21,7 @@ use bevy_app::{App, Plugin};
 use bevy_ecs::prelude::*;
 use bevy_math::{Mat4, Vec3};
 use bevy_transform::components::GlobalTransform;
+use bevy_utils::HashMap;
 
 pub struct ViewPlugin;
 
@@ -181,11 +182,15 @@ fn prepare_view_targets(
     mut texture_cache: ResMut<TextureCache>,
     cameras: Query<(Entity, &ExtractedCamera)>,
 ) {
+    let mut sampled_textures = HashMap::default();
     for (entity, camera) in cameras.iter() {
         if let Some(size) = camera.physical_size {
             if let Some(texture_view) = camera.target.get_texture_view(&windows, &images) {
                 let sampled_target = if msaa.samples > 1 {
-                    let sampled_texture = texture_cache.get(
+                    let sampled_texture = sampled_textures
+                        .entry(camera.target.clone())
+                        .or_insert_with(|| {
+                            texture_cache.get(
                                 &render_device,
                                 TextureDescriptor {
                                     label: Some("sampled_color_attachment_texture"),
@@ -200,7 +205,8 @@ fn prepare_view_targets(
                                     format: TextureFormat::bevy_default(),
                                     usage: TextureUsages::RENDER_ATTACHMENT,
                                 },
-                    );
+                            )
+                        });
                     Some(sampled_texture.default_view.clone())
                 } else {
                     None

examples/3d/two_passes.rs

@@ -0,0 +1,217 @@
+use bevy::{
+    core_pipeline::{draw_3d_graph, node, AlphaMask3d, Opaque3d, Transparent3d},
+    prelude::*,
+    render::{
+        camera::{ActiveCamera, Camera, CameraTypePlugin, RenderTarget},
+        render_graph::{Node, NodeRunError, RenderGraph, RenderGraphContext, SlotValue},
+        render_phase::RenderPhase,
+        renderer::RenderContext,
+        view::RenderLayers,
+        RenderApp, RenderStage,
+    },
+    window::WindowId,
+};
+
+// The name of the final node of the first pass.
+pub const FIRST_PASS_DRIVER: &str = "first_pass_driver";
+
+// Marks the camera that determines the view rendered in the first pass.
+#[derive(Component, Default)]
+struct FirstPassCamera;
+
+fn main() {
+    let mut app = App::new();
+    app.insert_resource(Msaa { samples: 4 })
+        .add_plugins(DefaultPlugins)
+        .add_plugin(CameraTypePlugin::<FirstPassCamera>::default())
+        .add_startup_system(setup)
+        .add_system(cube_rotator_system)
+        .add_system(rotator_system)
+        .add_system(toggle_msaa);
+
+    let render_app = app.sub_app_mut(RenderApp);
+    let driver = FirstPassCameraDriver::new(&mut render_app.world);
+
+    // This will add 3D render phases for the new camera.
+    render_app.add_system_to_stage(RenderStage::Extract, extract_first_pass_camera_phases);
+
+    let mut graph = render_app.world.get_resource_mut::<RenderGraph>().unwrap();
+
+    // Add a node for the first pass.
+    graph.add_node(FIRST_PASS_DRIVER, driver);
+
+    // The first pass's dependencies include those of the main pass.
+    graph
+        .add_node_edge(node::MAIN_PASS_DEPENDENCIES, FIRST_PASS_DRIVER)
+        .unwrap();
+
+    // Insert the first pass node: CLEAR_PASS_DRIVER -> FIRST_PASS_DRIVER -> MAIN_PASS_DRIVER
+    graph
+        .add_node_edge(node::CLEAR_PASS_DRIVER, FIRST_PASS_DRIVER)
+        .unwrap();
+    graph
+        .add_node_edge(FIRST_PASS_DRIVER, node::MAIN_PASS_DRIVER)
+        .unwrap();
+    app.run();
+}
+
+// Add 3D render phases for FirstPassCamera.
+fn extract_first_pass_camera_phases(
+    mut commands: Commands,
+    active: Res<ActiveCamera<FirstPassCamera>>,
+) {
+    if let Some(entity) = active.get() {
+        commands.get_or_spawn(entity).insert_bundle((
+            RenderPhase::<Opaque3d>::default(),
+            RenderPhase::<AlphaMask3d>::default(),
+            RenderPhase::<Transparent3d>::default(),
+        ));
+    }
+}
+// A node for the first pass camera that runs draw_3d_graph with this camera.
+struct FirstPassCameraDriver {
+    query: QueryState<Entity, With<FirstPassCamera>>,
+}
+
+impl FirstPassCameraDriver {
+    pub fn new(render_world: &mut World) -> Self {
+        Self {
+            query: QueryState::new(render_world),
+        }
+    }
+}
+
+impl Node for FirstPassCameraDriver {
+    fn update(&mut self, world: &mut World) {
+        self.query.update_archetypes(world);
+    }
+
+    fn run(
+        &self,
+        graph: &mut RenderGraphContext,
+        _render_context: &mut RenderContext,
+        world: &World,
+    ) -> Result<(), NodeRunError> {
+        for camera in self.query.iter_manual(world) {
+            graph.run_sub_graph(draw_3d_graph::NAME, vec![SlotValue::Entity(camera)])?;
+        }
+        Ok(())
+    }
+}
+
+// Marks the first pass cube.
+#[derive(Component)]
+struct FirstPassCube;
+
+// Marks the main pass cube.
+#[derive(Component)]
+struct MainPassCube;
+
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    let cube_handle = meshes.add(Mesh::from(shape::Cube { size: 4.0 }));
+    let cube_material_handle = materials.add(StandardMaterial {
+        base_color: Color::GREEN,
+        reflectance: 0.02,
+        unlit: false,
+        ..Default::default()
+    });
+
+    let split = 2.0;
+
+    // This specifies the layer used for the first pass, which will be attached to the first pass camera and cube.
+    let first_pass_layer = RenderLayers::layer(1);
+
+    // The first pass cube.
+    commands
+        .spawn_bundle(PbrBundle {
+            mesh: cube_handle,
+            material: cube_material_handle,
+            transform: Transform::from_translation(Vec3::new(-split, 0.0, 1.0)),
+            ..Default::default()
+        })
+        .insert(FirstPassCube)
+        .insert(first_pass_layer);
+
+    // Light
+    // NOTE: Currently lights are shared between passes - 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::default()
+    });
+
+    // First pass camera
+    commands
+        .spawn_bundle(PerspectiveCameraBundle::<FirstPassCamera> {
+            camera: Camera {
+                target: RenderTarget::Window(WindowId::primary()),
+                ..Default::default()
+            },
+            transform: Transform::from_translation(Vec3::new(0.0, 0.0, 15.0))
+                .looking_at(Vec3::default(), Vec3::Y),
+            ..PerspectiveCameraBundle::new()
+        })
+        .insert(first_pass_layer);
+
+    let cube_size = 4.0;
+    let cube_handle = meshes.add(Mesh::from(shape::Box::new(cube_size, cube_size, cube_size)));
+
+    let material_handle = materials.add(StandardMaterial {
+        base_color: Color::RED,
+        reflectance: 0.02,
+        unlit: false,
+        ..Default::default()
+    });
+
+    // Main pass cube.
+    commands
+        .spawn_bundle(PbrBundle {
+            mesh: cube_handle,
+            material: material_handle,
+            transform: Transform {
+                translation: Vec3::new(split, 0.0, -4.5),
+                rotation: Quat::from_rotation_x(-std::f32::consts::PI / 5.0),
+                ..Default::default()
+            },
+            ..Default::default()
+        })
+        .insert(MainPassCube);
+
+    // The main pass camera.
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        transform: Transform::from_translation(Vec3::new(0.0, 0.0, 15.0))
+            .looking_at(Vec3::default(), Vec3::Y),
+        ..Default::default()
+    });
+}
+
+/// Rotates the inner cube (first pass)
+fn rotator_system(time: Res<Time>, mut query: Query<&mut Transform, With<FirstPassCube>>) {
+    for mut transform in query.iter_mut() {
+        transform.rotation *= Quat::from_rotation_x(1.5 * time.delta_seconds());
+        transform.rotation *= Quat::from_rotation_z(1.3 * time.delta_seconds());
+    }
+}
+
+/// Rotates the outer cube (main pass)
+fn cube_rotator_system(time: Res<Time>, mut query: Query<&mut Transform, With<MainPassCube>>) {
+    for mut transform in query.iter_mut() {
+        transform.rotation *= Quat::from_rotation_x(1.0 * time.delta_seconds());
+        transform.rotation *= Quat::from_rotation_y(0.7 * time.delta_seconds());
+    }
+}
+
+fn toggle_msaa(input: Res<Input<KeyCode>>, mut msaa: ResMut<Msaa>) {
+    if input.just_pressed(KeyCode::M) {
+        if msaa.samples == 4 {
+            info!("Not using MSAA");
+            msaa.samples = 1;
+        } else {
+            info!("Using 4x MSAA");
+            msaa.samples = 4;
+        }
+    }
+}

examples/README.md

@@ -109,6 +109,7 @@ Example | File | Description
 `parenting` | [`3d/parenting.rs`](./3d/parenting.rs) | Demonstrates parent->child relationships and relative transformations
 `pbr` | [`3d/pbr.rs`](./3d/pbr.rs) | Demonstrates use of Physically Based Rendering (PBR) properties
 `render_to_texture` | [`3d/render_to_texture.rs`](./3d/render_to_texture.rs) | Shows how to render to a texture, useful for mirrors, UI, or exporting images
+`two_passes` | [`3d/two_passes.rs`](./3d/two_passes.rs) | Shows how to render multiple passes to the same window, useful for rendering different views or drawing an object on top regardless of depth
 `shadow_caster_receiver` | [`3d/shadow_caster_receiver.rs`](./3d/shadow_caster_receiver.rs) | Demonstrates how to prevent meshes from casting/receiving shadows in a 3d scene
 `shadow_biases` | [`3d/shadow_biases.rs`](./3d/shadow_biases.rs) | Demonstrates how shadow biases affect shadows in a 3d scene
 `spherical_area_lights` | [`3d/spherical_area_lights.rs`](./3d/spherical_area_lights.rs) | Demonstrates how point light radius values affect light behavior.