Option to enable deterministic rendering (#11248)

# Objective

Issue #10243: rendering multiple triangles in the same place results in
flickering.

## Solution

Considered these alternatives:
- `depth_bias` may not work, because of high number of entities, so
creating a material per entity is practically not possible
- rendering at slightly different positions does not work, because when
camera is far, float rounding causes the same issues (edit: assuming we
have to use the same `depth_bias`)
- considered implementing deterministic operation like
`query.par_iter().flat_map(...).collect()` to be used in
`check_visibility` system (which would solve the issue since query is
deterministic), and could not figure out how to make it as cheap as
current approach with thread-local collectors (#11249)

So adding an option to sort entities after `check_visibility` system
run.

Should not be too bad, because after visibility check, only a handful
entities remain.

This is probably not the only source of non-determinism in Bevy, but
this is one I could find so far. At least it fixes the repro example.

## Changelog

- `DeterministicRenderingConfig` option to enable deterministic
rendering

## Test

<img width="1392" alt="image"
src="https://github.com/bevyengine/bevy/assets/28969/c735bce1-3a71-44cd-8677-c19f6c0ee6bd">

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

Cargo.toml

@@ -598,6 +598,17 @@ description = "Showcases different blend modes"
 category = "3D Rendering"
 wasm = true
 
+[[example]]
+name = "deterministic"
+path = "examples/3d/deterministic.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.deterministic]
+name = "Deterministic rendering"
+description = "Stop flickering from z-fighting at a performance cost"
+category = "3D Rendering"
+wasm = true
+
 [[example]]
 name = "lighting"
 path = "examples/3d/lighting.rs"

crates/bevy_render/src/deterministic.rs

@@ -0,0 +1,14 @@
+use bevy_ecs::system::Resource;
+
+/// Configure deterministic rendering to fix flickering due to z-fighting.
+#[derive(Resource, Default)]
+pub struct DeterministicRenderingConfig {
+    /// Sort visible entities by id before rendering to avoid flickering.
+    ///
+    /// Render is parallel by default, and if there's z-fighting, it may cause flickering.
+    /// Default fix for the issue is to set `depth_bias` per material.
+    /// When it is not possible, entities sorting can be used.
+    ///
+    /// This option costs performance and disabled by default.
+    pub stable_sort_z_fighting: bool,
+}

crates/bevy_render/src/lib.rs

@@ -6,6 +6,7 @@ extern crate core;
 pub mod batching;
 pub mod camera;
 pub mod color;
+pub mod deterministic;
 pub mod extract_component;
 pub mod extract_instances;
 mod extract_param;
@@ -48,6 +49,7 @@ use bevy_window::{PrimaryWindow, RawHandleWrapper};
 use globals::GlobalsPlugin;
 use renderer::{RenderAdapter, RenderAdapterInfo, RenderDevice, RenderQueue};
 
+use crate::deterministic::DeterministicRenderingConfig;
 use crate::{
     camera::CameraPlugin,
     mesh::{morph::MorphPlugin, Mesh, MeshPlugin},
@@ -216,6 +218,8 @@ pub const MATHS_SHADER_HANDLE: Handle<Shader> = Handle::weak_from_u128(106653563
 impl Plugin for RenderPlugin {
     /// Initializes the renderer, sets up the [`RenderSet`] and creates the rendering sub-app.
     fn build(&self, app: &mut App) {
+        app.init_resource::<DeterministicRenderingConfig>();
+
         app.init_asset::<Shader>()
             .init_asset_loader::<ShaderLoader>();
 

crates/bevy_render/src/view/visibility/mod.rs

@@ -12,6 +12,7 @@ use bevy_transform::{components::GlobalTransform, TransformSystem};
 use std::cell::Cell;
 use thread_local::ThreadLocal;
 
+use crate::deterministic::DeterministicRenderingConfig;
 use crate::{
     camera::{
         camera_system, Camera, CameraProjection, OrthographicProjection, PerspectiveProjection,
@@ -392,6 +393,7 @@ pub fn check_visibility(
         &GlobalTransform,
         Has<NoFrustumCulling>,
     )>,
+    deterministic_rendering_config: Res<DeterministicRenderingConfig>,
 ) {
     for (mut visible_entities, frustum, maybe_view_mask, camera) in &mut view_query {
         if !camera.is_active {
@@ -452,6 +454,11 @@ pub fn check_visibility(
         for cell in &mut thread_queues {
             visible_entities.entities.append(cell.get_mut());
         }
+        if deterministic_rendering_config.stable_sort_z_fighting {
+            // We can use the faster unstable sort here because
+            // the values (`Entity`) are guaranteed to be unique.
+            visible_entities.entities.sort_unstable();
+        }
     }
 }
 

examples/3d/deterministic.rs

@@ -0,0 +1,87 @@
+//! Shows how to enable deterministic rendering which helps with flickering due to z-fighting.
+//! Rendering is not deterministic by default.
+//! Note most users don't need rendering to be deterministic, and should rely on depth bias instead.
+
+use bevy::app::App;
+use bevy::app::Startup;
+use bevy::prelude::shape::Plane;
+use bevy::prelude::*;
+use bevy::render::deterministic::DeterministicRenderingConfig;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_systems(Startup, setup)
+        .add_systems(Update, (keys, update_help).chain())
+        .run();
+}
+
+fn setup(
+    mut commands: Commands,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut deterministic_rendering_config: ResMut<DeterministicRenderingConfig>,
+) {
+    // Safe default.
+    deterministic_rendering_config.stable_sort_z_fighting = true;
+
+    // Help message will be rendered there.
+    commands.spawn(TextBundle::default());
+
+    commands.spawn(Camera3dBundle {
+        transform: Transform::from_xyz(3.0, 3.0, 3.0).looking_at(Vec3::new(0., 0., 0.), Vec3::Y),
+        ..default()
+    });
+
+    let mesh = meshes.add(Plane::from_size(2.0));
+    for i in 0..360 {
+        let color = Color::hsl(i as f32, 1.0, 0.5);
+        commands.spawn(PbrBundle {
+            mesh: mesh.clone(),
+            material: materials.add(StandardMaterial {
+                base_color: color,
+                // Setting depth bias would be a default choice to fix z-fighting.
+                // When it is not possible, deterministic rendering can be used.
+                // Here we intentionally don't use depth bias to demonstrate the issue.
+                depth_bias: 0.0,
+                unlit: true,
+                ..Default::default()
+            }),
+            ..default()
+        });
+    }
+}
+
+fn keys(
+    mut deterministic_rendering_config: ResMut<DeterministicRenderingConfig>,
+    keyboard_input: Res<ButtonInput<KeyCode>>,
+) {
+    if keyboard_input.just_pressed(KeyCode::KeyD) {
+        deterministic_rendering_config.stable_sort_z_fighting ^= true;
+    }
+}
+
+fn update_help(
+    mut text: Query<&mut Text>,
+    deterministic_rendering_config: Res<DeterministicRenderingConfig>,
+) {
+    if deterministic_rendering_config.is_changed() {
+        *text.single_mut() = Text::from_section(
+            format!(
+                "\
+            Press D to enable/disable deterministic rendering\n\
+            \n\
+            Deterministic rendering: {}\n\
+            \n\
+            When rendering is not deterministic, you may notice flickering due to z-fighting\n\
+            \n\
+            Warning: may cause seizures for people with photosensitive epilepsy",
+                deterministic_rendering_config.stable_sort_z_fighting
+            ),
+            TextStyle {
+                font_size: 20.,
+                ..default()
+            },
+        );
+    }
+}

examples/README.md

@@ -123,6 +123,7 @@ Example | Description
 [Atmospheric Fog](../examples/3d/atmospheric_fog.rs) | A scene showcasing the atmospheric fog effect
 [Blend Modes](../examples/3d/blend_modes.rs) | Showcases different blend modes
 [Deferred Rendering](../examples/3d/deferred_rendering.rs) | Renders meshes with both forward and deferred pipelines
+[Deterministic rendering](../examples/3d/deterministic.rs) | Stop flickering from z-fighting at a performance cost
 [Fog](../examples/3d/fog.rs) | A scene showcasing the distance fog effect
 [Generate Custom Mesh](../examples/3d/generate_custom_mesh.rs) | Simple showcase of how to generate a custom mesh with a custom texture
 [Lighting](../examples/3d/lighting.rs) | Illustrates various lighting options in a simple scene