Make some examples deterministic (#16488)

# Objective

- Improve reproducibility of examples

## Solution

- Use seeded rng when needed
- Use fixed z-ordering when needed

## Testing

```sh
steps=5;
echo "cpu_draw\nparallel_query\nanimated_fox\ntransparency_2d" > test
cargo run -p example-showcase -- run --stop-frame 250 --screenshot-frame 100 --fixed-frame-time 0.05 --example-list test --in-ci;
mv screenshots base;
for prefix in `seq 0 $steps`;
do
  echo step $prefix;
  cargo run -p example-showcase -- run --stop-frame 250 --screenshot-frame 100 --fixed-frame-time 0.05 --example-list test;
  mv screenshots $prefix-screenshots;
done;
mv base screenshots
for prefix in `seq 0 $steps`;
do
  echo check $prefix
  for file in screenshots/*/*;
  do
    echo $file;
    diff $file $prefix-$file;
  done;
done;
```

examples/2d/cpu_draw.rs

@@ -11,7 +11,8 @@ use bevy::render::{
     render_asset::RenderAssetUsages,
     render_resource::{Extent3d, TextureDimension, TextureFormat},
 };
-use rand::Rng;
+use rand::{Rng, SeedableRng};
+use rand_chacha::ChaCha8Rng;
 
 const IMAGE_WIDTH: u32 = 256;
 const IMAGE_HEIGHT: u32 = 256;
@@ -33,6 +34,9 @@ fn main() {
 #[derive(Resource)]
 struct MyProcGenImage(Handle<Image>);
 
+#[derive(Resource)]
+struct SeededRng(ChaCha8Rng);
+
 fn setup(mut commands: Commands, mut images: ResMut<Assets<Image>>) {
     // spawn a camera
     commands.spawn(Camera2d);
@@ -80,6 +84,11 @@ fn setup(mut commands: Commands, mut images: ResMut<Assets<Image>>) {
     // create a sprite entity using our image
     commands.spawn(Sprite::from_image(handle.clone()));
     commands.insert_resource(MyProcGenImage(handle));
+
+    // We're seeding the PRNG here to make this example deterministic for testing purposes.
+    // This isn't strictly required in practical use unless you need your app to be deterministic.
+    let seeded_rng = ChaCha8Rng::seed_from_u64(19878367467712);
+    commands.insert_resource(SeededRng(seeded_rng));
 }
 
 /// Every fixed update tick, draw one more pixel to make a spiral pattern
@@ -89,12 +98,11 @@ fn draw(
     // used to keep track of where we are
     mut i: Local<u32>,
     mut draw_color: Local<Color>,
+    mut seeded_rng: ResMut<SeededRng>,
 ) {
-    let mut rng = rand::thread_rng();
-
     if *i == 0 {
         // Generate a random color on first run.
-        *draw_color = Color::linear_rgb(rng.gen(), rng.gen(), rng.gen());
+        *draw_color = Color::linear_rgb(seeded_rng.0.gen(), seeded_rng.0.gen(), seeded_rng.0.gen());
     }
 
     // Get the image from Bevy's asset storage.
@@ -117,7 +125,7 @@ fn draw(
     // If the old color is our current color, change our drawing color.
     let tolerance = 1.0 / 255.0;
     if old_color.distance(&draw_color) <= tolerance {
-        *draw_color = Color::linear_rgb(rng.gen(), rng.gen(), rng.gen());
+        *draw_color = Color::linear_rgb(seeded_rng.0.gen(), seeded_rng.0.gen(), seeded_rng.0.gen());
     }
 
     // Set the new color, but keep old alpha value from image.

examples/2d/transparency_2d.rs

@@ -23,7 +23,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
             color: Color::srgba(0.0, 0.0, 1.0, 0.7),
             ..default()
         },
-        Transform::from_xyz(100.0, 0.0, 0.0),
+        Transform::from_xyz(100.0, 0.0, 0.1),
     ));
     commands.spawn((
         Sprite {
@@ -31,6 +31,6 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
             color: Color::srgba(0.0, 1.0, 0.0, 0.3),
             ..default()
         },
-        Transform::from_xyz(200.0, 0.0, 0.0),
+        Transform::from_xyz(200.0, 0.0, 0.2),
     ));
 }

examples/animation/animated_fox.rs

@@ -8,7 +8,8 @@ use bevy::{
     pbr::CascadeShadowConfigBuilder,
     prelude::*,
 };
-use rand::{thread_rng, Rng};
+use rand::{Rng, SeedableRng};
+use rand_chacha::ChaCha8Rng;
 
 const FOX_PATH: &str = "models/animated/Fox.glb";
 
@@ -28,6 +29,9 @@ fn main() {
         .run();
 }
 
+#[derive(Resource)]
+struct SeededRng(ChaCha8Rng);
+
 #[derive(Resource)]
 struct Animations {
     animations: Vec<AnimationNodeIndex>,
@@ -42,19 +46,19 @@ fn observe_on_step(
     particle: Res<ParticleAssets>,
     mut commands: Commands,
     transforms: Query<&GlobalTransform>,
+    mut seeded_rng: ResMut<SeededRng>,
 ) {
     let translation = transforms.get(trigger.entity()).unwrap().translation();
-    let mut rng = thread_rng();
     // Spawn a bunch of particles.
     for _ in 0..14 {
-        let horizontal = rng.gen::<Dir2>() * rng.gen_range(8.0..12.0);
-        let vertical = rng.gen_range(0.0..4.0);
-        let size = rng.gen_range(0.2..1.0);
+        let horizontal = seeded_rng.0.gen::<Dir2>() * seeded_rng.0.gen_range(8.0..12.0);
+        let vertical = seeded_rng.0.gen_range(0.0..4.0);
+        let size = seeded_rng.0.gen_range(0.2..1.0);
         commands.queue(spawn_particle(
             particle.mesh.clone(),
             particle.material.clone(),
             translation.reject_from_normalized(Vec3::Y),
-            rng.gen_range(0.2..0.6),
+            seeded_rng.0.gen_range(0.2..0.6),
             size,
             Vec3::new(horizontal.x, vertical, horizontal.y) * 10.0,
         ));
@@ -121,6 +125,11 @@ fn setup(
     println!("  - digit 1 / 3 / 5: play the animation <digit> times");
     println!("  - L: loop the animation forever");
     println!("  - return: change animation");
+
+    // We're seeding the PRNG here to make this example deterministic for testing purposes.
+    // This isn't strictly required in practical use unless you need your app to be deterministic.
+    let seeded_rng = ChaCha8Rng::seed_from_u64(19878367467712);
+    commands.insert_resource(SeededRng(seeded_rng));
 }
 
 // An `AnimationPlayer` is automatically added to the scene when it's ready.

examples/ecs/parallel_query.rs

@@ -14,10 +14,11 @@ fn spawn_system(mut commands: Commands, asset_server: Res<AssetServer>) {
     // We're seeding the PRNG here to make this example deterministic for testing purposes.
     // This isn't strictly required in practical use unless you need your app to be deterministic.
     let mut rng = ChaCha8Rng::seed_from_u64(19878367467713);
-    for _ in 0..128 {
+    for z in 0..128 {
         commands.spawn((
             Sprite::from_image(texture.clone()),
-            Transform::from_scale(Vec3::splat(0.1)),
+            Transform::from_scale(Vec3::splat(0.1))
+                .with_translation(Vec2::splat(0.0).extend(z as f32)),
             Velocity(20.0 * Vec2::new(rng.gen::<f32>() - 0.5, rng.gen::<f32>() - 0.5)),
         ));
     }