sky

assets/shaders/sky.wgsl

@@ -0,0 +1,49 @@
+#import bevy_sprite::mesh2d_vertex_output::VertexOutput
+#import bevy_sprite::mesh2d_view_bindings::globals
+
+fn hash(p: vec2<f32>) -> f32 {
+    return fract(sin(dot(p, vec2<f32>(127.1, 311.7))) * 43758.5453);
+}
+
+fn noise(p: vec2<f32>) -> f32 {
+    let i = floor(p);
+    let f = fract(p);
+    let u = f * f * (3.0 - 2.0 * f);
+    return mix(
+        mix(hash(i), hash(i + vec2<f32>(1.0, 0.0)), u.x),
+        mix(hash(i + vec2<f32>(0.0, 1.0)), hash(i + vec2<f32>(1.0, 1.0)), u.x),
+        u.y,
+    );
+}
+
+fn fbm(p: vec2<f32>) -> f32 {
+    var value = 0.0;
+    var amplitude = 0.5;
+    var q = p;
+    for (var i = 0; i < 5; i++) {
+        value += amplitude * noise(q);
+        q *= 2.0;
+        amplitude *= 0.5;
+    }
+    return value;
+}
+
+@fragment
+fn fragment(mesh: VertexOutput) -> @location(0) vec4<f32> {
+    let t = globals.time;
+    // uv.y is 0 at the top of the quad
+    let height = 1.0 - mesh.uv.y;
+
+    let horizon = vec3<f32>(0.98, 0.85, 0.72);
+    let zenith = vec3<f32>(0.45, 0.68, 0.92);
+    var color = mix(horizon, zenith, smoothstep(0.0, 1.0, height));
+
+    // Two layers of slowly drifting clouds
+    let p = mesh.world_position.xy / 300.0;
+    let clouds = fbm(p + vec2<f32>(t * 0.03, 0.0)) * 0.6
+        + fbm(p * 2.0 + vec2<f32>(t * 0.05, t * 0.01)) * 0.4;
+    let cover = smoothstep(0.45, 0.75, clouds);
+    color = mix(color, vec3<f32>(1.0), cover * 0.8);
+
+    return vec4<f32>(color, 1.0);
+}

src/boids.rs

@@ -51,7 +51,7 @@ fn setup(
             Vec2::new(-3.0, 3.0),
             Vec2::new(-3.0, -3.0),
         ))),
-        material: MeshMaterial2d(materials.add(Color::WHITE)),
+        material: MeshMaterial2d(materials.add(Color::BLACK)),
     };
     spawn_boids(
         &mut commands,

src/main.rs

@@ -1,6 +1,7 @@
 mod boids;
 mod config;
 mod pointer;
+mod sky;
 mod spatial_grid;
 mod ui;
 
@@ -22,6 +23,7 @@ fn main() {
         .add_plugins((
             config::ConfigPlugin,
             pointer::PointerPlugin,
+            sky::SkyPlugin,
             boids::BoidsPlugin,
             ui::ConfigUiPlugin,
         ))

src/sky.rs

@@ -0,0 +1,51 @@
+use bevy::{
+    prelude::*,
+    render::render_resource::AsBindGroup,
+    shader::ShaderRef,
+    sprite_render::{Material2d, Material2dPlugin},
+    window::PrimaryWindow,
+};
+
+pub struct SkyPlugin;
+
+impl Plugin for SkyPlugin {
+    fn build(&self, app: &mut App) {
+        app.add_plugins(Material2dPlugin::<SkyMaterial>::default())
+            .add_systems(Startup, setup_sky)
+            .add_systems(Update, fit_sky_to_window);
+    }
+}
+
+/// Animated sky, driven by the global shader time.
+#[derive(Asset, TypePath, AsBindGroup, Clone)]
+struct SkyMaterial {}
+
+impl Material2d for SkyMaterial {
+    fn fragment_shader() -> ShaderRef {
+        "shaders/sky.wgsl".into()
+    }
+}
+
+#[derive(Component)]
+struct Sky;
+
+fn setup_sky(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<SkyMaterial>>,
+) {
+    commands.spawn((
+        Sky,
+        Mesh2d(meshes.add(Rectangle::new(1.0, 1.0))),
+        MeshMaterial2d(materials.add(SkyMaterial {})),
+        // Behind the boids
+        Transform::from_xyz(0.0, 0.0, -10.0),
+    ));
+}
+
+fn fit_sky_to_window(
+    window: Single<&Window, (With<PrimaryWindow>, Changed<Window>)>,
+    mut sky: Single<&mut Transform, With<Sky>>,
+) {
+    sky.scale = window.size().extend(1.0);
+}