dithering in fullscreen tonemap

Cargo.toml

@@ -1349,6 +1349,16 @@ description = "Various test cases for hierarchy and transform propagation perfor
 category = "Stress Tests"
 wasm = true
 
+[[example]]
+name = "color_banding"
+path = "examples/stress_tests/color_banding.rs"
+
+[package.metadata.example.color_banding]
+name = "Color Banding"
+description = "Test cases for extreme color banding"
+category = "Stress Tests"
+wasm = false
+
 # Tools
 [[example]]
 name = "scene_viewer"

crates/bevy_core_pipeline/src/tonemapping/tonemapping.wgsl

@@ -10,5 +10,11 @@ var hdr_sampler: sampler;
 fn fragment(in: FullscreenVertexOutput) -> @location(0) vec4<f32> {
     let hdr_color = textureSample(hdr_texture, hdr_sampler, in.uv);
 
-    return vec4<f32>(reinhard_luminance(hdr_color.rgb), hdr_color.a);
+    var output_rgb = vec3<f32>(reinhard_luminance(hdr_color.rgb));
+    output_rgb = pow(output_rgb.rgb, vec3<f32>(1.0 / 2.2));
+    output_rgb = output_rgb + screen_space_dither(in.position.xy);
+    // This conversion back to linear space is required because our output texture format is
+    // SRGB; the GPU will assume our output is linear and will apply an SRGB conversion.
+    output_rgb = pow(output_rgb.rgb, vec3<f32>(2.2));
+    return vec4<f32>(output_rgb, hdr_color.a);
 }

crates/bevy_core_pipeline/src/tonemapping/tonemapping_shared.wgsl

@@ -27,3 +27,11 @@ fn reinhard_luminance(color: vec3<f32>) -> vec3<f32> {
     let l_new = l_old / (1.0 + l_old);
     return tonemapping_change_luminance(color, l_new);
 }
+
+// Source: Advanced VR Rendering, GDC 2015, Alex Vlachos, Valve, Slide 49
+// https://media.steampowered.com/apps/valve/2015/Alex_Vlachos_Advanced_VR_Rendering_GDC2015.pdf
+fn screen_space_dither(frag_coord: vec2<f32>) -> vec3<f32> {
+    var dither = vec3<f32>(dot(vec2<f32>(171.0, 231.0), frag_coord)).xxx;
+    dither = fract(dither.rgb / vec3<f32>(103.0, 71.0, 97.0));
+    return dither / 255.0;
+}

examples/stress_tests/color_banding.rs

@@ -0,0 +1,61 @@
+//! Useful for stress-testing rendering cases that exhibit extreme color banding.
+
+use bevy::prelude::*;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .insert_resource(AmbientLight {
+            color: Color::BLACK,
+            brightness: 0.0,
+        })
+        .add_startup_system(setup)
+        .run();
+}
+
+/// set up a simple 3D scene
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    let colors = [Color::WHITE, Color::RED, Color::GREEN, Color::BLUE];
+    let x = [-5.0, 5.0, -5.0, 5.0];
+    let z = [-5.0, -5.0, 5.0, 5.0];
+    let mesh = meshes.add(Mesh::from(shape::Plane { size: 10.0 }));
+
+    for i in 0..4 {
+        // plane
+        commands.spawn_bundle(PbrBundle {
+            mesh: mesh.clone(),
+            material: materials.add(custom_material(colors[i])),
+            transform: Transform::from_xyz(x[i], 0.0, z[i]),
+            ..default()
+        });
+    }
+    // light
+    commands.spawn_bundle(PointLightBundle {
+        point_light: PointLight {
+            intensity: 50.0,
+            shadows_enabled: false,
+            ..default()
+        },
+        transform: Transform::from_xyz(0.0, 5.0, 0.0),
+        ..default()
+    });
+    // camera
+    commands.spawn_bundle(Camera3dBundle {
+        transform: Transform::from_xyz(0.0, 6.0, 0.0).looking_at(Vec3::default(), -Vec3::Z),
+        ..default()
+    });
+}
+
+fn custom_material(color: Color) -> StandardMaterial {
+    StandardMaterial {
+        base_color: color,
+        perceptual_roughness: 1.0,
+        metallic: 0.0,
+        reflectance: 0.0,
+        ..default()
+    }
+}