Optimize grass rendering with GPU scatter and indirect draws

STATUS.md

@@ -575,6 +575,43 @@ As of 2026-08-31 · `cargo test --workspace`: **1136 tests green** · clippy and
   spent in query order, so a field five hundred metres away filled in while the one under
   the window stayed bare. And a hero card drew a painted card inside the model standing on
   it.
+- **Meadow grass on the GPU (2026-09-04, `world_render::grass`):** the default terrain's
+  grass was first grown like the crops — every 32 m cell within reach of the camera a mesh
+  of a million vertices, built on the main thread the frame the camera came near, uploaded
+  whole, and drawn through the full PBR path with all three LOD levels in one mesh. A train
+  at line speed entered a new row of cells every second, and each one was a visible hitch.
+  Nothing of the meadow is built on the CPU any more. **(1) A ground cache:** the terrain
+  tiles around the camera and the fields, roads and waters draped on them are drawn once,
+  top down, into a 1024² texture of (height, grass weight) by a pass of its own over the
+  meshes already on the GPU — highest surface wins, an excluded surface lifted a little so
+  it wins the coplanar fight against the ground under it — and drawn again only when the
+  camera has left a 64 m margin, a tile has streamed in or out, or the origin was rebased.
+  **(2) A scatter compute pass** each frame over a grid of 4 m patches: a patch out of the
+  frustum costs one box test; a patch in view lays its blades out on Roberts' R2 sequence
+  (any prefix of it is evenly spread, which is what lets a prefix be the thinned stand),
+  keeps a blade where its rank is below the density its own distance asks for — so coming
+  closer only ever adds blades and never moves one — reads its foot off the ground cache,
+  culls it against the frustum, and appends the survivors to one of three instance lists,
+  whose counts land straight in the indirect draw arguments. **(3) Three indirect draws**
+  in the opaque pass, one per level of detail (eleven, seven and three vertices), through
+  Bevy's own mesh pipeline for the view's key with the shaders and the mesh bind group
+  swapped — so MSAA, HDR, the shadow filter, the fog and the atmosphere are whatever the
+  camera has, with no second copy of any of it. The vertex shader grows each 32-byte
+  record into a quadratic Bézier bent by its own lean and the weather's wind (a gust front
+  travelling downwind, a ripple on it, each blade's own flutter, less of all three on a
+  stiff blade), tapers it, keeps it at least a pixel and a half wide so far grass does not
+  shimmer, and turns the normal across the blade so it lights as a rounded stalk; the
+  fragment shader lights it with diffuse transmission for the back-lit glow, a root-to-tip
+  occlusion gradient, and `weather_pbr` for the rain and the snow. Density is 450 blades/m²
+  at the camera falling as 1/(1+(d/10 m)²) out to the range — 220 m on High, 160 on
+  Medium, 110 on Low, the density scaled with it — the survivors widening by 1/√keep so
+  the sward keeps its cover, and a blade at the threshold growing in rather than popping.
+  Fields, roads and water carry `GroundSurface::Excluded`, so no blade stands on them; the
+  splat's gravel and rock take the grass weight away on the formation and on steep ground,
+  and the edge is dithered so a verge thins into the ballast instead of stopping at a
+  line. Deep winter takes the meadow away, and the grassland phenology sets its height, so
+  it is short after the cut. The crop cards and hero models of the fields are untouched.
+  Nothing is state: every blade is a function of the ground and a hash of its position.
 - **No two fields on one piece of ground (2026-08-31, `content::farmland`,
   `fields::geometry`):** a cadastral register's parcels are meant to tile the land and do
   not quite: neighbours are digitised from either side of a boundary and agree to a few

crates/app/src/settings.rs

@@ -96,8 +96,8 @@ pub struct Graphics {
     pub texture_quality: Quality,
     /// Rendered blade geometry over the painted ground material.
     pub grass: bool,
-    /// Reach and immediate-detail radius of rendered grass. High is the full
-    /// authored sward; the renderer's lossless batching applies to every level.
+    /// Reach and density of the rendered grass. High is the full authored
+    /// sward; the lower levels shorten the reach and thin the stand.
     pub grass_quality: Quality,
     /// Which anti-aliasing runs on the cab camera.
     pub anti_aliasing: AntiAliasing,
@@ -709,24 +709,16 @@ pub fn ground_quality(graphics: &Graphics) -> world_render::GroundQuality {
     world_render::GroundQuality { size, anisotropy }
 }
 
-/// Radial grass bands for the selected quality. High is the authored default;
-/// lower levels trade only grass reach and the radius of the densest local layer.
+/// Reach and density of the meadow for the selected quality. High is the
+/// authored default; lower levels trade reach and blades per square metre,
+/// which is what the scatter pass and the three draws cost.
 fn grass_quality(graphics: &Graphics) -> world_render::GrassRenderSettings {
-    let (bands, fades) = match graphics.grass_quality {
-        Quality::Low => (
-            Vec4::new(24.0, 52.0, 110.0, 14.0),
-            Vec4::new(10.0, 10.0, 12.0, 5.0),
-        ),
-        Quality::Medium => (
-            Vec4::new(28.0, 62.0, 145.0, 18.0),
-            Vec4::new(11.0, 11.0, 12.0, 6.0),
-        ),
-        Quality::High => (
-            Vec4::new(30.0, 70.0, 170.0, 22.0),
-            Vec4::new(12.0, 12.0, 12.0, 8.0),
-        ),
+    let (range, density) = match graphics.grass_quality {
+        Quality::Low => (110.0, 0.4),
+        Quality::Medium => (160.0, 0.7),
+        Quality::High => (220.0, 1.0),
     };
-    world_render::GrassRenderSettings::new(graphics.grass, bands, fades)
+    world_render::GrassRenderSettings::new(graphics.grass, range, density)
 }
 
 /// Generates the ground textures again into the handles the terrain material already
@@ -810,7 +802,6 @@ fn apply_scene(
     quality: Option<ResMut<world_render::mist::Quality>>,
     clouds: Option<ResMut<world_render::clouds::Quality>>,
     grass: Option<ResMut<world_render::GrassRenderSettings>>,
-    grass_materials: Option<ResMut<Assets<world_render::GrassMaterial>>>,
     cameras: Query<(Entity, Has<Bloom>), With<CabCamera>>,
     mut projections: Query<&mut bevy::camera::Projection, With<CabCamera>>,
 ) {
@@ -837,15 +828,6 @@ fn apply_scene(
     {
         *grass = wanted_grass;
     }
-    // Existing material instances update immediately; newly grown cells read
-    // the same resource in `update_field_plants` when their material is made.
-    if let Some(mut materials) = grass_materials {
-        for (_, material) in materials.iter_mut() {
-            if material.extension.grass != wanted_grass.params {
-                material.extension.grass = wanted_grass.params;
-            }
-        }
-    }
     if let Some(mut view) = view {
         view.0 = graphics.view_distance;
     }

crates/i18n/locales/de/main.ftl

@@ -1582,7 +1582,7 @@ set-texture-quality-hint = Größe und Filterung der erzeugten Bodentexturen. Gi
 set-grass = Gerendertes Gras
 set-grass-hint = Zeichnet echte, windbewegte Grashalme über dem Bodenmaterial. Änderungen gelten sofort.
 set-grass-quality = Grasqualität
-set-grass-quality-hint = Reichweite des gerenderten Grases und Größe des besonders dichten, detaillierten Bereichs direkt um die Kamera.
+set-grass-quality-hint = Reichweite des gerenderten Grases und wie viele Halme auf einem Quadratmeter stehen. Das Gras wird jedes Bild auf der Grafikkarte verteilt; eine niedrigere Stufe kostet dort weniger.
 set-shadow-quality = Schattenqualität
 set-shadow-quality-hint = Kantenlänge der Schattenkarte der Sonne: 1024, 2048 oder 4096 Texel. Eine Stufe kostet die vierfache Zahl an Texeln — die Einstellung, die man zuerst senkt.
 set-mist-quality = Nebelqualität

crates/i18n/locales/en/main.ftl

@@ -1579,7 +1579,7 @@ set-texture-quality-hint = Size and filtering of the generated ground textures.
 set-grass = Rendered grass
 set-grass-hint = Draws real, wind-animated grass blades over the ground material. Changes apply immediately.
 set-grass-quality = Grass quality
-set-grass-quality-hint = Reach of rendered grass and size of the especially dense, detailed area immediately around the camera.
+set-grass-quality-hint = Reach of the rendered grass and how many blades stand on a square metre. The grass is laid out on the graphics card every frame; a lower level costs less there.
 set-shadow-quality = Shadow quality
 set-shadow-quality-hint = Edge length of the sun's shadow map: 1024, 2048 or 4096 texels. Four times the texels a step, so it is the setting to lower first.
 set-mist-quality = Mist quality

crates/world-render/Cargo.toml

@@ -20,3 +20,6 @@ glam = { workspace = true }
 # train moves. A mip chain is the only cure, and DDS is the one format that
 # carries one and that ImageMagick can write (`tools/cars/import_cars.mjs`).
 bevy = { workspace = true, features = ["jpeg", "dds"] }
+# Only for `wgpu::Color`, the clear value of the grass ground cache; unifies
+# with the version Bevy pins.
+wgpu = { version = "29", default-features = false }

crates/world-render/src/farmland.rs

@@ -276,6 +276,9 @@ pub fn spawn_fields(
                 // level meshes with their own distances — the patch grows it
                 // when a camera comes near, and takes it back when none does.
                 crate::plants::FieldPlants::default(),
+                // No meadow grass grows through it: the patch cuts a hole into
+                // the grass ground cache (`crate::grass`).
+                crate::grass::GroundSurface::Excluded,
             ));
         }
     });