perf(scene): hatch LOD — solid-fill substitute + sub-pixel skip (Phase 3.3)

Hatch fills drew the full per-family pattern loop every frame regardless
of zoom. Two LOD tiers, neither of which existed:

  * **Shader-side solid substitute.** When the densest line family's
    spacing × scale projects to less than 2 px, individual lines blur
    into a solid color and the per-fragment family loop is wasted ALU
    (and produces moiré). Pass `world_per_pixel` through Uniforms; the
    hatch fragment shader now returns `h.color` directly in that case.
  * **CPU-side sub-pixel skip.** When the entire hatch AABB projects
    to less than 2 px, drop the draw call. Adds `world_aabb` to
    HatchGpu plus a per-frame `compute_hatch_lod` pass (mirrors the
    existing scissor pass) and `hatch_skip_flags`. The render pass
    skips flagged hatches.

Factored a shared `aabb_below_pixel` helper for future LOD callers.

The Uniforms layout is unchanged in size (96 B) — the trailing
`_pad: vec2<f32>` becomes `world_per_pixel: f32, _pad: f32`, so wgsl
modules that don't read the new field keep working with their old
`_pad: vec2<f32>` declarations.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/scene/pipeline/hatch_gpu.rs

@@ -106,6 +106,10 @@ pub struct HatchGpu {
     /// frame's `compute_hatch_scissors` can translate the world rect into
     /// a pixel scissor.
     pub vp_scissor: Option<[f32; 4]>,
+    /// World-space XY bounding rect [min_x, min_y, max_x, max_y] of the
+    /// boundary polygon. Used by the per-frame LOD pass to skip hatches
+    /// whose entire footprint projects to less than ~2 px (Phase 3.3).
+    pub world_aabb: [f32; 4],
     _uniform_buf: wgpu::Buffer,
     _boundary_buf: wgpu::Buffer,
     _family_buf: wgpu::Buffer,
@@ -273,6 +277,7 @@ impl HatchGpu {
             vertex_buffer,
             bind_group,
             vp_scissor: model.vp_scissor,
+            world_aabb: [min_x, min_y, max_x, max_y],
             _uniform_buf,
             _boundary_buf,
             _family_buf,

src/scene/pipeline/mod.rs

@@ -59,6 +59,10 @@ pub struct Pipeline {
     gpu_hatches: Vec<HatchGpu>,
     /// Pixel scissor rects [x, y, w, h] for viewport-clipped hatches. Recomputed each frame.
     hatch_pixel_scissors: Vec<Option<[u32; 4]>>,
+    /// `true` for hatches whose AABB projects to less than ~2 px at the
+    /// current zoom. Render pass skips the draw call. Recomputed each
+    /// frame alongside the scissor pass (Phase 3.3 hatch LOD).
+    hatch_skip_flags: Vec<bool>,
     /// Wipeout fills — rendered after wires in a separate pass.
     gpu_wipeouts: Vec<HatchGpu>,
     /// Pixel scissor rects [x, y, w, h] for viewport-clipped wipeouts. Recomputed each frame.
@@ -597,6 +601,7 @@ impl Pipeline {
             gpu_selected_wires: vec![],
             gpu_hatches: vec![],
             hatch_pixel_scissors: vec![],
+            hatch_skip_flags: vec![],
             gpu_wipeouts: vec![],
             wipeout_pixel_scissors: vec![],
             gpu_images: vec![],
@@ -640,6 +645,20 @@ impl Pipeline {
             .collect();
     }
 
+    /// Recompute per-hatch LOD skip flags. A hatch whose entire AABB
+    /// projects to less than ~2 px is invisible at this zoom — skip the
+    /// draw call entirely instead of running the polygon test for every
+    /// fragment in the bounding quad. Pairs with the shader-side
+    /// solid-fill substitution for hatches between 2 px and the dense
+    /// pattern threshold.
+    pub fn compute_hatch_lod(&mut self, view_proj: glam::Mat4, clip_w: u32, clip_h: u32) {
+        self.hatch_skip_flags = self
+            .gpu_hatches
+            .iter()
+            .map(|h| aabb_below_pixel(h.world_aabb, view_proj, clip_w, clip_h, 2.0))
+            .collect();
+    }
+
     /// Recompute pixel scissor rects for viewport-clipped wipeouts.
     pub fn compute_wipeout_scissors(&mut self, view_proj: glam::Mat4, clip_w: u32, clip_h: u32) {
         self.wipeout_pixel_scissors = self
@@ -767,6 +786,10 @@ impl Pipeline {
                 pass.set_bind_group(0, &self.uniform_bind_group, &[]);
                 let mut scissor_active = false;
                 for (i, hatch) in self.gpu_hatches.iter().enumerate() {
+                    // Phase 3.3 LOD: skip hatches projecting to sub-pixel size.
+                    if self.hatch_skip_flags.get(i).copied().unwrap_or(false) {
+                        continue;
+                    }
                     match self.hatch_pixel_scissors.get(i) {
                         Some(Some([x, y, w, h])) => {
                             pass.set_scissor_rect(*x, *y, *w, *h);
@@ -1161,6 +1184,43 @@ impl Pipeline {
     }
 }
 
+/// Return `true` when the world-XY AABB's screen-space size is below the
+/// given pixel threshold. Used by LOD passes (hatch skip, etc.) to drop
+/// draw calls that wouldn't contribute a visible pixel.
+fn aabb_below_pixel(
+    aabb: [f32; 4],
+    view_proj: glam::Mat4,
+    clip_w: u32,
+    clip_h: u32,
+    threshold_px: f32,
+) -> bool {
+    let [x0, y0, x1, y1] = aabb;
+    if !x0.is_finite() || !y0.is_finite() || !x1.is_finite() || !y1.is_finite() {
+        return false;
+    }
+    let w = clip_w as f32;
+    let h = clip_h as f32;
+    let corners = [
+        view_proj.project_point3(glam::Vec3::new(x0, y0, 0.0)),
+        view_proj.project_point3(glam::Vec3::new(x1, y0, 0.0)),
+        view_proj.project_point3(glam::Vec3::new(x0, y1, 0.0)),
+        view_proj.project_point3(glam::Vec3::new(x1, y1, 0.0)),
+    ];
+    let mut min_px = f32::INFINITY;
+    let mut max_px = f32::NEG_INFINITY;
+    let mut min_py = f32::INFINITY;
+    let mut max_py = f32::NEG_INFINITY;
+    for c in &corners {
+        let px = (c.x + 1.0) * 0.5 * w;
+        let py = (1.0 - c.y) * 0.5 * h;
+        if px < min_px { min_px = px; }
+        if px > max_px { max_px = px; }
+        if py < min_py { min_py = py; }
+        if py > max_py { max_py = py; }
+    }
+    (max_px - min_px).max(max_py - min_py) < threshold_px
+}
+
 /// Project a world-space XY scissor rect through `view_proj` into the four
 /// pixel-space corners and return the smallest aligned-rect that bounds them,
 /// clamped to the clip viewport. Returns `None` when the rect is missing or

src/scene/pipeline/uniforms.rs

@@ -7,16 +7,27 @@ pub struct Uniforms {
     pub view_proj: glam::Mat4,
     pub camera_pos: glam::Vec4,
     pub viewport_size: [f32; 2],
-    pub _pad: [f32; 2],
+    /// World units per screen pixel at the current zoom. Used by the
+    /// hatch shader to substitute solid fill when pattern line spacing
+    /// falls below ~2 px (Phase 3.3 LOD).
+    pub world_per_pixel: f32,
+    pub _pad: f32,
 }
 
 impl Uniforms {
     pub fn new(camera: &Camera, bounds: Rectangle) -> Self {
+        let half_h = camera.ortho_size();
+        let world_per_pixel = if bounds.height > 0.0 {
+            (2.0 * half_h) / bounds.height
+        } else {
+            0.0
+        };
         Self {
             view_proj: camera.view_proj(bounds),
             camera_pos: camera.position_vec4(),
             viewport_size: [bounds.width, bounds.height],
-            _pad: [0.0; 2],
+            world_per_pixel,
+            _pad: 0.0,
         }
     }
 }

src/scene/render.rs

@@ -147,6 +147,7 @@ impl shader::Primitive for Primitive {
         pipeline.compute_hatch_scissors(self.uniforms.view_proj, clip_size.width, clip_size.height);
         pipeline.compute_wipeout_scissors(self.uniforms.view_proj, clip_size.width, clip_size.height);
         pipeline.compute_image_scissors(self.uniforms.view_proj, clip_size.width, clip_size.height);
+        pipeline.compute_hatch_lod(self.uniforms.view_proj, clip_size.width, clip_size.height);
         if self.show_viewcube {
             pipeline.viewcube.upload(
                 queue,

src/shaders/hatch.wgsl

@@ -13,10 +13,11 @@
 // ── Group 0: frame uniforms (shared) ──────────────────────────────────────
 
 struct Uniforms {
-    view_proj:     mat4x4<f32>,
-    camera_pos:    vec4<f32>,
-    viewport_size: vec2<f32>,
-    _pad:          vec2<f32>,
+    view_proj:       mat4x4<f32>,
+    camera_pos:      vec4<f32>,
+    viewport_size:   vec2<f32>,
+    world_per_pixel: f32,
+    _pad:            f32,
 }
 @group(0) @binding(0) var<uniform> u: Uniforms;
 
@@ -195,7 +196,24 @@ fn check_family(
         return mix(h.color, h.color2, t);
     }
 
-    // 3. Pattern: evaluate each line family.
+    // 3. Pattern: LOD substitution — when the densest family's spacing
+    //    projects to less than 2 px, individual lines blur into a solid
+    //    fill and the per-family loop just wastes ALU. Return solid color
+    //    instead. (Phase 3.3 hatch LOD.)
+    if u.world_per_pixel > 0.0 && f.n_families > 0u {
+        var min_spacing_world: f32 = 1.0e30;
+        for (var i = 0u; i < f.n_families; i++) {
+            let s = abs(f.families[i].perp_step) * h.scale;
+            if s > 0.0 && s < min_spacing_world {
+                min_spacing_world = s;
+            }
+        }
+        if min_spacing_world / u.world_per_pixel < 2.0 {
+            return h.color;
+        }
+    }
+
+    // 4. Pattern: evaluate each line family.
     let cos_off = cos(h.angle_offset);
     let sin_off = sin(h.angle_offset);
     for (var i = 0u; i < f.n_families; i++) {