feat(pipeline): per-instance visibility for batched hatches (Phase 4-B, step 4)

Adds a dedicated `visibility: array<u32>` storage buffer (binding 4)
alongside the existing batched hatch buffers. Vertex shader reads
`visibility[v.instance_index]` — 0 → emit an out-of-NDC clip
position so the GPU rasterizer clips the primitive before the
fragment stage runs.

`compute_hatch_lod` now drives this buffer instead of (or alongside,
for the per-hatch fallback) `hatch_skip_flags`. Walks a per-instance
CPU AABB mirror that the builder fills at upload time (`instance_aabbs`),
applies the same `aabb_below_pixel` (sub-pixel LOD) OR `aabb_offscreen`
(frustum cull) rule the legacy path used, and pushes the resulting
0/1 flags via `queue.write_buffer` on a separate 4 B-per-hatch buffer
— ~40 KB per pan tick for 10 k hatches, far cheaper than re-writing
the 112 B-per-instance data.

Step 5 will remove the legacy `gpu_hatches` Vec, the per-hatch draw
loop, and `hatch_skip_flags` once the batched path has soaked.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/pipeline/hatch_batched_gpu.rs

@@ -136,14 +136,25 @@ pub struct HatchBatchedGpu {
     pub boundary_buffer: wgpu::Buffer,
     pub family_buffer: wgpu::Buffer,
     pub dash_buffer: wgpu::Buffer,
+    /// Per-instance visibility flag (1=draw, 0=skip). Stored in its
+    /// own small storage buffer so per-frame updates don't have to
+    /// touch the large `instance_buffer`. Vertex shader reads
+    /// `visibility[instance_index]` — when 0 it emits an out-of-NDC
+    /// clip position so the GPU clips the primitive before the
+    /// fragment stage runs.
+    pub visibility_buffer: wgpu::Buffer,
     pub bind_group: wgpu::BindGroup,
     pub instance_count: u32,
-    /// CPU-side mirror of `HatchInstance.visible` — `compute_hatch_lod`
+    /// CPU mirror — `update_visibility` re-uploads this whole slice
-    /// writes 0/1 here, then `upload_visibility` pushes the changed
+    /// when any flag changes. ~4 B per hatch, so 40 KB / 10 k hatches
-    /// `visible` field through to the GPU `instance_buffer`. Storing
+    /// per pan tick. Far cheaper than touching the 112 B-per-instance
-    /// just the flag (not the whole HatchInstance) keeps the per-frame
+    /// data.
-    /// upload small.
     pub visibility: Vec<u32>,
+    /// CPU-side mirror of each instance's local-space AABB (world-
+    /// offset-subtracted, world_origin already added back). Used by
+    /// `compute_hatch_lod` to evaluate the sub-pixel + frustum cull
+    /// without reading back from the GPU.
+    pub instance_aabbs: Vec<[f32; 4]>,
 }
 
 impl HatchBatchedGpu {
@@ -350,6 +361,13 @@ impl HatchBatchedGpu {
             usage: wgpu::BufferUsages::STORAGE,
         });
 
+        let visibility: Vec<u32> = instances.iter().map(|i| i.visible).collect();
+        let visibility_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
+            label: Some("hatch_batched.visibility"),
+            contents: bytemuck::cast_slice(&visibility),
+            usage: wgpu::BufferUsages::STORAGE | wgpu::BufferUsages::COPY_DST,
+        });
+
         let bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
             label: Some("hatch_batched.bg"),
             layout: bgl,
@@ -370,10 +388,30 @@ impl HatchBatchedGpu {
                     binding: 3,
                     resource: dash_buffer.as_entire_binding(),
                 },
+                wgpu::BindGroupEntry {
+                    binding: 4,
+                    resource: visibility_buffer.as_entire_binding(),
+                },
             ],
         });
 
-        let visibility = instances.iter().map(|i| i.visible).collect();
+        // CPU AABB mirror — local-space rect with world_origin added
+        // back, ready for `aabb_offscreen` / `aabb_below_pixel` to
+        // compare against the camera view_proj.
+        let instance_aabbs: Vec<[f32; 4]> = instances
+            .iter()
+            .map(|i| {
+                let ox = i.world_origin[0];
+                let oy = i.world_origin[1];
+                [
+                    i.aabb[0] + ox,
+                    i.aabb[1] + oy,
+                    i.aabb[2] + ox,
+                    i.aabb[3] + oy,
+                ]
+            })
+            .collect();
+
         Some(Self {
             vertex_buffer,
             vertex_count: verts.len() as u32,
@@ -381,12 +419,24 @@ impl HatchBatchedGpu {
             boundary_buffer,
             family_buffer,
             dash_buffer,
+            visibility_buffer,
             bind_group,
             instance_count: instances.len() as u32,
             visibility,
+            instance_aabbs,
         })
     }
 
+    /// Push the CPU `visibility` slice to GPU. Call when any
+    /// element changes (typically per-frame from compute_hatch_lod).
+    pub fn upload_visibility(&self, queue: &wgpu::Queue) {
+        queue.write_buffer(
+            &self.visibility_buffer,
+            0,
+            bytemuck::cast_slice(&self.visibility),
+        );
+    }
+
     /// Group-1 bind group layout — shared by the pipeline so it can be
     /// constructed once at startup. All four bindings are read-only
     /// storage and visible to both VS (AABB+visibility lookup) and FS
@@ -404,7 +454,7 @@ impl HatchBatchedGpu {
         };
         device.create_bind_group_layout(&wgpu::BindGroupLayoutDescriptor {
             label: Some("hatch_batched.bgl"),
-            entries: &[entry(0), entry(1), entry(2), entry(3)],
+            entries: &[entry(0), entry(1), entry(2), entry(3), entry(4)],
         })
     }
 }

src/scene/pipeline/mod.rs

@@ -745,7 +745,32 @@ impl Pipeline {
     /// 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) {
+    pub fn compute_hatch_lod(
+        &mut self,
+        queue: &wgpu::Queue,
+        view_proj: glam::Mat4,
+        clip_w: u32,
+        clip_h: u32,
+    ) {
+        // Phase 4-B batched path — write the same skip rule (sub-pixel
+        // LOD OR offscreen) into the GPU visibility buffer. The vertex
+        // shader maps 0 → out-of-NDC clip → primitive culled before
+        // fragment stage; equivalent to the per-hatch `skip_flags`
+        // gate but at GPU-side cost.
+        if let Some(batch) = &mut self.gpu_hatch_batched {
+            // Same skip rule as the legacy per-hatch path: sub-pixel
+            // LOD (`aabb_below_pixel`) OR offscreen (`aabb_offscreen`).
+            // Walks the per-instance AABB mirror — no GPU readback.
+            for (i, aabb) in batch.instance_aabbs.iter().enumerate() {
+                let skip = aabb_below_pixel(*aabb, view_proj, clip_w, clip_h, 2.0)
+                    || aabb_offscreen(*aabb, view_proj, clip_w, clip_h);
+                batch.visibility[i] = if skip { 0 } else { 1 };
+            }
+            batch.upload_visibility(queue);
+            return;
+        }
+
+        // Per-hatch (legacy) fallback path.
         self.hatch_skip_flags = self
             .gpu_hatches
             .iter()

src/scene/render.rs

@@ -147,7 +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);
+        pipeline.compute_hatch_lod(queue, self.uniforms.view_proj, clip_size.width, clip_size.height);
         pipeline.compute_wipeout_lod(self.uniforms.view_proj, clip_size.width, clip_size.height);
         pipeline.compute_mesh_lod(self.uniforms.view_proj, clip_size.width, clip_size.height);
         if self.show_viewcube {

src/shaders/hatch_batched.wgsl

@@ -67,6 +67,11 @@ struct LineFamily {
 @group(1) @binding(1) var<storage, read> boundary:   array<vec4<f32>>;
 @group(1) @binding(2) var<storage, read> families:   array<LineFamily>;
 @group(1) @binding(3) var<storage, read> dashes:     array<f32>;
+// Per-instance visibility (Phase 4-B sub-pixel + frustum skip).
+// CPU writes `1` to draw / `0` to skip every frame; vertex shader
+// emits an out-of-NDC clip position for 0-instances so the GPU
+// rasterizer culls the primitive before any fragment runs.
+@group(1) @binding(4) var<storage, read> visibility: array<u32>;
 
 // ── Vertex shader ────────────────────────────────────────────────────────
 
@@ -100,12 +105,12 @@ fn corner_xy(c: u32, aabb: vec4<f32>) -> vec2<f32> {
     var o: VOut;
     let inst = instances[v.instance_index];
 
-    // CPU-driven visibility flag (Phase 4-B frustum skip). Emit a clip
+    // Per-frame visibility (CPU-driven sub-pixel + frustum skip).
-    // position whose x/y exceed |w|, so the GPU frustum-culls the
+    // 0 → emit a clip position whose x/y exceed |w| so the GPU
-    // primitive and no fragment runs for it. (WGSL forbids literal NaN
+    // frustum-culls the primitive and no fragment runs. (WGSL
-    // so we can't go that route; pushing the vertex out of the unit
+    // forbids literal NaN so this out-of-NDC trick replaces the
-    // cube is the equivalent degenerate-triangle trick.)
+    // usual NaN-degenerate-triangle.)
-    if inst.visible == 0u {
+    if visibility[v.instance_index] == 0u {
         o.clip = vec4<f32>(2.0, 2.0, 2.0, 1.0);
         o.xz = vec2<f32>(0.0, 0.0);
         o.instance_index = v.instance_index;