docs(rendering): mark Phase 4 done, drop deferred / non-goal phases
Phase 4-B (batched hatch pipeline) is in; Phase 4.1 (multi-draw indirect / GPU compute cull) stays as a deliberate non-goal because iced 0.14 doesn't expose the required wgpu features to widget pipelines, and the single-draw batched path already collapses N bind-group swaps + N draw calls into one. Phase 4.2 (Hi-Z occlusion) only matters for perspective 3D, which H7CAD doesn't ship. The doc is now a short stub — the per-phase rationale lives at the call sites (search for "Phase N.X" comments). Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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# Rendering Optimization Roadmap: Culling & Level of Detail
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# Rendering Optimization Roadmap
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## Background
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Phases 1–4 have all landed. This document is kept as a stub so the
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git history points somewhere; the optimisations themselves are
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documented inline at the call sites:
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H7CAD renders all entities every frame regardless of camera position or zoom level. No spatial
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- **Phase 1** – per-frame LOD ladder, scissor cull (search for
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acceleration, no LOD, no frustum culling. Every wire, hatch, mesh, and image is uploaded and
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`Phase 1.*` in `scene/pipeline/mod.rs`).
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drawn unconditionally. This scales poorly for large drawings (100k+ entities) and dense 3D solids.
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- **Phase 2** – quadtree spatial index for top-level entities, plus
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draw-time frustum cull for hatches / wipeouts / meshes
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(`Phase 2.1`, `2.2`, `2.3`).
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- **Phase 3** – LOD substitution for far / sub-pixel content
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(`Phase 3.2`, `3.3`, `3.4`).
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- **Phase 4-B** – batched hatch pipeline. Every hatch is uploaded
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into shared storage buffers (`hatch_batched_gpu.rs`) and drawn in
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one `pass.draw(0..6N, 0..1)`; the per-instance `visibility` buffer
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doubles as a CPU-driven cull mask (sub-pixel + frustum) that the
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vertex shader honours by emitting an out-of-NDC clip position for
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skipped instances. Replaces N bind-group swaps + N draw calls with
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a single draw.
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Current pipeline order:
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## Deliberate non-goals
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1. Hatch fills → 2. Images → 3. Meshes → 4. Face3D fills → 5. Face3D edges →
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6. Wires → 7. Wipeouts → 8. Selection overlay → 9. MSAA resolve → 10. Blit
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---
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## Phase 4 — GPU-Side Culling (Advanced)
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**Goal:** Offload culling to the GPU; zero CPU cost for large entity counts.
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### 4.1 Indirect Draw + Compute Cull
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Convert per-entity draw calls to indirect draw calls (`draw_indirect` / `draw_indexed_indirect`).
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Run a compute shader pre-pass that tests each entity's AABB against the frustum and writes
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`DrawIndirectArgs` only for visible entities.
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```wgsl
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// cull.wgsl
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@compute @workgroup_size(64)
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fn cull_entities(@builtin(global_invocation_id) id: vec3<u32>) {
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let entity = entities[id.x];
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if frustum_test(entity.aabb) {
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// atomically append to indirect draw buffer
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let slot = atomicAdd(&draw_count, 1u);
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draw_args[slot] = entity.draw_args;
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}
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}
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```
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Requires restructuring entity data into GPU-side storage buffers. High complexity; tackle after
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Phases 1–3 prove insufficient.
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### 4.2 Hierarchical Z-Buffer Occlusion Culling (3D Only)
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For dense 3D solid scenes, use a Hi-Z buffer to cull occluded meshes:
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1. Render depth-only pass for large opaque solids.
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2. Downsample depth into mip chain (Hi-Z pyramid).
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3. Compute shader tests each mesh AABB against Hi-Z; skips occluded meshes.
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Relevant only for perspective (3D) mode with many overlapping solids.
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---
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## Implementation Order
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```
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Phase 4.1 Indirect draw + GPU cull high complexity, defer
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Phase 4.2 Hi-Z occlusion high complexity, 3D only, last
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```
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---
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## Key Files to Modify
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| File | Change |
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|------|--------|
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| `src/scene/mod.rs` | Quadtree/octree; LOD epoch tracking |
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| `src/scene/pipeline/mod.rs` | Cull hatch/image entities before upload loops |
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| `src/shaders/cull.wgsl` | New — Phase 4 compute culling shader |
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---
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## Success Metrics
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- **Phase 2 target:** Pan/zoom frame cost O(visible) not O(total).
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- **Phase 4 target:** GPU-cull overhead < 0.5 ms for 1M entity scene.
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- **Phase 4.1 (multi-draw indirect / compute cull)** — iced 0.14
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doesn't expose `Features::MULTI_DRAW_INDIRECT` to widget pipelines.
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Without it, single-draw indirect adds no real saving over the
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current batched draw. Revisit if iced surfaces the feature flag,
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or if H7CAD ever swaps the shader widget for a custom wgpu surface.
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- **Phase 4.2 (Hi-Z occlusion)** — pays off only for perspective 3D
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with many overlapping opaque meshes. H7CAD is orthographic
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top-down for every realistic workflow, so the win is theoretical.
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- **Per-mutation incremental quadtree updates** — `entity_index_cache`
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rebuilds lazily on `geometry_epoch` change. Full rebuild is ~50 ms
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on a 100 k-entity doc; promote to per-mutation `insert`/`remove`/
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`update` if profiling shows that as a hot spot during heavy edits.
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- **Per-hatch viewport scissor in the batched path** — paper-space
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MSPACE viewports can render hatches past the viewport border for
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now. Add a per-instance scissor rect (computed from `vp_scissor`)
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if it shows up as a visible artefact.
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