OneCAD-Draft/RENDERING_OPTIMIZATION.md

Rendering Optimization Roadmap

Phases 1–4 have all landed. This document is kept as a stub so the git history points somewhere; the optimisations themselves are documented inline at the call sites:

• Phase 1 – per-frame LOD ladder, scissor cull (search for Phase 1.* in scene/pipeline/mod.rs).
• Phase 2 – quadtree spatial index for top-level entities, plus draw-time frustum cull for hatches / wipeouts / meshes (Phase 2.1, 2.2, 2.3).
• Phase 3 – LOD substitution for far / sub-pixel content (Phase 3.2, 3.3, 3.4).
• Phase 4-B – batched hatch pipeline. Every hatch is uploaded into shared storage buffers (hatch_batched_gpu.rs) and drawn in one pass.draw(0..6N, 0..1); the per-instance visibility buffer doubles as a CPU-driven cull mask (sub-pixel + frustum) that the vertex shader honours by emitting an out-of-NDC clip position for skipped instances. Replaces N bind-group swaps + N draw calls with a single draw.

Deliberate non-goals

• Phase 4.1 (multi-draw indirect / compute cull) — iced 0.14 doesn't expose Features::MULTI_DRAW_INDIRECT to widget pipelines. Without it, single-draw indirect adds no real saving over the current batched draw. Revisit if iced surfaces the feature flag, or if H7CAD ever swaps the shader widget for a custom wgpu surface.
• Phase 4.2 (Hi-Z occlusion) — pays off only for perspective 3D with many overlapping opaque meshes. H7CAD is orthographic top-down for every realistic workflow, so the win is theoretical.
• Per-mutation incremental quadtree updates — entity_index_cache rebuilds lazily on geometry_epoch change. Full rebuild is ~50 ms on a 100 k-entity doc; promote to per-mutation insert/remove/ update if profiling shows that as a hot spot during heavy edits.
• Per-hatch viewport scissor in the batched path — paper-space MSPACE viewports can render hatches past the viewport border for now. Add a per-instance scissor rect (computed from vp_scissor) if it shows up as a visible artefact.