cad-editor/ROADMAP.md

Open CAD Studio — File Open & Render Speed Roadmap

This document lists the planned improvements for cutting file open time and on-screen draw (render) time. It builds on the already-landed Rendering Optimization work (Phase 1-4); what is left now sits on the open-time, allocation, and draw-call sides.

Source-scan summary (references):

• File open flow: src/io/mod.rs (open_path_with_phase, load_file, purge_corrupt_entities).
• Post-open UI work: src/app/update.rs:90-192 (FileOpened handler — xref resolve, second purge, linetype populate, etc.).
• Derived-cache build: src/scene/mod.rs:90-212 (build_derived_caches — rayon-parallel for hatch / image / mesh).
• Wire tessellation: src/scene/mod.rs:1328-1402 (rayon, zoom-adaptive curve tol).
• Block defn cache: src/scene/block_cache.rs:238 (build_defn — single-threaded today; nested expansion is topological).
• Pipeline: src/scene/pipeline/mod.rs (batched hatch, frustum cull, LOD — Phase 1-4 done).

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Phase 1 — File Open Time

Goal: measurably halve the wall time between "Open" click and "first frame" for a 50 MB DWG.

1.1 Drop the second purge_corrupt_entities

Today purge_corrupt_entities runs once on the background thread and again in the FileOpened handler after xref resolve. XREF content already comes from a separate document — fold the purge inline into xref resolve and delete the outer one. On large files walking doc.entities() again is a measurable cost.

Work: make resolve_xrefs call purge as it merges each xref; remove update.rs:138.

1.2 Move XREF resolution to the background thread

resolve_xrefs runs on the UI thread today — large external references freeze the UI. Move it into the open_path_with_phase worker; have DerivedCaches carry the resolved-xref list back. The UI thread only emits log lines.

New phase tag: PHASE_XREFS (we already have 3 phases; this is the 4th).

1.3 Single-pass entity walk (parse + purge + cache planning)

load_file → purge → build_derived_caches does three separate entities() walks. A single pass can produce:

• corrupt-entity detection,
• hatch / image / mesh handle lists,
• AABB accumulation for world_offset (currently a separate pass inside compute_world_offset).

Target: three O(N) passes → one.

1.4 Memory-mapped file reads (DWG / DXF)

DwgReader::from_file / DxfReader::from_file likely load the whole file into RAM with std::fs::read. Switching to memmap2:

• eliminates the cold-cache read syscall on large files,
• lets the DWG section index be walked on disk (if the acadrust API supports it).

Dependency: acadrust upstream may need a from_reader / from_slice API; add it in our patched fork (hakanaktt/acadrust).

1.5 Parallelize the acadrust parser (long-term)

acadrust's DWG parser is single-threaded. Section-based parallelism (header / classes / objects / blocks / entities — independent offsets) is the biggest unrealized win. Lives in the upstream fork.

Order: profile first — is this really the largest slice? Measure with puffin.

1.6 Defer raster image decode

build_derived_caches calls ImageModel::from_raster_image for every RasterImage entity — pixel decode happens up front. Wasted if the entity is off-screen. Defer the decode until first render (per-handle lazy OnceCell).

1.7 File-hash cache (warm re-open)

When re-opening the same file ((path, mtime, size) key) keep a disk snapshot of CadDocument + DerivedCaches (e.g. ~/.cache/OpenCADStudio/). Skip DWG parse entirely. Win: most-recently-opened file goes from 1-2 s to sub-100 ms.

Risk: cache invalidation. Stay conservative — load only on exact mtime + size match, otherwise normal parse.

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Phase 2 — First-Frame Wire Tessellation

After FileOpened, bump_geometry() fires; the first frame tessellates every model-space wire. Measurable hitch at ~100 k entities.

2.1 Parallelize block-definition build

block_cache::build is single-threaded — the comment says "nested expansion is fiddly". Fix: stratify blocks in topological order (leaf blocks → callers → …) and build each layer in parallel via rayon. Dependency order is preserved.

2.2 Incremental wire cache (delta tessellation)

bump_geometry() invalidates the whole wire cache today (scene/mod.rs:650). Edits usually touch 1-2 entities — re-tessellating the whole doc is waste.

Fix: wire cache becomes HashMap<Handle, (entity_version, Vec<WireModel>)>. The editing command bumps the version of the affected handles; the render path re-tessellates only those, reusing the rest.

Also useful on open: any partial cache (e.g. from block defns) can be re-used.

2.3 Progressive first render

On the first frame emit a coarse-tol wire pass (e.g. 4× the normal tol); refine to full tol on the second frame. The user sees something within 16 ms; detail snaps in smoothly afterwards.

2.4 Merge the world-offset scan into the single-pass walk

compute_world_offset walks the whole MSPACE AABB when the header is unreliable. That scan should join the single-pass walk from 1.3 (we are already iterating entities()).

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Phase 3 — Per-Frame Render Cost

After Phase 1-4 culling/LOD, what's left is upload bytes and draw call count.

3.1 Camera-only invalidation: don't re-tessellate

The wire cache key today is (geometry_epoch, camera_generation) (scene/mod.rs:414). A camera change should not force re-tessellation — only zoom-adaptive curve-tol changes need resampling, and only for curve entities (Arc / Spline / Ellipse). Straight geometry is camera-invariant.

Practical: split the wire cache in two:

• tess_cache[handle] → WireModel (rebuild only if tol-invariant content changed),
• frame_visible[handle] → bool (recomputed per camera_generation).

3.2 Persistent GPU buffer pool — diff upload

Today every wire GPU buffer is re-uploaded when cached_epoch changes. A persistent pool — HashMap<Handle, GpuSlot> — uploads only the slots that actually changed. Big win in CAD-edit scenarios.

3.3 Single-draw batched wire pipeline (Phase 4-B-style)

Every WireModel today costs one draw call plus a bind-group swap. Port the batched hatch pipeline (hatch_batched_gpu.rs) to wires:

• pack all wire vertices into one storage buffer,
• per-instance (color, pattern_id, lw_px, visibility) in a side buffer,
• vertex shader pulls instance data via instance_index,
• a single pass.draw(0..V, 0..N) covers everything.

At 100 k wires that collapses thousands of draw calls into one. If iced 0.14's widget-pipeline limits allow, immediate win.

3.4 Hardware instancing for repeated block inserts

When the same block defn is INSERT-ed N times (every door / window in an architectural drawing) each instance currently renders as its own wire set. Hardware instancing:

• upload the block defn vertex buffer once,
• one 4×4 transform row per Insert in an instance buffer,
• pass.draw_indexed(0..V, 0..N_instances).

Typical architectural DWGs: 10-100× faster.

3.5 Glyph-stroke batching

tessellate.rs produces one WireModel per glyph stroke today — one text entity = dozens of models. Cache stroke geometry per font once (HashMap<(font, glyph), Vec<Point2>>), then per-text only a transform matters.

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Phase 4 — Allocation & Memory

4.1 Swap HashMap for rustc-hash::FxHashMap

Handle is an integer wrapper; the default SipHash is overkill. FxHashMap gives 20-40 % in hash-heavy sites (block_cache, hatches / images / meshes, viewport_wire_cache).

4.2 Arena (bumpalo) for transient wire vertices

Tessellation allocates millions of small Vec<Vec3>s. A bump arena — single allocation, frame-end reset — kills the per-vertex malloc cost. bumpalo plays well with rayon (per-thread arenas).

4.3 SmallVec for small collections

Polyline.vertices, Hatch.boundary_paths, glyph-stroke lists are typically < 8-16 entries. SmallVec<[T; 8]> skips the heap on the common case.

4.4 Compact entity-ID representation

Handle is 8 bytes. 100 k entities → 800 KB just in keys. Hot handle HashSet / HashMap usage can be flattened to Vec<u32> indices plus a single FxHashMap<Handle, u32> translation table — cache-friendlier.

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Phase 5 — Profiling Infrastructure (prerequisite)

Don't start any of the above without measuring first.

5.1 Add puffin or tracy spans

• io::open_path_with_phase → parse, purge, caches spans.
• Scene::wires_for_block → block_cache, tess, sort spans.
• Pipeline::prepare → upload, cull, draw spans.

Gate behind debug_assertions or a --features profile flag.

5.2 Open-time breakdown log

When open completes, push to the command line:

Opened "x.dwg" — 84321 entities — parse 1.2s, purge 80ms, caches 340ms, xref 60ms, first frame 210ms

Regressions are visible immediately.

5.3 Frame-budget HUD

Add a CLI PERF toggle (or F12) that shows per-frame breakdown: tess ms, upload ms, draw ms, GPU wait ms. Makes PR-to-PR comparison trivial.

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Priority Order

Phase 5 first (profiling) — avoids speculation.

Then, measurement-guided:

1. Phase 1.1 + 1.2 (cheap, low-risk, certain win).
2. Phase 1.3 + 1.6 (single-pass + lazy image).
3. Phase 2.2 (incremental wire cache — wins on both edit and open).
4. Phase 3.1 (camera-only invalidation — users pan/zoom constantly).
5. Phase 3.3 (batched wire pipeline) and Phase 3.4 (instancing) — biggest render win, highest complexity.
6. Phase 1.7 (warm cache) — dramatic UX, but invalidation must be correct or it creates nasty bugs.
7. Phase 1.5 (acadrust parallel parse) — hardest, longest-term; only worth it if profiling confirms it is the dominant slice.

Deliberate non-goals (for now)

• GPU compute culling: for orthographic 2D CAD the CPU quadtree is enough. Already covered by Phase 1-4.
• Out-of-core entity streaming: meaningful for 100 MB+ single files; typical Open CAD Studio files are not there yet.
• Multi-frame async tessellation pipeline: if 2.3 progressive render works cleanly, this isn't needed.