research: Astra low review of per-block bit-flag masks (read-only)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

Standalone/Research/WorldAssemblerM3/Astra3/PROMPT.md

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+# VoxelForge — read-only design review: per-block bit flags (masks) to skip work
+
+You are reviewing an idea against real code. **Do not edit, create or delete any file, and do not build or run
+anything.** Read code and reason. Be thorough; mark each claim as [CODE] (with file:line), [MEASURED] (from a report),
+or [INFERENCE].
+
+## Context
+
+VoxelForge Core (`Standalone/Core`) generates a density field: negative = solid, positive = air, 25 cm voxels, 32³ tiles,
+LOD0–8 clipmap, marching cubes. The production mesher for the standalone path is in `Standalone/Source/...`, and Core is
+also compiled into Unreal. Same seed must give the same world on every machine and worker count; byte-identity with
+older versions is NOT required.
+
+Branch `assembler/world` (HEAD `7022d02`'s parent line, currently `4041d4b`) has the smooth inter-strate tunnels.
+Readiness target: LOD0–2 near terrain in ~1 s on arrival. Measured today: SW ~0.7–0.8 s, VS ~0.9 s, FI/TN ~1.0 s
+headless. In Linux UE, start→LOD0–2 is ~2.2–2.5 s. At 30 m/s in UE, near coverage is ~40%.
+
+Reports to consult: `Standalone/Research/WorldAssemblerM3/Linux1*/REPORT.md`, especially:
+- L77/L87/L117, cost census;
+- L124, shape gate and per-strate timings;
+- L125/L126/L128, tunnel prepare: per-segment work, 67–73% pair overlap, support up to 7.9 m;
+- L128, where an exact one-owner slab scheme halved pairs but got slower because of per-sample ownership tests.
+
+Tile skipping already exists at tile level: box verdicts / classify, where all-solid or all-air tiles are skipped.
+
+## The idea (from the owner)
+
+Keep bit flags per block (1 bit per voxel, or per small brick of e.g. 4³ or 8³ voxels): for example "certainly deep
+solid", "certainly deep air", "near a surface", "touched by a tunnel", "already evaluated". Use them to skip work in
+generation and meshing, and possibly elsewhere (collision, streaming, edits, AI/nav, lighting).
+
+## Questions
+
+1. Where does time actually go per tile today, and which of that work a mask could skip? Cover density fill,
+   classification, mesher (marching cubes, normals/gradients, skirts, seam refinement), passage prepare, and composition.
+   Use measured census numbers where they exist.
+2. Which flags are sound? A flag must be a proof, never a guess. A mask "certainly solid" must be derived from a
+   conservative bound (the bound must be the supremum of what the evaluator produces), or the mesher will delete
+   surfaces. Which bounds already exist in the code (box verdicts, interval bounds) that a brick-level mask could reuse?
+   Name the existing hazards. For example, `Identity` means `Sdf >= T`, and `ClassifyTile` proving below-TerrainZ solid
+   would delete 3D caves.
+3. Granularity: per-voxel bits vs per-brick (4³/8³) vs per-column. What does each cost to build, versus what it
+   saves? Consider SIMD and cache behaviour. Marching cubes needs only cells whose corner signs differ, and gradients
+   need a halo.
+4. "Already evaluated" or "checked" flags: where would they be correct (memoisation of identical queries), and where
+   would they be wrong (nearest-of-many queries like the tunnel segments)? Is there repeated identical evaluation today,
+   for example halo samples shared between neighbouring tiles, or skirt/seam re-queries?
+5. Other uses: player edits (diff layer), collision-only server tiles, streaming priority, nav. Which would a mask
+   genuinely help, given the multiplayer rule (replicate seed + layout + diff, never geometry)?
+6. Recommendation: the one or two mask designs most likely to pay off, with the expected gain (a range, marked
+   inference), the measurement that would prove or kill each, and the risks. Keep it short.
+
+Write plain Markdown, at most ~1,200 words.