# BRIEFING — 2026-10-01T20:34:00Z

## Mission
Investigate and formulate tight isometric diamond culling math to eliminate phantom chunk draw calls, and resolve edge cases (0x0 viewport early exit, negative coords, extreme pans, NaN).

## 🔒 My Identity
- Archetype: explorer
- Roles: Frustum Culling & Edge Cases Explorer
- Working directory: c:\Projects\FreeExile\.agents\teamwork\explorer_m2_fix_2
- Original parent: 1cc48fc5-ce57-4f48-8964-24cab4bfcacc
- Milestone: M2 Fix (Tile Map Renderer Frustum Culling & Edge Cases)

## 🔒 Key Constraints
- Read-only investigation — do NOT implement directly in source files
- Strict diamond polygon culling math vs rectangular canvas AABB to eliminate phantom chunk draws
- Fix 0x0 viewport early exit without fallback frame rendering
- Handle negative coordinates, extreme pans, and NaN safely
- Coordinate with explorer_m2_fix_1 recommendations ($8 \times 8$ / $16 \times 16$)

## Current Parent
- Conversation ID: 1cc48fc5-ce57-4f48-8964-24cab4bfcacc
- Updated: 2026-10-01T20:34:00Z

## Investigation State
- **Explored paths**:
  - `ORIGINAL_REQUEST.md` (section R2: 16x16 chunking, draw call ceilings, mobile RAM)
  - `reviewer_m2_2\handoff.md` and `challenger_m2_1\handoff.md` (LRU thrashing root cause, synthetic benchmark bias)
  - `client/webapp/js/engine/tile_map_renderer.js` and `iso_math.js`
  - `tools/perf/map_render_benchmark.js` and `stress_test_lru_cache.js`
  - `explorer_m2_fix_1` empirical chunk capacity and diamond tests
- **Key findings**:
  - Coarse rectangular canvas AABB ($1024 \times 512$) includes empty corner triangles, admitting 2 phantom chunks at $(30, 30)$ and $(50, 50)$, pushing visibleCount to 8.
  - Closed-form 4-plane SAT culling ($X, Y, D_1 = 2Y + X, D_2 = 2Y - X$) completely eliminates phantom chunks: $v$ cancels out on $D_1$ leaving $64u$, $u$ cancels out on $D_2$ leaving $64v$.
  - 61,008 chunk configurations tested: 0 false negatives, 100% elimination of phantom chunks.
  - Viewport 0x0 early exit fixed (`viewport.clientWidth <= 0 || viewport.clientHeight <= 0`).
  - Extreme pan OOB guard added (`minTx > maxTx || minTy > maxTy`).
  - $16 \times 16$ chunks proved superior to $8 \times 8$ chunks: avg 5.23 blits vs 11.76 blits; static camera draws 4-6 chunks vs 10-17 chunks.
- **Unexplored areas**: None. Investigation complete.

## Key Decisions Made
- Formulated closed-form 4-plane SAT diamond intersection test with 0 allocations.
- Derived exact code snippet for Worker M2 drop-in replacement.
- Validated performance over 10,000 pan frames: avg 5.23 blits/frame.

## Artifact Index
- `DISPATCH.md` — Assignment instructions
- `BRIEFING.md` — Persistent working memory
- `progress.md` — Liveness heartbeat file
- `report.md` — Full technical analysis and code recommendation for Worker M2
- `handoff.md` — 5-component handoff report