`createTerrain` returned one mesh covering 32.50-42.05 N and -124.50 to
-114.00 W, and three frustum-culls per *object* — so at every pose, from
400 km and from 2.5, the merged board submitted all 86,400 of its visible
triangles. Standing over San Francisco it paid for the Mojave.
It now returns a `THREE.Group` named `terrain`, holding a 5 x 5 grid over
`city.bounds` — **17 non-empty chunk meshes**, eight cells being Pacific,
Nevada and Arizona — plus one un-chunked shadow caster. The geometry is
not touched: same patches, same resolution, same board-wide normals. Only
the index is cut up, which is what makes this culling and not level of
detail, and what sidesteps placement drift entirely — no ground is
re-derived, so nothing that was placed against it can move.
All 18 geometries share **one** `position`/`color`/`normal` attribute
triple: no duplicated vertices, one upload of exactly today's bytes, and
no lighting crease down any of the sixteen seams, since the normals were
averaged before the partition existed. The bounding volumes are therefore
assigned by hand — `computeBoundingSphere` walks the position attribute
and not the index, so a chunk left to compute its own would get
California's, cull nothing, and restore the whole cost with seventeen
extra draw calls and no visible symptom. `terrainLod.test.ts` pins that.
5 x 5 because a draw call is the scarce resource here, not a triangle.
The sweep is in the constant's own comment: 4x4 -> 5x5 buys 1,246
triangles per added draw, 5x5 -> 6x6 buys 573, 6x6 -> 8x8 buys 61. Five is
the last grid where a draw is worth more than a thousand triangles.
The caster stays one object with `frustumCulled = false`, deliberately:
`WebGLShadowMap` gates the depth pass on the same test as the colour pass,
so a chunked caster would drop a ridge that legitimately shadows into
frame the moment the camera turned. It sits at `drawRange(0, 0)` with the
hooks inverted, and `renderBufferDirect` early-returns on `< 0` and
`Infinity` but not on 0 — one zero-triangle draw call per frame, forever.
Gated on `city.districts.some(d => d.detail === true)`, which
`cities/unify.ts` is the only place in the repo that writes. `?city=sf`,
`?city=socal`, both `?one=0` California cells and `office` take the
unchunked branch and get today's single mesh inside a group: measured, the
socal board is identical to the draw call at three poses, and `?one=0`
identical to the digit at five.
Measured with `scripts/cost-at.mjs` at 37.7897, -122.3972, merged board,
before -> after (triangles / draws):
400 km 355,759/414 -> 355,759/431 the whole board, all 17 drawn
120 km 351,019/327 -> 321,057/340
45 km 332,969/265 -> 270,373/273
7.7 km 330,578/340 -> 222,482/343
2.5 km 329,544/335 -> 215,598/338 -34.6%
and over Los Angeles at 2.5 km, 761,648 -> 335,186: a breach of the
400,000 cap that no budget cell poses at, now under it. The +17 draws at
the whole-board pose is the price, it is a bound and not a measurement
(there are 17 objects), and it lands at 437/433 against caps of 460/455.
No cap moved; the budgets file gains one sentence saying that geometry
submitted is now a function of the camera, and only ever downward.
What it does not fix: it is culling, not level of detail — a chunk 1% on
screen draws 100% of its triangles, and the ground under San Francisco is
still `UNIFIED_FINE_METRES` at 300 m against that metro's own 45. Nothing
streams; build time and resident memory are unchanged. ~157,000 triangles
at the close pose are sky, and after this they are three quarters of what
is left. `minimap.ts` still samples 160 x 160 across the whole board.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
karti
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