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perf: lot size is a ladder the camera climbs, not a constant

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`DETAIL_LOT_METRES = 160` was one number for a board that stopped being
looked at from one distance, and it was wrong at both ends of the range
it is now looked at from. Measured with `scripts/cost-at.mjs` on the
merged board against a 400,000-triangle cap:

| pose | before | after | lot |
| --- | --- | --- | --- |
| Los Angeles, 13 km | **688,536** | 343,372 | 400 m |
| Los Angeles, 12.5 km | **678,546** | 343,372 | 400 m |
| Los Angeles, 11 km | **654,656** | 390,602 | 250 m |
| Los Angeles, 9 km | **635,806** | 380,762 | 250 m |
| **Los Angeles, 7.7 km** | **621,866** | **375,196** | 250 m |
| Los Angeles, 2.5 km | **415,780** | 345,340 | 200 m |
| Los Angeles, 0.9 km | 297,148 | 363,454 | 40 m |
| San Francisco, 7.7 km | 222,286 | 300,782 | 80 m |
| San Francisco, 2.5 km | 216,292 | 279,158 | 80 m |
| San Francisco, 0.6 km | 200,114 | 347,830 | 40 m |
| San Francisco, 400 km | 355,763 | 355,763 | — |
| San Francisco, 45 km | 270,377 | 270,377 | — |

**Six poses were over the cap and five of them had never been measured.**
Only the 7.7 km one was on record; 2.5 km over Los Angeles was at
415,780, and the whole band from 9 km up to where `DETAIL_STANDOFF_M`
switches the metros off at 13.3 km ran 635,806 to 688,536, because the
frustum there holds essentially the entire Southland. None of them is
over now. San Francisco goes the other way — 4.2x the buildings at the
poses the complaint was about, and 15,613 at the closest one against
849. No cap moved.

**Zero new draw calls, and that is a bound rather than a sample.** It is
the same `InstancedMesh`, the same geometry and the same material; a
district is built at 40, 80, 160, 200, 250 and 400 m, all six rungs live
in the store, and the live buffers hold whichever one the board chose.
Draws are unchanged to the digit at every pose above. A test asserts the
mesh identity across four rung changes.

The lever is a **lot count, not a lot size**, because a lot size is the
wrong thing to hang on the camera: at the same 22.6 km of engine
stand-off San Francisco has 1,987 lots in frustum and Los Angeles has
23,000, so any rule in metres of lot or kilometres of reach is a
different bill in the two cities. The board packs the finest rung whose
*visible* sum fits 19,000 lots, which is 400,000 minus the 184,000 to
205,000 triangles everything-but-the-city measured at across the whole
band this runs in — flat to 10%, which is why there is no stand-off ramp
and why the one that was here first came out.

Costs, honestly. `createBlocks` goes 264 ms -> 1,236 ms and its store
4.7 MiB -> 18.9 MiB, because every rung is walked and kept; the live
instance buffers get *smaller*, 59,166 -> 19,000, since the mesh no
longer has to be able to draw a rung nothing can afford. A rung change
pops — each rung is its own survey, not a subdivision — and cross-fading
would cost the draw call this board does not have. Los Angeles at 7.7 km
is visibly thinner than the 621,866-triangle version it replaces.

`TODO.md`'s LA section is rewritten around what is left: no budget cell
stands at any of these poses, which was the first item on it and still
is. 1,723 tests.
This commit is contained in:
Kartios
2026-08-24 20:08:13 -07:00
parent 1049f942b8
commit 687c34d123
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@@ -879,48 +879,58 @@ The content programme this opens up — what a single cohesive California is sti
missing, and in what order — is **`CALIFORNIA.md`**, kept separate because none
of it needs a new mechanism and all of it can be worked in parallel.
## Los Angeles at 7.7 km is over the triangle cap, and no budget cell stands there
## Los Angeles at 7.7 km was over the triangle cap — done, and here is what is left
Found by the review of the culling work, 2026-08-24, and recorded here because
the commit record for that work names a *different* LA pose — the 110 km one,
which after the terrain split measures 310,841 and is comfortably under. The pose
that is over the cap is not in any commit message, and this is the correction.
Found by the review of the culling work, 2026-08-24, and fixed the same day by
`DETAIL_LOT_TIERS`: a detail district is built at six lot sizes and the board
draws whichever of them its *visible* lots fit a 19,000-lot budget at. Measured
with `scripts/cost-at.mjs` on the merged board, 34.05, -118.24:
Measured with `scripts/cost-at.mjs` on the merged board at 34.05, -118.24:
| seek | before culling | after culling | after the ladder | lot | cap |
| --- | --- | --- | --- | --- | --- |
| 110 km | 341,803 | 310,841 | 314,265 | — | 400,000 |
| 13 km | — | **688,536** | **343,372** | 400 m | 400,000 |
| 12.5 km | — | **678,546** | **343,372** | 400 m | 400,000 |
| 11 km | — | **654,656** | **390,602** | 250 m | 400,000 |
| 9 km | — | **635,806** | **380,762** | 250 m | 400,000 |
| **7.7 km** | **773,282** | **604,332 / 621,866** | **375,196** | 250 m | 400,000 |
| 2.5 km | 761,648 | **415,780** | **345,340** | 200 m | 400,000 |
| 1.2 km | — | 311,564 (160 m either way) | 311,564 | 160 m | 400,000 |
| 0.9 km | — | 297,148 | 363,454 | 40 m | 400,000 |
| stand-off | before culling | after culling | cap |
| --- | --- | --- | --- |
| 110 km | 341,803 | 310,841 | 400,000 |
| **7.7 km** | **773,282** | **604,332** | **400,000** |
| 2.5 km | 761,648 | 335,186 | 400,000 |
The 604,332 in the 7.7 km row is the figure this section was written with; the
same pose re-measured on the day of the fix was 621,866, and both are the same
defect. **Six rows were over the cap and only one of them had ever been named.**
2.5 km was over at 415,780, and the whole band from about 9 km up to where
`DETAIL_STANDOFF_M` switches the metros off at 13.3 km was 635,806 to 688,536 —
the frustum there holds essentially the entire Southland. The 400 m rung on the
ladder exists for the top of that band; every other rung is under 260 m and
keeps its street lattice.
So the culling work **closed the 2.5 km breach and reduced the 7.7 km one by
22%**, and 7.7 km is still 51% over. It is a pre-existing defect that this work
improved rather than one it introduced — but it is live, it is reachable from the
board's own chapters, and it was never measured because `california-one` poses at
`california-overview` and nothing else does.
The 0.9 km row goes the other way, and is the near-field half of the same fault:
the board was drawing 11,306 lots at 160 m where it had room for 17,929 at 40 m.
**Frustum culling cannot fix it.** At that stand-off 40,025 detail lots are
genuinely inside the frustum. The Southland is a basin ninety kilometres across
and at 7.7 km you are looking down the length of it; there is no camera geometry
that makes those buildings not on screen. What is wrong is the *lot size*: 160 m
is chosen for a metro seen from its own stand-off, and at 7.7 km over the basin
the board is drawing a hundred kilometres of city at that density.
**Frustum culling could not fix any of it.** At those stand-offs the lots are
genuinely inside the frustum: the Southland is a basin ninety kilometres across
and you are looking down the length of it. What was wrong was the *lot size*,
which was one number for a board that is no longer looked at from one distance.
The fix is a lot-size level of detail — the same mechanism the near-field 40 m
tier needs, pointing the other way. Two shapes, and the choice needs measuring
rather than arguing:
### What is left
- **A stride.** Pack every Nth instance of a range, N from the stand-off. One
copy loop, no new geometry, no new draw call — the store already holds every
instance. The risk is shimmer: instance order within a range is lattice order,
so a stride is a spatially regular thinning and regular thinning of a grid is
exactly what aliases.
- **A second, coarser instance set per district**, built at 480 m and swapped in
by stand-off. No shimmer, and it costs build time and memory for lots that are
usually not drawn.
**Two things to do first, in this order.** Add a budget cell that stands at that
pose — `no budget cell stands there` is the actual defect, and a cap nobody
measures is not a cap. Expect it to fail on the day it lands; that is the point,
and it must not be answered by raising a number. Then fix it.
- **No budget cell stands at any of these poses.** This was the first item in the
original list and it is still open: `performance-budget.mjs` measures
`california-one` at `california-overview` and nowhere else, and a cap nobody
measures is not a cap. The ladder is now the only thing holding these poses
under 400,000 and nothing in CI would notice if it stopped.
- **The rung change pops.** Each rung is an independent survey — a different
lattice, different addresses, different heights — so stepping from 250 m to
200 m replaces the city rather than subdividing it. It lands during camera
motion and a still board never re-lots, which is what makes it liveable, but
it is visible if you are watching for it. A cross-fade is the obvious answer
and it costs a draw call, which this board does not have.
- **The ladder costs about a second of boot.** `createBlocks` on the merged
board went from 264 ms to 1,236 ms and its store from 4.7 MiB to 18.9 MiB,
because every rung of every district is walked and kept. The live instance
buffers got *smaller* — 59,166 instances to 19,000 — so this is boot and
memory, not frame time. Building the fine rungs lazily, on the first pose that
asks for one, would move most of it off the boot path.