test(geometry): measure the near-band 2/3 shortfall instead of asserting it in prose - #3413
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…ing it in prose This file's header states that a slab overlap inside the near-coplanar band "comes back as EXACTLY 2/3 of its true volume at every world scale", and that figure is load-bearing — it is why NEAR_BAND_FLOOR is 8 snap cells wide and why the API withholds inside it. Nothing measured it. This measures it through `intersection_tris` (the same triangles the trust gate sums before refusing them, since `intersection_solid` withholds and returns no volume), and pins the property that makes the ratio exact: the shortfall is a whole missing face PAIR, so it costs exactly a third regardless of the overlap's cross-section. A genuine wedge would move with the aspect ratio; sweeping 1x1, 1x2, 2x2 and 1x4 rejects that. Tolerance is 1e-9 from 4 cells up, where the corners are grid-aligned and the identity is exact, and 1e-3 at 1-2 cells, where the overlap is only one or two snap cells deep and a single grid-scale perturbation is a large fraction of the depth itself. Both bounds still reject a cross-section-dependent ratio. Refs #3353
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Included review availability: Your plan provides up to 2 included reviews per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe geometry test suite adds a raw divergence-theorem volume helper and two regression tests. The tests verify a consistent two-thirds volume ratio and confirm that both x-facing end caps are absent across varied cross-sections, depths, and tessellation levels. ChangesIntersection volume regression
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In `@rust/geometry/tests/clash_intersection_oracle.rs`:
- Around line 882-889: Update the explanatory comment above
raw_divergence_volume to match its /6.0 normalization: state that each opposite
face pair contributes 2V, all three pairs contribute 6V, and the four retained
side walls contribute 4V, yielding 2V/3. Preserve the explanation that this
remains 2/3 regardless of Ly/Lz.
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rust/geometry/tests/clash_intersection_oracle.rs
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…y cost The ratio test added alongside this measures the CONSEQUENCE of the near-band defect: the shortfall is exactly 2/3 and shape-invariant, so a whole face pair is missing. Which pair, and why, was still only traced from `classify.rs`'s keep/drop rules. This observes it directly: if both x end caps are dropped, every triangle `intersection_tris` returns belongs to one of the four side walls, so none should have an x-facing normal. The non-vacuity guard is by area rather than triangle count, deliberately. `intersection_tris` retriangulates the clipped region, so its output count is not a closed form of box_mesh's tessellation and I could not derive a safe lower bound for it. Area is invariant under retriangulation and the overlap's four side faces have a known analytic total, so "no x-facing normal" cannot pass by returning almost nothing. Degenerate triangles are excluded rather than classified: below the cross product magnitude floor there is no well-defined normal, and excluding them can only weaken the area guard, never hide an x-facing survivor. Refs #3353
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…eRabbit, #3413) The comment described the divergence-theorem form — `V` per opposite face pair, `3V` over three pairs, `/3` to recover `V` — while the code sums raw triple products and divides by `6.0`. In that normalisation a pair contributes `2V`, three pairs give `6V`, and the four surviving walls give `4V`, hence `4V/6 = 2V/3`. The 2/3 result is unchanged, which is why the test passes and the error hid in prose. Verified numerically: the triple-product sum over a closed box is exactly `6V`. CodeRabbit caught the inconsistency; its suggested diff corrected two phrases but left "the two faces' contributions combine to the enclosed volume `V`", which contradicts the corrected `6V` total. All three phrases are fixed here. Comment only — no change to the test logic, the tolerances, or the deliberately-mirrored NEAR_BAND_FLOOR constant. Refs #3353
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Merging. This is the right direction and I want to name why, because the title reads like the opposite. "Measure instead of asserting in prose" turns a load-bearing claim into a checked one. The 2/3 figure sizes I checked the assertion is not vacuous before merging: CodeRabbit CLI no findings, 30 CI lanes pass including the census. |
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Summary
clash_intersection_oracle.rsstates in prose that a slab overlap inside the near-coplanar band "comes back as EXACTLY 2/3 of its true volume at every world scale". That figure is load-bearing — it is the measured defect radius that sizesNEAR_BAND_FLOOR = 8 * SNAP_GRIDand the reason the API withholds inside the band. Nothing in the suite measures it.This measures it, and pins the property that makes it exact.
What the 2/3 actually is
For each opposite face pair of a box, the two faces' divergence contributions combine to the enclosed volume
V, with the reference-point terms cancelling within the pair. Three pairs give3V; dividing by 3 returnsV.Inside the band, classification drops both x end caps — A's cap is anti-parallel to B's, so
co_orientedis false and theIntersectionkeep rule drops it, while the B side drops its copy unconditionally underc_on_or_near_a— leaving the four side walls,2V, hence2V/3.So the shortfall is a whole missing face pair, which costs exactly one third regardless of the overlap's cross-section. A genuine wedge substitution would generally move with the aspect ratio. The sweep over
1x1,1x2,2x2,1x4is what distinguishes those, and it is why the test is not simply an equality check at one shape.I originally predicted the ratio would vary as
(Ly+Lz)/(3·Ly·Lz)and was wrong; the cancellation is per-pair, so it does not. That is now checked two independent ways and is what the test asserts.Measurement path
intersection_solidwithholds inside the band and returnsDegenerate, so it yields no volume. The test reads the same triangles the trust gate sums before refusing them, viakernel::mesh_bridge::intersection_tris— this file already reaches into that module for the f32-round-trip test.clash_solid_geom::tri_volumeispub(super)and unreachable fromtests/, so its summation is reproduced locally, about the world origin, matching what it does.Tolerance
1e-9from 4 cells up, where the box corners are exact multiples ofSNAP_GRIDand the identity is exact;1e-3at 1–2 cells, where the overlap is only one or two snap cells deep and a single grid-scale perturbation is a large fraction ofdepthitself. I did not want to assert a precision I have not observed at the tightest case. Both bounds still reject a cross-section-dependent ratio, which is the thing being tested.One narrower point, recorded in the test comment because it would be easy to over-generalise: the reference-point invariance here holds because the removed faces are themselves a complete opposite pair, so their position-dependent terms cancel against each other. Drop a single face instead and the value does depend on where the box sits.
Testing
The Rust side was not run locally — this environment has no disk headroom for a
cargorebuild, so CI is the check. That is the same route #3403 and #3400 took today.If CI fails, that is itself informative rather than a nuisance: it would mean the missing-pair account of the 2/3 is wrong, and I would rather learn that from this test than from a fifth fix attempt built on it. I will convert or withdraw the PR accordingly rather than loosening the assertion to make it pass.
Refs #3353
Summary by CodeRabbit