VoxelForge/CODEX-TASK-004-minmax-radius-envelope.md

Codex task 004 — two box verdicts assume MinRadius ≤ MaxRadius; a third one already doesn't

Owner: Codex (Model Luna, xHigh) · Orchestrator: Claude · Branch: experimental Status: specified, not started Kind: ⚠️ correctness of a box verdict. Same class as task 003. Small fix, closes a class.

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Why this exists

Three operators roll a primitive radius from a hash between two designer-set params:

Out.R   = FMath::Lerp(MinRadius,          MaxRadius,          hash01);   // FGridColumnMod   ~1129
Out.R   = FMath::Lerp(P.ShaftMinRadius,   P.ShaftMaxRadius,   hash01);   // FShaftFieldSource ~1560
Out.Rxy = FMath::Lerp(P.IslandMinRadius,  P.IslandMaxRadius,  hash01);   // FIslandBlobSource ~1850

FMath::Lerp(A, B, t) with t ∈ [0,1] lands anywhere in [min(A,B), max(A,B)] — it does not require A ≤ B.

Each op's EffectOverBox then sweeps a range of lattice cells around the query box, padded by the largest radius a cell could hold, and tests each rolled primitive exactly. The pad decides which cells are looked at at all, so a pad smaller than the true maximum radius means cells are never examined, their primitives are never tested, and the op reports Identity for a box that its own Eval will carve or fill.

op	pad used for the cell sweep	correct?
FGridColumnMod ~1086	FMath::Max(MaxRadius, 0.0f) + ColBlend	⛔ exposed
FShaftFieldSource ~1436	FMath::Max(P.ShaftMaxRadius, P.ConnectorRadius) + ExtraReach	⛔ exposed
FIslandBlobSource ~1803	const float MaxR = FMath::Max(P.IslandMinRadius, P.IslandMaxRadius);	✅ already correct

The third one is the point. Someone hit this exact concern while writing the island source and guarded it. The other two shipped without the guard. This task makes the three consistent.

How exploitable, stated honestly

The shipped defaults are correctly ordered (2/5, 2/7, 5/11), so nothing is broken out of the box. It needs a mis-ordered asset value — ColumnMinRadius = 8, ColumnMaxRadius = 4.

Nothing prevents that. The UPROPERTY metas carry ClampMin = "1.0", which is a per-property floor; Unreal has no declarative way to say "must be ≤ that other property". And ColumnMinRadius is also settable per-room through UVoxelTerrainOpDefinition, so it is not only the strate asset.

What makes it worth the five lines: when it does happen, the failure is invisible and maddening. Eval still draws the fat column perfectly, so every tile that gets meshed looks correct; only the tiles the classifier skipped are missing — no geometry, no collision, in a world that otherwise looks right.

The fix — five lines, and one thing you must NOT do

Make each pad use the true envelope:

// FGridColumnMod ~1086
const float Reach = FMath::Max3(MinRadius, MaxRadius, 0.0f) + ColBlend;

// FShaftFieldSource ~1436
const float Pad = FMath::Max3(P.ShaftMinRadius, P.ShaftMaxRadius, P.ConnectorRadius) + ExtraReach;

(Use whatever spelling is idiomatic here — check that FMath::Max3 is already used in this codebase before reaching for it; nested FMath::Max is fine and matches FIslandBlobSource's existing line.)

⛔ DO NOT "fix it properly" by normalising the params

The tempting larger fix — swap Min/Max at resolution time so Min ≤ Max always — is wrong and will break the build's tests. Eval computes Lerp(Min, Max, t); swapping the endpoints maps the same hash t to a different radius for the same cell. That changes generated geometry and breaks the eight bit-for-bit equivalence tests against the switch path.

Only the BOUND may become conservative. Eval stays byte-identical. This is the same rule as task 003 and the same reason.

⚠️ Invariants

1. No Eval, RollColumn, RollShaft, GetCells, or GetCellsAt body may change. If your diff touches one, you have the wrong site — stop and say so.
2. Do not touch FIslandBlobSource. It is already correct and is the reference for this fix.
3. The change direction is strictly conservative: a wider sweep examines more cells, so a verdict can only move from Identity toward CarveOnly/FillOnly, never the reverse. Do not add any compensating tightening.
4. Comments are French + English; match the surrounding file. Say why the envelope is max(Min, Max) and not Max — the next reader must not "simplify" it back.

Acceptance

• git diff --stat shows exactly one file: Source/VoxelForge/Private/VoxelDensityOpStack.cpp, and a handful of lines.
• The three ops now agree on the pattern.
• After the build: every box-verdict line and every equivalence test is unchanged, because the shipped defaults are correctly ordered and the envelope only differs when they are not. A change in any of those numbers means the diff did something it should not have. That is this task's whole acceptance signal — a no-op at defaults is the expected, correct result.

Notes for the reviewer (Claude)

• Confirm both pads changed and FIslandBlobSource did not.
• Confirm no Lerp argument order was touched anywhere — that is the failure mode that would look like a tidy-up and silently change the world.