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fix(terrain): align per-cell triangle geometry with ACE's ConstructPolygons convention

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Our LandblockMesh, terrain.vert corner tables, and TerrainSurface.SampleZ
used the OPPOSITE diagonal for each CellSplitDirection enum value from
what ACE (and the decompiled retail client at FUN_00532a50) picks for the
same sign bit. Same formula, same sign-bit mapping, inverted geometry.

Symptom: remote players rendered at server-broadcast Z hovered or clipped
by up to ~1m on sloped cells. Flat cells masked the bug because all four
corner heights were equal so any triangle pair returned the same Z. Live
diagnostic confirmed +0.79m hover on cell (7,5) at lb(AA,B4) — a ~20°
slope — while flat neighbors agreed to floating-point noise.

Three coordinated edits so CPU mesh + GPU corner lookup + CPU sampler all
agree on the retail geometry:
 - LandblockMesh: SWtoNE branch now emits {BL,BR,TR}+{BL,TR,TL} (y=x cut),
   SEtoNW emits {BL,BR,TL}+{BR,TR,TL} (x+y=1 cut).
 - terrain.vert: corner-index tables updated to match.
 - TerrainSurface.SampleZ: swapped the two branches' interpolation.

After the fix, 19 live DIAG samples across flat + two slope transitions
all land within 0.01m of server Z. Staircase pattern during remote motion
on slopes is a separate bug (no per-frame collision resolution) and will
be addressed via the transition/FindValidPosition port.

Cross-verified against: ACE LandblockStruct.ConstructPolygons lines 221-
244, decompiled retail FUN_00532a50 (chunk_00530000.c:2235), ClientReference
IsSWtoNECut (tests/AcDream.Core.Tests/Terrain/ClientReference.cs).

Updated test SplitDirection_TerrainSurface_AgreesWith_TerrainBlending
with corrected expectations (Z values swap between the two branches).
All 717 tests green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Erik 2026-04-21 13:20:59 +02:00
parent beffdf477e
commit 56975f8919
5 changed files with 96 additions and 71 deletions
Download patch file Download diff file Expand all files Collapse all files

35
tests/AcDream.Core.Tests/Terrain/ClientConformanceTests.cs
View file

@ -79,7 +79,7 @@ public class ClientConformanceTests
public void SplitDirection_TerrainSurface_AgreesWith_TerrainBlending()
{
// Build an asymmetric heightmap where the two split directions
// produce different Z at the cell center (0.5, 0.5).
// produce different Z off-center.
// Heights: BL=0, BR=100, TL=0, TR=0 (steep slope along X only at Y=0)
var heights = new byte[81];
var heightTable = new float[256];
@ -91,32 +91,33 @@ public class ClientConformanceTests
heights[0 * 9 + 1] = 0; // TL
heights[1 * 9 + 1] = 0; // TR
// Sample at cell center (12, 12) = (0.5, 0.5) in cell coords.
// For SWtoNE split (tx+ty=1 boundary): both triangles give Z=50 at center.
// For SEtoNW split (ty=tx boundary): both triangles give Z=50 at center.
// So at exact center both agree. Sample at (18, 6) = (0.75, 0.25) instead.
// SWtoNE: tx+ty=1.0 → tx=0.75, ty=0.25, 0.75+0.25=1.0 → boundary
// Use (20, 4) = (0.833, 0.167): tx+ty=1.0 → still boundary
// Use (20, 2) = (0.833, 0.083): tx+ty=0.917 < 1 → BL+BR+TL triangle
// Z = 0 + (100-0)*0.833 + (0-0)*0.083 = 83.3
// For SEtoNW: ty=0.083 < tx=0.833 → BL+BR+TR triangle
// Z = 0 + (100-0)*0.833 + (0-100)*0.083 = 83.3 - 8.3 = 75.0
// These differ! So we can distinguish.
// Sample at (20, 2) = (0.833, 0.083) in cell-local coords.
//
// 2026-04-21 fix: matched to ACE's ConstructPolygons geometry.
// Our enum values now mean:
// SWtoNE → diagonal BL→TR (y=x), triangles {BL,BR,TR} + {BL,TR,TL}
// SEtoNW → diagonal BR→TL (x+y=1), triangles {BL,BR,TL} + {BR,TR,TL}
//
// At (tx=0.833, ty=0.083):
// SWtoNE: tx > ty → {BL,BR,TR} triangle
// Z = hBL + tx*(hBR-hBL) + ty*(hTR-hBR) = 0.833*100 + 0.083*(-100) ≈ 75.0
// SEtoNW: tx+ty=0.917 ≤ 1 → {BL,BR,TL} triangle
// Z = hBL + tx*(hBR-hBL) + ty*(hTL-hBL) = 0.833*100 + 0 ≈ 83.3
// Expectations swapped vs pre-fix.
// Use landblock (0,0) and check what the client says the split is.
bool clientSplit = ClientReference.IsSWtoNECut(0, 0);
var surface = new TerrainSurface(heights, heightTable, 0, 0);
float z = surface.SampleZ(20f, 2f);
if (clientSplit)
{
// SWtoNE → BL+BR+TL triangle at (0.833, 0.083) → Z ≈ 83.3
Assert.InRange(z, 82f, 85f);
// SWtoNE → {BL,BR,TR} triangle at (0.833, 0.083) → Z ≈ 75.0
Assert.InRange(z, 74f, 77f);
}
else
{
// SEtoNW → BL+BR+TR triangle at (0.833, 0.083) → Z ≈ 75.0
Assert.InRange(z, 74f, 77f);
// SEtoNW → {BL,BR,TL} triangle at (0.833, 0.083) → Z ≈ 83.3
Assert.InRange(z, 82f, 85f);
}
}