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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

2
src/AcDream.App/Rendering/GameWindow.cs
View file

@ -1970,7 +1970,7 @@ public sealed class GameWindow : IDisposable
entity.Position = worldPos;
entity.Rotation = rot;
// Track remote-entity motion for stop detection. Only record the
// Track remote-entity motion for stop detection. Only record the
// timestamp when position moved MEANINGFULLY (> 0.05m). Updates
// that report the same position keep the old Time, so the
// TickAnimations check can see when motion last changed.

32
src/AcDream.App/Rendering/Shaders/terrain.vert
View file

@ -76,26 +76,30 @@ void main() {
// Derive which of the 4 cell corners this vertex represents from
// gl_VertexID % 6. The CPU-side LandblockMesh emits vertices in a
// specific order for each split direction; the table below must stay
// specific order for each split direction; the tables below must stay
// in lockstep with LandblockMesh.Build's SWtoNE/SEtoNW branches.
// 2026-04-21 fix: geometry re-derived to match ACE's ConstructPolygons
// convention. SWtoNE (cut BL→TR, y=x diagonal) now maps to the {BL,BR,TR}
// + {BL,TR,TL} triangle pair; SEtoNW (cut BR→TL, x+y=1 diagonal) maps to
// {BL,BR,TL} + {BR,TR,TL}.
int vIdx = gl_VertexID % 6;
int corner = 0;
if (splitDir == 0u) {
// SWtoNE order: BL, TL, BR, BR, TL, TR → corners 0, 3, 1, 1, 3, 2
// SWtoNE order: BL, BR, TR, BL, TR, TL → corners 0, 1, 2, 0, 2, 3
if (vIdx == 0) corner = 0;
else if (vIdx == 1) corner = 3;
else if (vIdx == 2) corner = 1;
else if (vIdx == 3) corner = 1;
else if (vIdx == 4) corner = 3;
else corner = 2;
} else {
// SEtoNW order: BL, TR, BR, BL, TL, TR → corners 0, 2, 1, 0, 3, 2
if (vIdx == 0) corner = 0;
else if (vIdx == 1) corner = 2;
else if (vIdx == 2) corner = 1;
else if (vIdx == 1) corner = 1;
else if (vIdx == 2) corner = 2;
else if (vIdx == 3) corner = 0;
else if (vIdx == 4) corner = 3;
else corner = 2;
else if (vIdx == 4) corner = 2;
else corner = 3;
} else {
// SEtoNW order: BL, BR, TL, BR, TR, TL → corners 0, 1, 3, 1, 2, 3
if (vIdx == 0) corner = 0;
else if (vIdx == 1) corner = 1;
else if (vIdx == 2) corner = 3;
else if (vIdx == 3) corner = 1;
else if (vIdx == 4) corner = 2;
else corner = 3;
}
vec2 baseUV;