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fix #108-residual (root cause): terrain drew DOUBLE-SIDED - port retail landPolysDraw eye-side gate as terrain backface cull

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The cellar-ascent grass window was the UNDERSIDE of the z~94 grade
sheet. Retail terrain is single-sided: ACRender::landPolysDraw
(0x006b7040) draws each land triangle ONLY when the camera is on the
POSITIVE (upper) side of its plane (Plane::which_side2 vs
Render::FrameCurrent, zFightTerrainAdjust bias) - a below-grade eye
gets NO terrain, so retail shows sky through the cellar door.

We inherited WB's frame-global cull DISABLE (WB GameScene.cs:841 - an
editor camera goes underground by design) and TerrainModernRenderer.Draw
set no cull state of its own -> terrain rasterized both sides. From a
below-grade eye every aperture sight-ray RISES, so the only 'terrain'
it can see is the grade sheet's underside - which painted the exit-door
aperture (the landscape slice's 2D NDC clip planes (nx,ny,0,dw) have no
depth axis and cannot exclude between-eye-and-portal geometry) and slid
off the door exactly as the eye crossed grade. Membership/viewer was
exonerated by the harness in the previous commit.

Fix: TerrainModernRenderer.Draw owns its cull state (the 7th
self-contained-GL-state instance): Enable(CullFace) + CullFace(Back) +
FrontFace(Ccw), set -> draw -> restore the frame-global CW + cull-off
baseline. GL backface culling evaluates retail's per-triangle eye-side
predicate at rasterization; no shader change.

Pins:
- LandblockMeshTests.Build_AllTriangles_WindCounterClockwiseInWorldXY:
  every emitted triangle CCW in world XY across both FSplitNESW split
  directions - the winding invariant culling depends on.
- TerrainCullOrientationTests: under the production camera convention
  (LookAt up=+Z, Numerics perspective) an up-facing triangle winds CCW
  in window space from above (kept) and CW from below (culled) - guards
  FrontFace inversion, which would blank terrain from above.

Oracle note: retail's through-portal clip has NO portal-face near plane
(PView::GetClip / Render::set_view install edge planes only); nearer-
than-portal exclusion comes from the eye-side cull + cell-level
admission. No register row: this PORTS the retail mechanism, retiring
an undocumented WB-heritage deviation.

Gate pending: cellar climb (grass window gone) + outdoor sanity glance
(terrain intact from above).

Suites: App 263+1skip / Core 1443+2skip / UI 420 / Net 294.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
Erik 2026-06-12 22:05:31 +02:00
parent 007af1391c
commit 96a425a9a5
4 changed files with 180 additions and 19 deletions
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35
tests/AcDream.Core.Tests/Terrain/LandblockMeshTests.cs
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@ -169,6 +169,41 @@ public class LandblockMeshTests
Assert.True(cache.Count >= 2, $"Expected mix of palette codes, got {cache.Count}");
}
[Fact]
public void Build_AllTriangles_WindCounterClockwiseInWorldXY()
{
// #108-residual winding pin: TerrainModernRenderer enables backface
// culling with FrontFace(Ccw) — the GL port of retail's single-sided
// terrain (ACRender::landPolysDraw 0x006b7040 draws a land triangle
// only when the eye is on the POSITIVE side of its plane). That cull
// is only correct if EVERY emitted triangle winds the same way:
// counter-clockwise in world XY viewed from above (+Z toward the
// viewer), i.e. cross2D(v1-v0, v2-v0) > 0. Varied heights + several
// landblock coords exercise both FSplitNESW split directions across
// the 64 cells. A future emission-order change that flips any
// triangle would silently punch terrain holes under culling.
var block = BuildFlatLandBlock();
for (int i = 0; i < 81; i++)
block.Height[i] = (byte)((i * 37) % 64); // varied, deterministic slopes
foreach (var (lbx, lby) in new[] { (0u, 0u), (0xA9u, 0xB4u), (3u, 7u) })
{
var cache = new Dictionary<uint, SurfaceInfo>();
var mesh = LandblockMesh.Build(block, lbx, lby, IdentityHeightTable, MakeContext(), cache);
for (int t = 0; t < mesh.Indices.Length; t += 3)
{
var p0 = mesh.Vertices[mesh.Indices[t + 0]].Position;
var p1 = mesh.Vertices[mesh.Indices[t + 1]].Position;
var p2 = mesh.Vertices[mesh.Indices[t + 2]].Position;
float crossZ = (p1.X - p0.X) * (p2.Y - p0.Y) - (p1.Y - p0.Y) * (p2.X - p0.X);
Assert.True(crossZ > 0f,
$"lb=({lbx},{lby}) triangle {t / 3} winds CW in world XY (crossZ={crossZ}) — " +
"backface culling in TerrainModernRenderer would cull its TOP side");
}
}
}
[Fact]
public void Build_HeightmapPackedAsXMajor_NotYMajor()
{