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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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24
src/AcDream.App/Rendering/TerrainModernRenderer.cs
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@ -283,6 +283,27 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
// when wired, else the no-clip fallback (count 0 = ungated terrain).
BindClipUboBinding2();
// #108-residual: 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). GL backface culling
// evaluates the same per-triangle eye-side predicate at rasterization.
// LandblockMesh emits every triangle CCW in world XY seen from above
// (LandblockMeshTests winding pin), which the unified camera chain
// (CreateLookAt up=+Z + Numerics perspective) maps to CCW window
// winding from above / CW from below (TerrainCullOrientationTests) —
// so FrontFace(Ccw)+Cull(Back) keeps the top side and culls the
// underside. WB drew the whole world with culling DISABLED
// frame-globally (WB GameScene.cs:841 — an editor camera goes
// underground); inheriting that drew terrain DOUBLE-SIDED, and a
// below-grade eye (cellar ascent) saw the UNDERSIDE of the grade
// sheet through the exit-door aperture — the #108 grass window.
// Self-contained state per feedback_render_self_contained_gl_state;
// the frame-global CW + cull-off baseline is restored after the draw.
_gl.Enable(EnableCap.CullFace);
_gl.CullFace(TriangleFace.Back);
_gl.FrontFace(FrontFaceDirection.Ccw);
_gl.BindVertexArray(_globalVao);
_gl.MemoryBarrier(MemoryBarrierMask.CommandBarrierBit);
_gl.MultiDrawElementsIndirect(
@ -292,6 +313,9 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
(uint)sizeof(DrawElementsIndirectCommand));
_gl.BindVertexArray(0);
_gl.BindBuffer(GLEnum.DrawIndirectBuffer, 0);
_gl.FrontFace(FrontFaceDirection.CW);
_gl.Disable(EnableCap.CullFace);
}
public void Dispose()