Adds the GPU mechanism to clip drawing to a per-cell screen-space convex
region via gl_ClipDistance, consumed by the mesh + terrain vertex shaders.
This is the MECHANISM only — every instance defaults to slot 0 (no-clip /
pass-all) and terrain to count 0, so the running game renders IDENTICALLY to
pre-U.3 (verified: offline launch compiles both shaders and reaches steady
state; no GL errors). U.4 populates real clip data from portal visibility.
Binding contract (define once, both sides obey):
- mesh_modern.vert: SSBO binding=2 CellClip[] (shared per-frame regions, slot 0
reserved no-clip) + SSBO binding=3 uint[] per-instance slot, indexed by the
IDENTICAL gl_BaseInstanceARB+gl_InstanceID used for binding=0. binding=0/1
untouched.
- terrain_modern.vert: UBO binding=2 TerrainClip { int count; vec4 planes[8]; }
for the single OutsideView region (UBO namespace; SceneLighting is UBO
binding=1, so binding=2 is free and does not collide with the mesh SSBO
binding=2). count 0 = ungated.
- Both redeclare out gl_PerVertex { vec4 gl_Position; float gl_ClipDistance[8]; }
and set unused planes (i >= count) to +1.0 so they pass everything.
CellClip std430 layout (144 bytes/slot): count@0, 3 pad uints@4/8/12,
planes[8]@16 (vec4 stride 16). Terrain UBO std140: count@0 (padded to 16),
planes[8]@16 → 144 bytes. Verified by ClipFrameLayoutTests (8 new tests).
Pieces:
- ClipFrame: per-frame container + uploader for the SHARED clip data (binding=2
SSBO + terrain UBO). NoClip() = slot 0 + terrain count 0. AppendSlot /
SetTerrainClip pack std430/std140 bytes for U.4. UploadShared binds both.
- WbDrawDispatcher + EnvCellRenderer: each owns its binding=3 zero buffer
(all-zeros sized to its instance count → slot 0), re-binds binding=2 from the
shared ClipFrame id (or an internal no-clip fallback if unwired) before MDI.
gl_ClipDistance is per-vertex, so the single glMultiDrawElementsIndirect per
group is preserved — no draw splitting.
- TerrainModernRenderer: binds the terrain clip UBO (shared or no-clip fallback)
before its draw.
- GameWindow: glEnable(GL_CLIP_DISTANCE0..7) once at init (unused planes pass-all
so always-on avoids per-draw thrash); per frame builds ClipFrame.NoClip(),
UploadShared, and hands the buffer ids to the three renderers (tiny diff; U.4
swaps NoClip() for the real portal-visibility frame).
Gate: dotnet build green; App suite 134/134; offline launch confirms both
shaders compile + link with no GL errors.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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