46 lines
2.8 KiB
Markdown
46 lines
2.8 KiB
Markdown
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---
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name: project-island-terrain-design
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description: Design decisions for the downloadable-island terrain system (replaces Unreal Landscape)
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metadata:
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node_type: memory
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type: project
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originSessionId: 38ffbfc6-93fe-4670-9015-1be594a56944
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---
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Game concept: like visiting Animal Crossing islands — client downloads an island
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(heightfield + weightmaps) from a server and builds terrain at runtime in a
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packaged build. Stock Landscape is unusable (editor-only construction). Terrain
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is smooth sculpted hills (heightfield, NOT voxels, NOT tile-based). Islands can
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be large (multi-km); ~5s load budget is fine.
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Decisions as of 2026-06-11:
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- **Patches**: 256×256 quads = 257×257 vertices, driven by a 257×257 non-mipped
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height texture region (possibly a sub-rect of one big island texture — left open).
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- **LOD**: 3 levels (interiors 256/128/64 segments), but patch *edges always stay
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at full 257-vertex resolution* — only interiors decimate. Any two neighbor LODs
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fit seamlessly by construction; no stitching, no neighbor bookkeeping. Transition
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ring fans coarse interior to full-res edge (the tricky indexing part).
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- **One shared mesh**: a single flat (z=0) UStaticMesh with 3 LODs, built at
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runtime via FMeshDescription + BuildFromMeshDescriptions(bFastBuild=true),
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cached lazily in a GameInstanceSubsystem. Rendered via HISM instances (one per
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patch); HISM auto per-instance LOD selection is safe thanks to edge-preserving LODs.
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- **Displacement**: material WPO samples height texture (explicit mip 0, point
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filter, +half-texel offset). UVs NOT stored in mesh — derived from world position
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minus island origin (material params). No per-instance custom data needed.
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Pixel shader: splat weightmaps + heightfield-derived normal map (those DO get mips;
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weightmaps sized 4096², not 4097, so BC compression works).
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- **Collision**: custom UPrimitiveComponent wrapping Chaos::FHeightField (no cook,
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~ms build, built from the same uint16 samples; crib from LandscapeCollision.cpp:1049).
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Nobody public has done this (verified via Sourcegraph 2026-06); trimesh PMC path is
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scaffolding only. Heightfield wins on memory (34MB vs GBs at 4097²) and query speed.
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- **Height data**: uint16 grid, Landscape convention ((worldZ/scale)+32768).
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- **Lumen**: project doesn't use it (no RendererSettings overrides; default GI=None).
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Flat-proxy WPO terrain is incompatible-ish with Lumen — accepted, documented risk.
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- Heightfield texture is NOT mipmapped (LODs sample full-res at sparse points —
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exact vs collision; Landscape instead resamples mips + geomorphs, [[its choice]]).
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Code so far: `Source/Integration/TerrainPatch.h/cpp` — USTRUCT FlxTerrainPatch with
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stubbed static MakePatch() (returns UStaticMesh*) and editor-only SavePatch()
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(writes uasset for debugging; game always generates dynamically).
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