PolyTerrain materials and bake

The MPD owns materials, channels, layers, and build variants. Bake turns Base + modifiers into compiled section and weight assets through one evaluator plus the Transformer Pipeline.

MPD four sections

Asset: .polypartitiondefinition (naming tip: MPD_<Name>).

Material

Setting	Purpose
Material hint	Atom material for preview / compiled display (e.g. `PolyPartitionSplat`)
Channel texel size	World meters per weight texel (smaller = finer)
Weight channels	Name, format (typically R8), bake-to-runtime, debug color
Physical material channels	Weight channel → PhysX material mapping
Default physical material	Fallback when no positive weight wins

Undeclared channels can be painted in the editor but are not baked to runtime textures. Display splat supports up to 16 packed channels (four RGBA planes); more than 16 needs a custom material consuming the slice-array path.

Default gem material: Gems/PolyTerrain/Assets/Materials/PolyTerrain/PolyPartitionSplat_Default.material. Material Canvas nodes under PolyTerrain include Channel Sample, Channel Inspector, and Sample Vt * helpers.

Modifiers

Ordered Priority Layer names (index 0 evaluates first) and a default sub-priority step (e.g. 10) for insertions.

Build

Setting	Purpose
Variants	Named compiled configs (e.g. HighEnd / LowEnd) with optional platform maps
Transformer Pipeline	Ordered stages after evaluation (asset or embedded)

Preview

Max section complexity (vertex budget for Preview / Refine refusal) and optional lighter preview pipeline.

MPD hot-reload clears preview caches and dirties all sections. Optional partition Auto build on save runs incremental bake on level save.

Transformer Pipeline stages

Typical compiled order:

Evaluated mesh + weights
 → StaticMesh (LOD chain)
 → Subsection (optional split)
 → Skirt (border walls / seam cover)
 → Collision (triangle / convex)
 → FarField (HLOD proxy)
 → WS Actor Properties (streaming metadata)
 → VT (optional metadata / write hints)

Stage	Preview	Compiled	Notes
StaticMesh	yes	yes	LOD count from `lodDistances`; border verts locked when simplifying
Subsection	optional	yes	Can respect Subsection Hint AABBs
Skirt	yes	yes	Covers Refine / density seams; optional thickness
Collision	simplified	yes	Triangle budget, dominant-channel or per-chunk material blend, optional convex hulls
FarField	no	yes	Coarse mesh for World Streaming HLOD
WS properties	no	yes	Cell / layer metadata for PrefabProcessor
VT	metadata	yes	Hints consumed when `VtEnabled`

LOD distance ladder: when distances are a strict ascending ladder, runtime can pin LOD by camera distance; otherwise Atom screen-coverage selection applies.

Bake path

1 Build Compiled Sections (editor)
 → export .polypartition (skip write if bytes unchanged)
2 PolyPartitionBuilder (AssetProcessor)
 → evaluate full stack + pipeline
 → .polypartitionsection + .polypartitionweights
3 PolyPartitionPrefabProcessor
 → inject runtime section (+ collision) entities
 → optional WorldStreamingMember (before WS processors)

Flag	Behavior
Incremental `true`	Skip source rewrite when snapshot matches disk; AP still driven by product needs
Incremental `false`	Force export

ContentHash includes Base, modifier params, MPD material/channel/physics settings, and hash-sensitive Transformer fields. Changing only Preview-only UI state does not invalidate bake.

Physics materials from weights

Dominant channel: per-triangle material index at cook time.
Per-chunk blend: optional chunk size on Collision Transformer; runtime can reblend friction when weight rewrite is on.
Convex hulls: optional V-HACD path; per-hull materials can follow rewrite when CollisionConvexReweightEnabled is on.

See Optional features for runtime weight rewrite gates.

Checklist before Play

MPD layers and channels match modifier / paint names.
Build variant includes StaticMesh + Collision (and Skirt for multi-section).
AssetProcessor finished without builder errors.
PrefabProcessor expanded runtime entities (or you placed Runtime Section components manually).
For WS levels, section entities are in the expected cells (Focus Load WS Cells helps in editor).