World Streaming HLOD

HLOD provides distant proxy cells so the world stays readable when content cells are Unloaded. Proxies hide when child content activates and reappear before content unloads.

Declare an HLOD layer

Add WorldStreamingHlodLayerDesc entries on WorldConfig (m_hlodLayers).

Field	Default	Description
`m_hlodLayerId` / `m_name`	—	Identity
`m_sourceGridLevel` → `m_targetGridLevel`	0 → 1	Which grid’s content to aggregate, and which coarser grid hosts proxies
`m_loadingRange`	2048 m	Proxy streaming range; resolved independently only when `Runtime/EnforceHlodLoadingRange` is on
`m_generationPolicyId`	0 (Merge)	Policies 0–4 below
`m_requiresRegionMask`	`true`	`false` = indivisible whole-block proxy (hides only when all child cells activate)
`m_mergeAcrossContentLayers` + `m_mergedContentLayerId`	off	Merge across layers into one proxy layer id
`m_alwaysLoaded`	`false`	Proxy stays resident; never unloaded by distance
`m_fallbackPolicyId`	none (`0xFFFFFFFF`)	Retry once with fallback when primary produces nothing

Exclude individual entities from proxy geometry with ExcludeFromHlod on WorldStreamingMemberComponent (doors, destructibles, and so on).

Five generation policies

Id	Policy	Product	Fit
0	Merge	Per-material merged Atom Model	Near / mid distance
1	Simplify	Weld-decimate reduced mesh	Mid distance
2	Impostor	CPU atlas + three orthogonal billboards	Cheap far distance
3	OctahedralImpostor	CPU N×N octahedral atlas + single card	Far, view-correct
4	GpuOctahedralImpostor	GPU capture albedo+normal(+depth); relightable	Highest quality; missing atlas falls back to 3

Runtime knobs: HlodWarmupFrames (default 0), EnforceHlodLoadingRange (default false), EnableHlodDowngradeUnderBudget (default false).

Material switches for octahedral materials (when present): octahedral.tileInset / blendTiles / useParallax / parallaxScale.

Show / hide at runtime

Distant unloaded content shows proxy cells.
When child content reaches Activated, matching proxy regions hide (m_requiresRegionMask) or the whole block waits until all children activate.
On unload, proxies appear before content unloads to avoid holes.

Fragment visibility must reach Atom:

Set Bridge/AtomMode to Bridge (recommended for rendering projects), or
Handle OnHlodFragmentVisibilityChanged yourself.

With default AtomMode = Off, decisions still resolve but mesh visibility is not driven automatically. See Bridges.

Authoring and bake notes

Declare HLOD layers before bake; rebake after policy or source/target grid changes.
HLOD bake failure leaves a hole and does not block content streaming; optional m_fallbackPolicyId retries once.
Deterministic parallel HLOD: /Dawn Engine/WorldStreaming/Bake/HlodWorkerThreads (see Baking).

GPU octahedral workflow

Partition view → Bake GPU Octahedral Impostors (BakeGpuOctahedralImpostors; optionally IsGpuCaptureAvailable).
Bake World Streaming (missing atlas → policy 3).
Optional CaptureOctahedralImpostorScreenshots(outputDir).

Next steps

Bridges: Atom mode matrix.
Content Layers: Loaded-only layers vs proxy visibility.
Configuration and debugging: WsDumpAtomBridgeStats, HLOD Runtime keys.