The 3D Handbook
Part 4 — Modeling OptimisationPage 103 of 248 on the reading route
Key term

Hierarchical LOD (HLOD)

Level of detail for groups of objects. Distant clusters of meshes are replaced by one merged, simplified or instanced proxy, cutting draw calls and keeping unloaded areas visible.

optimisationlodopen-worldunreal

Hierarchical LOD (HLOD) is level of detail for groups of objects. A normal LOD swaps one mesh for a cheaper version of itself. HLOD swaps a whole cluster of meshes, such as every building in a city block, for one combined proxy when the cluster is far away.

In Unreal Engine's World Partition, the HLOD system "uses custom HLOD Layers to organize large amounts of Static Mesh Actors" and generates "a single proxy mesh and Material" from them (Unreal Engine documentation: World Partition - Hierarchical Level of Detail).

#Why HLOD exists

Two problems grow with world size:

  1. Draw calls. Every separate mesh and material costs the CPU a draw call, even if the object is a few pixels wide. Epic uses HLOD "to reduce the number of draw calls per frame" (Unreal Engine documentation: World Partition - Hierarchical Level of Detail).
  2. Streaming. Open worlds load and unload areas as the player moves. HLOD is how unloaded World Partition cells stay visible: distant, non-interactive actors such as mountains, trees and cliffs remain on screen as proxies (Unreal Engine documentation: World Partition - Hierarchical Level of Detail).
Top-down grid of square world cells. Cells inside a dashed circle around the camera show six small individual buildings each; cells outside show a single orange rounded block each.
Near the camera, cells are loaded and each building is its own actor. Beyond the loading range, each cell is drawn as one HLOD proxy.Diagram by the handbook author

#Types of HLOD proxy

Unreal's World Partition offers three layer types (Unreal Engine documentation: World Partition - Hierarchical Level of Detail):

Layer type What it builds Good for
Instancing Replaces meshes with Instanced Static Mesh components Trees, foliage and their impostors (Unreal Engine documentation)
Merged Mesh Merges meshes into a single proxy mesh Clusters that already have modest triangle counts
Simplified Mesh Merges, then simplifies the proxy Dense architecture seen only from far away

Merged and Simplified layers can also "Generate Nanite Enabled Mesh" for the proxy (Unreal Engine documentation: World Partition - Hierarchical Level of Detail). Each layer has a Cell Size for its runtime grid and a Loading Range at which its cells stream (Unreal Engine documentation: World Partition - Hierarchical Level of Detail). You build HLODs from Build > Build HLODs or with the WorldPartitionHLODsBuilder commandlet (Unreal Engine documentation: World Partition - Hierarchical Level of Detail). Changing a source actor marks its HLOD for a rebuild the next time you run the build (Unreal Engine documentation: World Partition - Hierarchical Level of Detail).

The Instancing type ties HLOD to instancing: Epic notes that an instance uses far less memory than a full primitive (64 bytes against 672) (Unreal Engine documentation: Instanced Static Mesh Component in Unreal…).

#In different tools

level of detail · impostors and billboards · instancing · draw calls · virtualised geometry · terrain and landscapes

#Further reading and watching

References

  1. Epic Games · World Partition - Hierarchical Level of Detail · Unreal Engine documentation, 2026 · docs · accessed 2026-10-21
  2. Epic Games · Instanced Static Mesh Component in Unreal Engine · Unreal Engine documentation, 2026 · docs · accessed 2026-10-21

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Last updated 2026-10-22