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Hierarchical Level-of-Detail

Hierarchical level of detail (HLOD) in complimentary method that involves the following:

  • Offline grouping of large variety of meshes together (usually in close proximity) in hierarchical manner and generating proxy geometry and material.
  • Rendering the hierarchical data based on a switching metric ( i.e area covered on screen or distance from camera).

Unreal Engine Specific Terms

  • Hierarchical Level Of Detail levels - The hierarchy level deciding when to switch in the clusters.
  • Clusters - a group of actors which are representing the source meshes fed to Simplygon when creating the proxy mesh.

When to use

  • Ideal for large open worlds.
  • When you have high triangle density leading in sub optimal hardware performance due to overdraw.
  • When you have many objects on screen and draw submission is the bottleneck.
  • When LOD switching mechanics are required.

How to enable Hierarchical LODSystem

Before using HLOD you need to enable it in World Settings. Navigate to Windows / World Settings / Enable Hierarchical LODSystem. After enabling this feature you can navigate to: Window / Hierarchical LOD Outliner in order to bring up the HLOD window below.

Hierarchical LOD Window

HLOD Window Split
HLOD Window Split
NoColorDescription
1——Hierarchical LOD Actions
2——Status bar
3——Settings Widget
4——LODLevels/Cluster(LODActors) Scene Hierarchy
5——Enable Simplygon for Proxy Generation.

See Epic's HLOD Outliner Reference for more details.

Gotchas

Generating proxies adds cost to your texture memory budget. Depending on your implementation of HLOD system you need to strike a balance in the number of HLOD level.

Tips

To minimize pops rendering engines can use the following techniques:

  • Screen door fading: a screen space technique where a dither pattern is used to blend between LODs when switching.
  • Geomorphing: a techniques that requires an extra set of UV coordinate to blend between the vertices of LODs.

To generate better clusters:

  • Use HLOD Volumes and for building assets and generate cluster generation based on only volumes. i.e. Metropolises
  • If you have a Landscape Actor in your level. Enable landscape culling to clip away back facing triangles. This also allows for better usage of texture space for baked material.

See Also