Mass LOD

The Mass LOD system consists of several different elements working together.

• UMassLODDistanceCollectorProcessor
  • Determines closest distance to any viewer for relevant Entities
• UMassLODCollectorProcessor
  • Determines closest distance to any visible viewer for relevant Entities
• UMassSimulationLODProcessor
  • Computes and updates LOD settings for Entities
    • Every tick unless you enable variable ticking per LOD
• UMassLODSubsystem
  • Tracks Viewers: Locations, Rotations, FOV, Aspect Ratio
• LOD Traits
  • LOD Collector
    • Either: UMassDistanceLODCollectorTrait
    • Or: UMassLODCollectorTrait
  • UMassSimulationLODTrait
    • with bEnableVariableTicking

Different Ways to Compute LOD

1. Proximity to Viewers (regardless of Viewer perspective)
   • Add UMassDistanceLODCollectorTrait to Entity
2. Visible by Viewers
   • Add UMassLODCollectorTrait to Entity

For any given Entity, choose which type of LOD calculation you want it to use, and add the appropriate LOD Collector Trait.

DO NOT add both traits to a single Entity.

LOD Terms

Hysteresis

A multiplier applied to distances to differentiate between a visible entity moving out of visibility range (where this ratio is applied), as compared to an invisible entity moving into visibility range.

This helps to prevent visibility oscillation at the edge of visible range.

VisibleLOD

Settings used when the entity is visible.

BaseLOD

Settings used when the entity is not visible.

UMassLODDistanceCollectorProcessor

This variant of the LODCollector sets Entity LOD purely based on distance from the Entity to the nearest viewer. It doesn’t care which direction the viewer is looking.

Server Execution logic:

• Update FMassViewerInfoFragment for each Entity (determine if in range of any viewer)

Client Execution logic:

• Same as server, but only tests Local Player visibility

UMassLODCollectorProcessor

This variant of the LODCollector sets Entity LOD based on both distance from the Entity to the nearest viewer and based on the direction the viewer is looking (if the Entity behind the player, even if it’s nearby, it’s Off LOD).

Server Execution logic:

• Update FMassViewerInfoFragment for each Entity (determine if visible by any viewer)

Client Execution logic:

• Same as server, but only tests Local Player visibility

UMassSimulationLODProcessor

Execution logic:

• Update viewers info
  • Gather Viewers info from UMassLODSubsystem
  • Update each FMassSimulationLODSharedFragment with updated viewers
• For every LOD entity chunk who should be recalculated (all, unless optional variable tick frequency says otherwise):
  • Calculate LOD
    • For every entity in chunk: Set new visibility status
• When count-based LOD adjustments are enabled (always, unless you disable it in INI):
  • For each FMassSimulationLODSharedFragment:
    • Determine if adjustments are needed
  • For every LOD entity chunk having a shared fragment that requires LOD adjustments:
    • Adjust previously computed LOD settings to reduce LOD as needed
      • The entities that are degraded are… ??? Probably the ones in later chunks.
• When variable tick rates are enabled:
  • For each chunk that was updated this frame:
    • Compute a randomish time until the next compute frame for this chunk, based on the FMassSimulationVariableTickParameters.TickRates setting for the chunk’s LOD.
• Change LOD tags on any entities that require it

Important Data Types

• FMassSimulationLODSharedFragment
  • One of these will exist for each unique value of FMassSimulationLODParameters
  • Uses TMassLODCalculator<FMassSimulationLODLogic>
• FMassSimulationLODFragment
  • Distance to the closest viewer
  • Current & Previous LOD
• FMassViewerInfoFragment
  • Distance to the closest viewer
• FMassSimulationVariableTickSharedFragment
  • TMassLODTickRateController
    • TickRate is not really a rate, it’s a time in seconds.
      • Unless running in deterministic mode, randomness is applied to this “rate” in an attempt to spread updates over multiple frames at approximately the time interval specified.
      • This comes from FMassSimulationVariableTickParameters.TickRates

UMassLODSubsystem

The Mass LOD Subsystem is responsible for determining which players or other actors exist at any given moment. It standardizes information such as their Location, Rotation, Field of View and camera aspect ratio to be used by various Mass processors who need this info.

This hooks into the UMassSimulationSubsystem phase processor to receive callbacks on every PrePhysics phase start. At the start of each PrePhysics phase, the Viewer info is updated.

Important Data Types

• FViewerInfo
  • Location, Rotation, FOV, AspectRatio for each Player
    • Also in some cases world partition streaming sources are “viewers”
    • Other actors may also be considered “viewers”

LOD Traits

UMassDistanceLODCollectorTrait

This trait marks an Entity to have its LOD computed based ONLY on whether it is in range of any viewer, regardless of whether it is visible.

Requires:

• Fragment: FTransformFragment

Adds:

• Fragment: FMassViewerInfoFragment
• AddTag: FMassCollectDistanceLODViewerInfoTag

UMassLODCollectorTrait

This trait marks an Entity to have its LOD computed based on whether it is visible by any viewer.

Requires:

• Fragment: FTransformFragment

Adds:

• Fragment: FMassViewerInfoFragment
• AddTag: FMassCollectLODViewerInfoTag

UMassSimulationLODTrait

Requires:

• FMassViewerInfoFragment
• FTransformFragment

Adds:

• Fragment: FMassSimulationLODFragment
• Shared Fragment: FMassSimulationLODSharedFragment
• Const Shared Fragment: FMassSimulationLODParameters

Manages Tags: (when bSetLODTags is true)

• FMassOffLODTag (added by default for all Entities)

UMassSimulationLODTrait with bEnableVariableTicking

In addition to the base fragments, when you enable bEnableVariableTicking, this trait also adds:

• Fragment: FMassSimulationVariableTickFragment
• Chunk Fragment: FMassSimulationVariableTickChunkFragment
• Shared Fragment: FMassSimulationVariableTickSharedFragment
• Const Shared Fragment: FMassSimulationVariableTickParameters