SimGroup

SimGroup is a container object used to organize other level objects in the scene tree and in items.level.json files.

Most levels use a root group named:

MissionGroup

Nested groups are commonly used to keep props, lights, roads, terrain objects, markers, and generated content in manageable folders.

Basic example

A simple group entry in items.level.json:

{
  "class": "SimGroup",
  "name": "road_props",
  "__parent": "MissionGroup"
}

Objects can then use that group as their parent:

{
  "class": "TSStatic",
  "name": "barrier_01",
  "__parent": "road_props",
  "shapeName": "/levels/example/art/shapes/barrier.dae",
  "position": [10, 20, 0],
  "rotationMatrix": [1, 0, 0, 0, 1, 0, 0, 0, 1],
  "scale": [1, 1, 1]
}

Important fields

Relationship with __parent

Level item files commonly store hierarchy through __parent.

Example:

{
  "name": "lights_night",
  "class": "SimGroup",
  "__parent": "MissionGroup"
}

Child object:

{
  "name": "street_lamp_01",
  "class": "SpotLight",
  "__parent": "lights_night"
}

The group must be loaded before children that reference it, or the loading tool must rebuild the parent relationship.

MissionGroup

MissionGroup is the main level group used by the scene tree.

A typical main item file contains:

{
  "name": "MissionGroup",
  "class": "SimGroup",
  "enabled": "1"
}

Most level objects are direct or indirect children of MissionGroup.

Tool notes

For tools that generate or transform levels:

• Preserve group names used by __parent.
• Preserve persistentId values unless creating a new group.
• Keep generated groups close to the content they own.
• Avoid flattening the whole scene into MissionGroup; large levels rely on nested groups for editing.
• Do not delete a group without moving or deleting its children intentionally.

Common issues

• Objects appear in the wrong scene tree location because __parent references the wrong group.
• Generated objects fail to load under the intended group because the group entry is missing.
• Renaming a group breaks child object placement if __parent values are not updated.

Validation criteria

• MissionGroup exists.
• Every generated __parent value resolves to an existing SimGroup.
• Large object sets are organized into meaningful groups.
• Group names are stable enough for tools, scripts, and editor workflows.

See also: Level object files (items.level.json) .