Unit Dictionary Keys

Metadata describing the unit of measurement used in a scene file, used with the SCNSceneSourceAssetUnitKey key.


By default, units of measurement for SceneKit are entirely arbitrary. A geometry’s vertex data is defined in whatever unit system and coordinate space is convenient for its author. The nodes in a scene each define their own space as is convenient for organizing the scene’s contents, and the camera that views a scene and the renderer that draws it determine the correspondence between the scene’s world and the pixels on a display.

Nevertheless, it can be useful for authors of 3D assets to establish a unit scale for the art they create. A scene file can identify the intended unit of measure for the world space their contents are defined in. If you load assets from multiple sources and combine them in a single scene, you can use the units specified in each source to determine how to scale each asset in the combined scene. When working with units from a scene file, the arbitrary unit of distance that SceneKit’s world space corresponds to the meter defined in the International System of Units (SI).

For example, one scene file might contain geometry representing a vehicle, having no designated unit but designed on a scale where one unit equals one meter. Another scene file might contain geometry representing a person, with a designated unit of US—the unit dictionary for the scene file contains the string inch for the SCNSceneSourceAssetUnitNameKey key, and the number 0.0254 for the SCNSceneSourceAssetUnitMeterKey key. To place these objects together in the same scene at appropriate sizes, set the scale property of the node containing the person geometry to 0.0254.




The name of the unit used in the scene file.


The conversion factor from the unit used in the scene file to the SI meter.