# Preliminary_ReferenceImage A schema that defines the properties of an image in the physical environment. ## Discussion This schema defines the criteria that the runtime uses to recognize an image in the physical environment, including pixel data and width. When RealityKit opens a USDZ file that defines a reference image, it instantiates an with a component type . ### Declaration ```other class Preliminary_ReferenceImage "Preliminary_ReferenceImage" ( inherits = ) ``` ### Define a reference image The following example defines a prim named `ImageReference` that instructs the runtime to scan for an image described by `image.png`. ```other def Preliminary_ReferenceImage "ImageReference" { uniform asset image = @image.png@ uniform double physicalWidth = 12 } ``` ## Topics ### Properties [image](/documentation/USD/image) An image file for which the runtime should search. [physicalWidth](/documentation/USD/physicalwidth) An image’s width in centimeters. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)