# init(named:in:) Creates a geometry modifier object from a named function in a Metal library. ``` init(named name: String, in library: any MTLLibrary) ``` ## Parameters `name` The name of the geometry modifier function. `library` The Metal library that contains the function. ## Discussion To create a geometry modifier function for a custom material, create a Metal file in your Xcode project. Prefix the function with the keyword `[[visible]]`. The function needs to take a single parameter of type `realitykit::geometry_parameters`. Here’s what a minimal geometry modifier function looks like: ```cpp #include #include using namespace metal; [[visible]] void myGeometryModifier(realitykit::geometry_parameters params) { // Use params.geometry().set_model_position_offset() to move the vertex. } ``` To create a custom material using this shader, get a reference to your app’s Metal library. You can do that like this: ```swift guard let device = MTLCreateSystemDefaultDevice() else { fatalError("Error creating default metal device.") } guard let library = maybeDevice.makeDefaultLibrary() else { fatalError("Error creating default metal library") } ``` Once you’ve got a reference to your Metal library, use it to create the surface shader reference like this: ```swift let geometryModifier = CustomMaterial.GeometryModifier( named: "myGeometryModifier", in: library ) ``` > Important: RealityKit loads surface shader functions by name, so > the name of your surface shader function needs to be unique and > exactly match the `name` parameter. For more information on creating custom materials and writing shader functions, see [Modifying RealityKit rendering using custom materials](/documentation/RealityKit/modifying-realitykit-rendering-using-custom-materials). --- 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)