# LowLevelMesh A container for vertex data that you can use to create and update meshes using your own format. ``` @MainActor class LowLevelMesh ``` ## Overview Use `LowLevelMesh` when you want to bring your own vertex format to RealityKit or update your data frequently. To update your data in `LowLevelMesh`, you can either use Swift for CPU processing, or Metal Compute Shaders for GPU processing. > Note: Use ``doc://com.apple.RealityKit/documentation/RealityKit/MeshDescriptor`` for a simpler alternative to `LowLevelMesh`. > For information on loading a model from a USD or Reality file, > see . Express your vertex by creating a [`LowLevelMesh.Descriptor`](/documentation/RealityKit/LowLevelMesh/Descriptor-swift.struct) that describes your layout, along with the required index and vertex capacities. This descriptor is similar to , with additional semantics that make the data available in your shaders. To use `LowLevelMesh`, first define your own vertex structure, either in a Metal header or using a Swift structure: ```swift struct MyVertex { var position: SIMD3 = .zero var color: UInt32 = .zero } ``` Next, describe your structure to `LowLevelMesh` by creating a list of vertex attributes and a vertex layout. This description informs `LowLevelMesh` how to represent the vertex data in memory: ```swift extension MyVertex { static var vertexAttributes: [LowLevelMesh.Attribute] = [ .init(semantic: .position, format: .float3, offset: MemoryLayout.offset(of: \.position)!), .init(semantic: .color, format: .uchar4Normalized_bgra, offset: MemoryLayout.offset(of: \.color)!) ] static var vertexLayouts: [LowLevelMesh.Layout] = [ .init(bufferIndex: 0, bufferStride: MemoryLayout.stride) ] static var descriptor: LowLevelMesh.Descriptor { var desc = LowLevelMesh.Descriptor() desc.vertexAttributes = MyVertex.vertexAttributes desc.vertexLayouts = MyVertex.vertexLayouts desc.indexType = .uint32 return desc } } ``` Create a [`LowLevelMesh.Descriptor`](/documentation/RealityKit/LowLevelMesh/Descriptor-swift.struct) and `LowLevelMesh`, and assign your mesh data and parts: ```swift func triangleMesh() throws -> LowLevelMesh { var desc = MyVertex.descriptor desc.vertexCapacity = 3 desc.indexCapacity = 3 let mesh = try LowLevelMesh(descriptor: desc) mesh.withUnsafeMutableBytes(bufferIndex: 0) { rawBytes in let vertices = rawBytes.bindMemory(to: MyVertex.self) vertices[0] = MyVertex(position: [-1, -1, 0], color: 0xFF00FF00) vertices[1] = MyVertex(position: [ 1, -1, 0], color: 0xFFFF0000) vertices[2] = MyVertex(position: [ 0, 1, 0], color: 0xFF0000FF) } mesh.withUnsafeMutableIndices { rawIndices in let indices = rawIndices.bindMemory(to: UInt32.self) indices[0] = 0 indices[1] = 1 indices[2] = 2 } let meshBounds = BoundingBox(min: [-1, -1, 0], max: [1, 1, 0]) mesh.parts.replaceAll([ LowLevelMesh.Part( indexCount: 3, topology: .triangle, bounds: meshBounds ) ]) return mesh } ``` To finish, create a [`MeshResource`](/documentation/RealityKit/MeshResource) from the `LowLevelMesh`, and add it to a [`ModelComponent`](/documentation/RealityKit/ModelComponent). You can then add this model to any [`Entity`](/documentation/RealityKit/Entity) in your scene: ```swift func triangleEntity() throws -> Entity { let lowLevelMesh = try triangleMesh() let resource = try MeshResource(from: lowLevelMesh) let modelComponent = ModelComponent(mesh: resource, materials: [UnlitMaterial()]) let entity = Entity() entity.name = "Triangle" entity.components.set(modelComponent) entity.scale *= 0.1 return entity } ``` The low-level mesh creates a triangular shape in your RealityKit scene:  The [`MeshResource`](/documentation/RealityKit/MeshResource) retains a reference to the `LowLevelMesh`, reflecting any changes when the renderer updates. ## Topics ### Creating a low-level mesh [`init(descriptor:)`](/documentation/RealityKit/LowLevelMesh/init(descriptor:)) Constructs a low-level mesh from a descriptor. ### Describing a low-level mesh [`descriptor`](/documentation/RealityKit/LowLevelMesh/descriptor-swift.property) The definition of the structure of this low-level mesh. [`parts`](/documentation/RealityKit/LowLevelMesh/parts) A mutable collection of parts. [`indexCapacity`](/documentation/RealityKit/LowLevelMesh/indexCapacity) The capacity of the index buffer, measured in indices. [`vertexCapacity`](/documentation/RealityKit/LowLevelMesh/vertexCapacity) The capacity of the vertex buffer, measured in vertices. ### Accessing mesh data on the CPU with Swift [`withUnsafeBytes(bufferIndex:_:)`](/documentation/RealityKit/LowLevelMesh/withUnsafeBytes(bufferIndex:_:)) Reads a Metal vertex buffer synchronously on the CPU. [`withUnsafeMutableBytes(bufferIndex:_:)`](/documentation/RealityKit/LowLevelMesh/withUnsafeMutableBytes(bufferIndex:_:)) Updates a Metal vertex buffer synchronously on the CPU. [`withUnsafeIndices(_:)`](/documentation/RealityKit/LowLevelMesh/withUnsafeIndices(_:)) Reads the index buffer synchronously on the CPU. [`withUnsafeMutableIndices(_:)`](/documentation/RealityKit/LowLevelMesh/withUnsafeMutableIndices(_:)) Updates the index buffer synchronously on the CPU. [`replaceUnsafeMutableBytes(bufferIndex:_:)`](/documentation/RealityKit/LowLevelMesh/replaceUnsafeMutableBytes(bufferIndex:_:)) Replaces a Metal vertex buffer synchronously on the CPU. [`replaceUnsafeMutableIndices(_:)`](/documentation/RealityKit/LowLevelMesh/replaceUnsafeMutableIndices(_:)) Replaces the index buffer synchronously on the CPU. ### Accessing mesh data on the GPU with Metal [`read(bufferIndex:using:)`](/documentation/RealityKit/LowLevelMesh/read(bufferIndex:using:)) Retrieves a Metal vertex buffer at the specified index, for GPU reading. [`readIndices(using:)`](/documentation/RealityKit/LowLevelMesh/readIndices(using:)) Retrieves the Metal index buffer for GPU reading. [`replace(bufferIndex:using:)`](/documentation/RealityKit/LowLevelMesh/replace(bufferIndex:using:)) Retrieves a Metal vertex buffer you can use to replace the contents of the specified buffer on the GPU using Metal. [`replaceIndices(using:)`](/documentation/RealityKit/LowLevelMesh/replaceIndices(using:)) Retrieves a Metal index buffer that you can use to replace the indices of this low-level mesh. --- 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)