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# replace(bufferIndex:using:)

Retrieves a Metal vertex buffer you can use to replace the contents of the specified buffer
on the GPU using Metal.

```
@MainActor func replace(bufferIndex index: Int, using commandBuffer: any MTLCommandBuffer) -> any MTLBuffer
```

## Parameters

`index`

The index of the buffer to modify.
Use a value that is less than [`vertexBufferCount`](/documentation/RealityKit/LowLevelMesh/Descriptor-swift.struct/vertexBufferCount).

`commandBuffer`

The <doc://com.apple.documentation/documentation/Metal/MTLCommandBuffer>
you intend to use for buffer modifications.
RealityKit waits for the command buffer to complete before utilizing the buffer for rendering.

## Discussion

The buffer’s contents are in an uninitialized state.

---

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