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# MTL4InstanceAccelerationStructureDescriptor

Descriptor for an instance acceleration structure.

```
class MTL4InstanceAccelerationStructureDescriptor
```

## Overview

An instance acceleration structure references other acceleration structures, and provides the ability to
“instantiate” them multiple times, each one with potentially a different transformation matrix.

You specify the properties of the instances in the acceleration structure this descriptor builds by providing a
buffer of `structs` via its [`instanceDescriptorBuffer`](/documentation/Metal/MTL4InstanceAccelerationStructureDescriptor/instanceDescriptorBuffer) property.

Use a [`MTLResidencySet`](/documentation/Metal/MTLResidencySet) to mark residency of all buffers and acceleration structures this descriptor references
when you build this acceleration structure.

---

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