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# MTL4IndirectInstanceAccelerationStructureDescriptor

Descriptor for an “indirect” instance acceleration structure that allows providing the instance count and
motion transform count indirectly, through buffer references.

```
class MTL4IndirectInstanceAccelerationStructureDescriptor
```

## 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/MTL4IndirectInstanceAccelerationStructureDescriptor/instanceDescriptorBuffer) property.

Compared to [`MTL4InstanceAccelerationStructureDescriptor`](/documentation/Metal/MTL4InstanceAccelerationStructureDescriptor), this descriptor allows you to provide the number
of instances it references indirectly through a buffer reference, as well as the number of motion transforms.

This enables you to determine these counts indirectly in the GPU timeline via a compute pipeline.
Metal needs only to know the maximum possible number of instances and motion transforms to support,
which you specify via the [`maxInstanceCount`](/documentation/Metal/MTL4IndirectInstanceAccelerationStructureDescriptor/maxInstanceCount) and [`maxMotionTransformCount`](/documentation/Metal/MTL4IndirectInstanceAccelerationStructureDescriptor/maxMotionTransformCount) properties.

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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