<!--
{
  "availability" : [
    "iOS: 15.0.0 -",
    "iPadOS: 15.0.0 -",
    "macCatalyst: 15.0.0 -",
    "macOS: 12.0.0 -",
    "tvOS: 16.0.0 -",
    "visionOS: 1.0.0 -"
  ],
  "documentType" : "symbol",
  "framework" : "Metal",
  "identifier" : "/documentation/Metal/MTLAccelerationStructureMotionInstanceDescriptor/motionTransformsStartIndex",
  "metadataVersion" : "0.1.0",
  "role" : "Instance Property",
  "symbol" : {
    "kind" : "Instance Property",
    "modules" : [
      "Metal"
    ],
    "preciseIdentifier" : "c:@SA@MTLAccelerationStructureMotionInstanceDescriptor@FI@motionTransformsStartIndex"
  },
  "title" : "motionTransformsStartIndex"
}
-->

# motionTransformsStartIndex

The index of motion data that represents the first key-frame motion data, which applies to
the next acceleration-structure motion instance you create with the descriptor.

```
var motionTransformsStartIndex: UInt32
```

## Discussion

The index points to an entry in an [`MTLInstanceAccelerationStructureDescriptor`](/documentation/Metal/MTLInstanceAccelerationStructureDescriptor)
instance’s transform data, which it stores in its
[`motionTransformBuffer`](/documentation/Metal/MTLInstanceAccelerationStructureDescriptor/motionTransformBuffer) property.

---

Copyright &copy; 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)