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# stepFunction

The circumstances under which the vertex and its attributes are presented to the vertex function.

```
var stepFunction: MTLVertexStepFunction { get set }
```

## Discussion

The default value is [`MTLVertexStepFunction.perVertex`](/documentation/Metal/MTLVertexStepFunction/perVertex).

If `stepFunction` is [`MTLVertexStepFunction.perVertex`](/documentation/Metal/MTLVertexStepFunction/perVertex), the function fetches new attribute data based on the `[[ vertex_id ]]` attribute qualifier. The function fetches new attribute data each time a new vertex is processed. In this case, `stepRate` needs to be set to `1`, which is its default value.

If `stepFunction` is [`MTLVertexStepFunction.perInstance`](/documentation/Metal/MTLVertexStepFunction/perInstance), the function fetches new attribute data based on the `[[ instance_id ]]` attribute qualifier.  In this case, `stepRate` needs to be greater than `0` and its value determines how often the function fetches new attribute data.

If `stepFunction` is [`MTLVertexStepFunction.constant`](/documentation/Metal/MTLVertexStepFunction/constant), the function fetches attribute data just once, and that attribute data is used for every vertex. In this case,`stepRate` needs to be set to `0`.

## See Also

  [Metal Shading Language Guide](https://developer.apple.com/library/archive/documentation/Metal/Reference/MetalShadingLanguageGuide/Introduction/Introduction.html#//apple_ref/doc/uid/TP40014364)

  [Metal Programming Guide](https://developer.apple.com/library/archive/documentation/Miscellaneous/Conceptual/MetalProgrammingGuide/Introduction/Introduction.html#//apple_ref/doc/uid/TP40014221)


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