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# checkpointDirectory

The directory that a the photogrammetry session uses for
checkpoints during reconstruction.

```
var checkpointDirectory: URL?
```

## Discussion

For macOS reconstruction, the `checkpointDirectory` serves
as a temporary reconstruction space if it is not `nil`.
Reconstruction starts from scratch, and does not resume
from previous checkpoints.

For iOS reconstruction, if you pass the same `checkpointDirectory`
used by an [`ObjectCaptureSession`](/documentation/RealityKit/ObjectCaptureSession) or an earlier interrupted
[`PhotogrammetrySession`](/documentation/RealityKit/PhotogrammetrySession), the [`PhotogrammetrySession`](/documentation/RealityKit/PhotogrammetrySession)
tries to use the saved checkpoint instead of starting from scratch.
Ensure that each `checkpointDirectory` is unique for each
images folder.

If set to an empty folder, the `checkpointDirectory` saves
checkpoints during processing for reuse in a subsequent restart.
If it is `nil`, the session does not use or save checkpoints, and
every reconstruction starts from scratch.

If a `checkpointDirectory` is has a non-nil value, but
the latest checkpoint is not compatible with the images folder
or other configuration settings, the session starts from scratch
and writes new checkpoints to this folder.

---

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