46 lines
2.3 KiB
Markdown
46 lines
2.3 KiB
Markdown
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---
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uid: arsubsystems-object-tracking-subsystem
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---
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# XR object tracking subsystem
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The object tracking subsystem attempts to detect three-dimensional objects in the environment that have previously been scanned and stored in a reference objects library.
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## Terminology
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|**Term**|**Description**|
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|--------|---------------|
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|**Reference object**|A reference object is a previously scanned object. The object tracking subsystem attempts to find instances of the object and report on their poses.|
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|**Reference object library**|A set of reference objects. When you start an object tracking subsystem, you must first provide it with a library of reference objects so it knows what to search for.|
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|**Provider**|The object tracking subsystem is an interface which is implemented in other packages. Each implementation is called a "provider". For example, you might have a different provider package for each AR platform.|
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## Creating a reference object library
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You create reference object libraries in the Unity Editor, then fill it with reference objects. Each reference object requires a provider-specific representation for each provider package you have in your project.
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From Unity's main menu, go to **Assets** > **Create** > **XR** > **Reference Object Library**.
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This creates a new asset in your project. To create reference objects, select this asset, then select **Add Entry**:
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Reference objects have a **Name**, followed by a list of provider-specific entries. In the example above, the object only has one entry for ARKit.
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You need to populate the reference object entries with provider-specific assets. For instructions on how to do this, refer to the provider's documentation.
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## Using the library at runtime
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To use the library at runtime, set it on the subsystem. For example:
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```csharp
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XRReferenceObjectLibrary myLibrary = ...
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XRObjectTrackingSubsystem subsystem = ...
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subsystem.library = myLibrary;
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subsystem.Start();
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```
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> [!NOTE]
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> You must set `imageLibrary` to a non-null reference before starting the subsystem.
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Query for changes to tracked objects with `XRImageTrackingSubsystem.GetChanges`. This returns all changes to tracked objects (added, updated, and removed) since the last call to this method.
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