Image tracking

This page is a supplement to the AR Foundation Image tracking manual. The following sections only contain information about APIs where ARCore exhibits unique platform-specific behavior.

Tip

When developing an AR app, refer to both the AR Foundation documentation as well as the required packages for each platform you support.

Optional feature support

ARCore implements the following optional features of AR Foundation's XRImageTrackingSubsystem:

Feature	Descriptor Property	Supported
Moving images	supportsMovingImages	Yes
Requires physical image dimensions	requiresPhysicalImageDimensions
Mutable library	supportsMutableLibrary	Yes
Image validation	supportsImageValidation	Yes

Reference image libraries

When you build an ARCore app for the Android platform, this package creates an imgdb file for each reference image library. ARCore creates these files in your project's StreamingAssets folder, in a subdirectory called HiddenARCore, so Unity can access them at runtime.

Texture formats

You can use .jpg or .png files as AR reference images in ARCore. If a reference image in the XRReferenceImageLibrary isn't a .jpg or .png, a script in this package called the ARCoreBuildProcessor will attempt to convert the texture to a .png so that ARCore can use it.

When you export a Texture2D to .png, it can fail if the texture's Texture Import Settings have Read/Write Enabled disabled and Compression is set to None.

To use the texture at runtime (not as a source Asset for the reference image), create a separate .jpg or .png copy for the source Asset. This reduces the performance impact of the Texture Import Settings at runtime.

AssetBundles

Reference image libraries can be stored in AssetBundles and loaded at runtime, but setting up your project to build the AssetBundles correctly requires special instructions. Refer to Use reference image libraries with AssetBundles in AR Foundation for more information.

Reference image dimensions

To improve image detection in ARCore you can specify the image dimensions. When you specify the dimensions for a reference image, ARCore receives the image's width, and then determines the height from the image's aspect ratio.

Native pointer

XRTrackedImage.nativePtr values returned by this package contain a pointer to the following struct:

typedef struct UnityXRNativeImage
{
    int version;
    void* imageTrackable;
} UnityXRNativeImage;

This package also provides a header file containing the definitions of various native data structs including UnityXRNativeImage. It can be found in the package directory under Includes~/UnityXRNativePtrs.h.

Cast void* imageTrackable to an ArAugmentedImage handle in C++ using the following example code:

// Marshal the native image data from the XRTrackedImage.nativePtr in C#
UnityXRNativeImage nativeImageData;
ArAugmentedImage* augmentedImageHandle = static_cast<ArAugmentedImage*>(nativeImageData.imageTrackable);

To learn more about native pointers and their usage, refer to Extending AR Foundation.