Use a projected context to access hardware on audio glasses and display glasses

Applicable XR devices

This guidance helps you build experiences for these types of XR devices.

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After you've requested and been granted the necessary permissions, your app can access the hardware on the audio glasses or display glasses. The key to accessing the glasses' hardware (instead of the phone's hardware), is to use a projected context.

There are two primary ways to get a projected context, depending on where your code is executing:

Get a projected context if your code is running in an projected activity

If your app's code is running from within your projected activity, its own activity context is already a projected context. In this scenario, calls made within that activity can already access the glasses' hardware.

Get a projected context for code running in a phone app component

If a part of your app outside of your projected activity (such as a phone activity or a service) needs to access the glasses' hardware, it must explicitly obtain a projected context. To do this, use the createProjectedDeviceContext method:

@OptIn(ExperimentalProjectedApi::class)
private fun getGlassesContext(context: Context): Context? {
    return try {
        // From a phone Activity or Service, get a context for the AI glasses.
        ProjectedContext.createProjectedDeviceContext(context)
    } catch (e: IllegalStateException) {
        Log.e(TAG, "Failed to create projected device context", e)
        null
    }
}
ProjectedHardware.kt

Check for validity

Wrap the createProjectedDeviceContext call within the ProjectedContext.isProjectedDeviceConnected. While this method returns true, the projected context remains valid to the connected device, and your phone app activity or service (such as a CameraManager) can access the AI glasses hardware.

Clean up on disconnect

The projected context is tied to the lifecycle of the connected device, so it is destroyed when the device disconnects. When the device disconnects, ProjectedContext.isProjectedDeviceConnected returns false. Your app should listen for this change and clean up any system services (such as a CameraManager) or resources that your app created using that projected context.

Re-initialize on reconnect

When the glasses reconnect, your app can obtain another projected context instance using createProjectedDeviceContext, and then re-initialize any system services or resources using the new projected context.

Record audio with the glasses' microphone

You can record audio from the glasses using two distinct methods:

• Use a projected context.
• Use Bluetooth Hands-Free Profile (HFP).

Choose a recording methods

The method you choose depends on whether you need high-fidelity, XR-specific audio processing, or standard Bluetooth audio input.

Record audio using a projected context

To record audio using a projected context, first request the required runtime permissions, and then record the audio using the AudioRecord API, as described in the following sections.

Request runtime permissions

To access multiple microphones on the glasses, you must request audio permissions specifically for the projected device. The standard, phone-scoped RECORD_AUDIO permission that a user has granted for your app on their mobile device is insufficient.

Follow these steps to request the permissions:

1. Declare the RECORD_AUDIO permission in your app's manifest file.

2. Request the projected-device-scoped permissions in one of the following ways, depending on where your code is executing:

   • Code executing from a projected activity: Use the ActivityResultLauncher with the ProjectedPermissionsResultContract. For more information on using this method, see the register the permissions launcher section and subsequent sections in the guide for requesting hardware permissions.
   • Code executing from a host phone activity: Use Activity#requestPermissions(permissions, requestCode, deviceId) and provide the device ID obtained from your projectedDeviceContext, as described in the understand the permission request user flow section of the guide for requesting hardware permissions.

Initialize AudioRecord with a projected context

To ensure that audio is recorded from the glasses rather than the host phone, you must associate the AudioRecord object with the projected device context.

The following code uses the AudioRecord.Builder and passes the projectedDeviceContext to the setContext method:

// Initialize AudioRecord with projected device context
val audioRecord = AudioRecord.Builder()
    .setAudioSource(MediaRecorder.AudioSource.CAMCORDER)
    .setAudioFormat(audioFormat)
    .setBufferSizeInBytes(bufferSize)
    // pass in the projected device context
    .setContext(projectedDeviceContext)
    .build()

audioRecord.startRecording()
ProjectedHardware.kt

Key points about the code

• You can set the audio source to CAMCORDER, VOICE_RECOGNITION, VOICE_COMMUNICATION, or UNPROCESSED to tailor the audio processing to your specific use case.

  For example, use VOICE_COMMUNICATION if your use case needs automatic noise reduction. VOICE_RECOGNITION is processed with acoustic echo cancellation (AEC). And if you need raw, unaltered audio, select UNPROCESSED or CAMCORDER.

• To ensure compatibility with the glasses, the audioFormat object must define a sample rate of 16kHz and a channel configuration of either mono or stereo (using CHANNEL_IN_MONO or CHANNEL_IN_STEREO).

• While there is no fixed requirement for buffer size, get the minimum buffer size to minimize perceived latency.

Clean up after use

When your app no longer needs the microphone, or when the activity is stopped, call stop and release on the AudioRecord object.

Check for runtime permissions before recording

Before calling startRecording, verify that the user has granted the microphone permission for the glasses using the projected context.

Record audio using Bluetooth HFP

To record audio using Bluetooth HFP, first request the required runtime permissions, and then record the audio using the AudioManager API, as described in the following sections.

Request permissions

As with any standard Bluetooth audio device, the RECORD_AUDIO, BLUETOOTH_CONNECT, and other related permissions are controlled by the phone and not the connected device (such as audio glasses or display glasses).

Follow these steps to request the permissions:

1. Declare following permissions in your app's manifest file:

   • RECORD_AUDIO
   • BLUETOOTH_CONNECT
   • MODIFY_AUDIO_SETTINGS

2. Request both the RECORD_AUDIO and BLUETOOTH_CONNECT permissions at runtime using the standard Android permission flow.

Use AudioManager to route audio

After the user has granted your app the necessary runtime permissions, use the AudioManager API to set the communication device to TYPE_BLUETOOTH_SCO to route the audio through Bluetooth HFP. This directs the system to retrieve audio from the Bluetooth peripheral.

val audioManager = context.getSystemService(AudioManager::class.java) ?: return
val devices = audioManager.getDevices(AudioManager.GET_DEVICES_INPUTS)
val hfpDevice = devices.find { it.type == AudioDeviceInfo.TYPE_BLUETOOTH_SCO }

hfpDevice?.let { device ->
    val audioRecord = AudioRecord.Builder()
        .setAudioSource(MediaRecorder.AudioSource.VOICE_COMMUNICATION)
        .setAudioFormat(audioFormat)
        .setBufferSizeInBytes(bufferSize)
        .build()

    // Route recording to the Bluetooth device
    audioRecord.setPreferredDevice(device)
    audioManager.setCommunicationDevice(device)

    audioRecord.startRecording()
ProjectedHardware.kt

Capture an image with the glasses' camera

To capture an image with the glasses' camera, set up and bind the CameraX's ImageCapture use case to the glasses' camera using the correct context for your app:

private fun startCameraOnGlasses(activity: ComponentActivity) {
    // 1. Get the CameraProvider using the projected context.
    // When using the projected context, DEFAULT_BACK_CAMERA maps to the AI glasses' camera.
    val projectedContext = try {
        ProjectedContext.createProjectedDeviceContext(activity)
    } catch (e: IllegalStateException) {
        Log.e(TAG, "AI Glasses context could not be created", e)
        return
    }

    val cameraProviderFuture = ProcessCameraProvider.getInstance(projectedContext)

    cameraProviderFuture.addListener({
        val cameraProvider: ProcessCameraProvider = cameraProviderFuture.get()
        val cameraSelector = CameraSelector.DEFAULT_BACK_CAMERA

        // 2. Check for the presence of a camera.
        if (!cameraProvider.hasCamera(cameraSelector)) {
            Log.w(TAG, "The selected camera is not available.")
            return@addListener
        }

        // 3. Query supported streaming resolutions using Camera2 Interop.
        val cameraInfo = cameraProvider.getCameraInfo(cameraSelector)
        val camera2CameraInfo = Camera2CameraInfo.from(cameraInfo)
        val cameraCharacteristics = camera2CameraInfo.getCameraCharacteristic(
            CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP
        )

        // 4. Define the resolution strategy.
        val targetResolution = Size(1920, 1080)
        val resolutionStrategy = ResolutionStrategy(
            targetResolution,
            ResolutionStrategy.FALLBACK_RULE_CLOSEST_LOWER
        )
        val resolutionSelector = ResolutionSelector.Builder()
            .setResolutionStrategy(resolutionStrategy)
            .build()

        // 5. If you have other continuous use cases bound, such as Preview or ImageAnalysis,
        // you can use  Camera2 Interop's CaptureRequestOptions to set the FPS
        val fpsRange = Range(30, 60)
        val captureRequestOptions = CaptureRequestOptions.Builder()
            .setCaptureRequestOption(CaptureRequest.CONTROL_AE_TARGET_FPS_RANGE, fpsRange)
            .build()

        // 6. Initialize the ImageCapture use case with options.
        val imageCapture = ImageCapture.Builder()
            // Optional: Configure resolution, format, etc.
            .setResolutionSelector(resolutionSelector)
            .build()

        try {
            // Unbind use cases before rebinding.
            cameraProvider.unbindAll()

            // Bind use cases to camera using the Activity as the LifecycleOwner.
            cameraProvider.bindToLifecycle(
                activity,
                cameraSelector,
                imageCapture
            )
        } catch (exc: Exception) {
            Log.e(TAG, "Use case binding failed", exc)
        }
    }, ContextCompat.getMainExecutor(activity))
}
ProjectedHardware.kt

Key points about the code

• Obtains an instance of the ProcessCameraProvider using the projected device context.
• Within the projected context's scope, the glasses' primary, outward-pointing camera maps to the DEFAULT_BACK_CAMERA when selecting a camera.
• A pre-binding check uses cameraProvider.hasCamera(cameraSelector) to verify that the selected camera is available on the device before proceeding.
• Uses Camera2 Interop with Camera2CameraInfo to read the underlying CameraCharacteristics#SCALER_STREAM_CONFIGURATION_MAP, which can be useful for advanced checks on supported resolutions.
• A custom ResolutionSelector is built to precisely control the output image resolution for ImageCapture.
• Creates an ImageCapture use case that is configured with a custom ResolutionSelector.
• Binds the ImageCapture use case to the activity's lifecycle. This automatically manages the opening and closing of the camera based on the activity's state (for example, stopping the camera when the activity is paused).

After the glasses' camera is set up, you can capture an image with the CameraX's ImageCapture class. Refer to the CameraX's documentation to learn about using takePicture to capture an image.

Capture a video with the glasses' camera

To capture a video instead of an image with the glasses' camera, replace the ImageCapture components with the corresponding VideoCapture components and modify the capture execution logic.

The main changes involve using a different use case, creating a different output file, and initiating the capture using the appropriate video recording method. For more information about the VideoCapture API and how to use it, see the CameraX's video capture documentation.

The following table shows the recommended resolution and frame rate depending on your app's use case:

Access a phone's hardware from a projected activity

An projected activity can also access the phone's hardware (such as the camera or microphone) by using createHostDeviceContext(context) to get the host device's (phone) context:

@OptIn(ExperimentalProjectedApi::class)
private fun getPhoneContext(activity: ComponentActivity): Context? {
    return try {
        // From an AI glasses Activity, get a context for the phone.
        ProjectedContext.createHostDeviceContext(activity)
    } catch (e: IllegalStateException) {
        Log.e(TAG, "Failed to create host device context", e)
        null
    }
}
ProjectedHardware.kt

When accessing hardware or resources that are specific to the host device (phone) in a hybrid app (an app containing both mobile and glasses experiences), you must explicitly select the correct context to make sure your app can access the correct hardware:

• Use the Activity context from the phone Activity or the ProjectedContext.createHostDeviceContext to get the phone's context.
• Don't use getApplicationContext because the application context can incorrectly return the glasses' context if a projected activity was the most-recently-launched component.