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.
Learn about XR device types →
Audio &
Display Glasses
Learn about XR device types →

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:

@RequiresApi(Build.VERSION_CODES.BAKLAVA)
@OptIn(ExperimentalProjectedApi::class, ExperimentalCoroutinesApi::class)
private fun monitorProjectedConnectivity(activity: ComponentActivity) {
    activity.lifecycleScope.launch {
        // Before creating a projected context, check to see if the projected device is connected.
        // While this method returns true, the projected context remains valid.
        ProjectedContext.isProjectedDeviceConnected(activity, coroutineContext)
            .collectLatest { isConnected ->
                if (isConnected) {
                    // From a phone Activity or Service, get a context for the audio and display glasses.
                    // Re-initialize on reconnect: Obtain another context instance.
                    val projectedContext = try {
                        ProjectedContext.createProjectedDeviceContext(activity)
                    } catch (e: IllegalStateException) {
                        Log.e(TAG, "Failed to create projected context", e)
                        return@collectLatest
                    }

                    // Use the projectedContext to initialize system services (e.g., CameraManager).
                    Log.i(TAG, "Projected device connected. Initializing hardware...")
                } else {
                    // The projected context is destroyed when the device disconnects.
                    // Clean up on disconnect: Listen for 'false' and release resources.
                    Log.i(TAG, "Projected device disconnected. Cleaning up hardware resources...")
                }
            }
    }
}

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 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 Bluetooth Hands-Free Profile (HFP). In this mode, the glasses connect to the phone using standard Headset and Advanced Audio Distribution Profile (A2DP) profiles, functioning like a typical Bluetooth peripheral. If your app is already designed for standard Bluetooth recording, you can use this method to record audio from the glasses without any additional development.

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:

  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 hfpDevice = audioManager.availableCommunicationDevices
    .firstOrNull { it.type == AudioDeviceInfo.TYPE_BLUETOOTH_SCO } ?: return

// HFP input is limited to 16 kHz, with mono or stereo channel configurations.
val sampleRate = 16_000
val channelMask = AudioFormat.CHANNEL_IN_MONO
val encoding = AudioFormat.ENCODING_PCM_16BIT

// Use a small multiple of the minimum buffer size to keep chunks short for low latency.
val minBufferSize = AudioRecord.getMinBufferSize(sampleRate, channelMask, encoding)
if (minBufferSize <= 0) return

val audioRecord = AudioRecord.Builder()
    .setAudioSource(MediaRecorder.AudioSource.VOICE_COMMUNICATION)
    .setAudioFormat(
        AudioFormat.Builder()
            .setSampleRate(sampleRate)
            .setChannelMask(channelMask)
            .setEncoding(encoding)
            .build()
    )
    .setBufferSizeInBytes(minBufferSize * 2)
    .build()

try {
    // Route voice communication audio through the Bluetooth device.
    if (!audioManager.setCommunicationDevice(hfpDevice)) return
    audioRecord.startRecording()
    // Read audio with audioRecord.read() on a background thread.
} finally {
    // Stop and release when done.
    if (audioRecord.recordingState == AudioRecord.RECORDSTATE_RECORDING) audioRecord.stop()
    audioRecord.release()
    audioManager.clearCommunicationDevice()
}

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 setUpGlassesCamera(activity: ComponentActivity) {
    activity.lifecycleScope.launch {
        // Before creating a projected context, check to see if the projected device is connected.
        ProjectedContext.isProjectedDeviceConnected(activity, coroutineContext)
            .collectLatest { isConnected ->
                if (isConnected) {
                    // 1. Get the CameraProvider using the projected context.
                    // When using the projected context, DEFAULT_BACK_CAMERA maps to the audio and display glasses' camera.
                    val projectedContext = try {
                        ProjectedContext.createProjectedDeviceContext(activity)
                    } catch (e: IllegalStateException) {
                        Log.e(TAG, "Projected context could not be created", e)
                        return@collectLatest
                    }

                    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 dynamically using Camera2 Interop.
                        val cameraInfo = cameraProvider.getCameraInfo(cameraSelector)
                        val map = Camera2CameraInfo.from(cameraInfo).getCameraCharacteristic(
                            CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP
                        )
                        val supportedResolutions = map?.getOutputSizes(ImageFormat.JPEG)?.toList() ?: emptyList()
                        Log.i(TAG, "Supported resolutions for JPEG: ${supportedResolutions.joinToString()}")

                        // 4. Dynamically select the best target resolution from the queried list.
                        val preferredResolution = Size(1920, 1080)
                        val dynamicTargetResolution = when {
                            // Case A: The preferred resolution is explicitly supported by the glasses.
                            supportedResolutions.contains(preferredResolution) -> preferredResolution

                            // Case B: Find the highest supported 16:9 resolution as a fallback.
                            else -> {
                                val targetAspectRatio = 16.0 / 9.0
                                val tolerance = 0.01
                                supportedResolutions
                                    .filter { abs((it.width.toDouble() / it.height) - targetAspectRatio) < tolerance }
                                    .maxByOrNull { it.width * it.height }
                                    // Case C: Absolute fallback if no 16:9 resolutions are found.
                                    ?: supportedResolutions.firstOrNull()
                                    ?: preferredResolution
                            }
                        }
                        Log.i(TAG, "Selected target resolution: $dynamicTargetResolution")

                        val resolutionStrategy = ResolutionStrategy(
                            dynamicTargetResolution,
                            ResolutionStrategy.FALLBACK_RULE_CLOSEST_LOWER
                        )
                        val resolutionSelector = ResolutionSelector.Builder()
                            .setResolutionStrategy(resolutionStrategy)
                            .build()

                        // 5. 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))
                }
            }
    }
}

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).
  • Configuring for single-stream hardware: The glasses camera pipeline is constrained to a single active stream at a time. When ImageCapture is bound alone, CameraX attaches an internal repeating stream (MeteringRepeating) for focus and metering by default, which causes the single-stream pipeline to stall. To bind ImageCapture by itself without a preview stream, disable the forced repeating stream by setting setRepeatingStreamForced(false) on CameraXConfig.Builder (for example, by implementing CameraXConfig.Provider in your Application class or configuring ProcessCameraProvider before initialization).

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 stream with the glasses' camera

To capture streams instead of an image, replace the ImageCapture component from the capture image example with VideoCapture or ImageAnalysis components and change the capture logic.

For more information about these APIs how to use them, see the CameraX video capture and CameraX iamge analysis documentation.

Choose a resolution and frame rate

You should query the device to find its supported resolutions and their corresponding maximum frame rates. The following table lists examples of values you might see, depending on your app's use case. To conserve battery power and reduce heat on the glasses, choose the lowest resolution and frame rate that work for your use case.

Use case Resolution Frame rate
Text recognition (OCR) or world sensing 1920 x 1080 1 FPS
AI video streaming (multimodal assistants) 1024 x 768 1 FPS
Localization and visual positioning (VPS) 640 x 360 or 640 x 480 10 FPS
Video calling 640 x 360 or 640 x 480 24 FPS

Query the maximum frame rate for each resolution

CONTROL_AE_AVAILABLE_TARGET_FPS_RANGES reports the device's overall frame rate bounds, which can be higher than the streaming rate that a given resolution supports. To get the actual maximum for each resolution on audio and display glasses, read its minimum frame duration and convert it to FPS with StreamConfigurationMap.getOutputMinFrameDuration(format, size):

@androidx.annotation.OptIn(ExperimentalCamera2Interop::class)
fun getMaxFpsBySize(cameraInfo: CameraInfo, format: Int): Map<Size, Int> {
    val map = Camera2CameraInfo.from(cameraInfo).getCameraCharacteristic(
        CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP
    ) ?: return emptyMap()

    return map.getOutputSizes(format).associateWith { size ->
        val minFrameDurationNs = map.getOutputMinFrameDuration(format, size)
        if (minFrameDurationNs > 0) (1_000_000_000L / minFrameDurationNs).toInt() else 0
    }
}

Next, use that value to calculate the minFps and maxFps bounds for your target frame rate range. Cap the lower bound (minFps) at the resolution's maximum streaming frame rate calculated from getOutputMinFrameDuration, and set a higher upper bound (maxFps) as a catch-up window:

val maxFpsBySize = getMaxFpsBySize(cameraInfo, ImageFormat.YUV_420_888)
val maxSupportedFps = maxFpsBySize[Size(640, 480)] ?: return ImageAnalysis.Builder().build()

// minFps is the target streaming frame rate and should be at most maxSupportedFps.
val minFps = minOf(desiredFps, maxSupportedFps)
// Set the upper bound to 2x minFps so the stream can catch up if frames are delayed.
val maxFps = minFps * 2

Key points about the code

  • Use the format that matches your use case. For ImageAnalysis, that's YUV_420_888.
  • For VideoCapture, use the Class overloads instead: getOutputSizes(MediaRecorder::class.java) and getOutputMinFrameDuration(MediaRecorder::class.java, size).
  • Specify a range where maxFps is greater than minFps (such as minFps*2) rather than setting both bounds to the same value like Range(minFps, minFps). The lower bound (minFps) sets the target streaming frame rate and should be at most the value calculated from getOutputMinFrameDuration, while the upper bound (maxFps) acts as a catch-up window for delayed frames to prevent persistent latency. For example, [1,2] or [1,3] for a 1 FPS stream, and [24,30] or [24,48] for a 24 FPS stream.

Configure the frame rate

To configure the frame rate range based on your CameraX use case:

// Configure Resolution Strategy for the stream
val resolutionStrategy = ResolutionStrategy(
    size,
    ResolutionStrategy.FALLBACK_RULE_CLOSEST_LOWER
)
val resolutionSelector = ResolutionSelector.Builder()
    .setResolutionStrategy(resolutionStrategy)
    .build()

// Create and return the ImageAnalysis builder with proper frame rate ranges
// ImageAnalysis: OCR, AI streaming, and VPS
val analysisBuilder = ImageAnalysis.Builder()
    .setResolutionSelector(resolutionSelector)

Camera2Interop.Extender(analysisBuilder)
    .setCaptureRequestOption(
        CaptureRequest.CONTROL_AE_TARGET_FPS_RANGE,
        Range(minFps, maxFps)
    )
return analysisBuilder.build()

This example shows configuration for both VideoCapture and ImageAnalysis, but you can implement whichever use cases your app supports.

Understand how the stream behaves

Keep the following system behaviors in mind while working with capture streams:

  • Unsupported resolutions are rejected: If you request a size that isn't in the list, Camera2 rejects the configuration before the stream opens.
  • Requesting frame rates above the maximum: If you request a minimum frame rate above the maximum supported for your resolution, the session still opens, but the target frame rate from the glasses is clamped to that supported maximum while the upper bound continues to act as the catch-up rate limit. For example, requesting [30,60] at 640 x 480 clamps the target frame rate to 24 FPS (behaving like [24,60]).
  • Thermal throttling: When the glasses get warm, the frame rate drops but the resolution stays the same. Build your app to handle lower or uneven frame rates, for example by checking frame timestamps rather than assuming a fixed rate.

Access a phone's hardware from a projected activity

A 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 a projected Activity, get a context for the phone.
        ProjectedContext.createHostDeviceContext(activity)
    } catch (e: IllegalStateException) {
        Log.e(TAG, "Failed to create host device context", e)
        null
    }
}

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:

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