屏幕闪烁

屏幕闪烁（也称为前置闪光灯或自拍闪光灯）是指在弱光条件下使用前置摄像头拍摄图像时，利用手机屏幕亮度照亮拍摄对象。许多原生相机应用和社交媒体应用都支持此功能。由于大多数人在自拍时都会将手机拿得足够近，因此这种方法很有效。

不过，开发者很难正确实现该功能，并在各种设备上始终保持良好的捕获质量。本指南介绍了如何使用 Camera2（低级 Android 相机框架 API）正确实现此功能。

常规工作流程

为了正确实现该功能，有两个关键因素：预拍摄测光序列（自动曝光预拍摄）的使用和操作的时机。一般工作流程如图 1 所示。

图 1. 实现屏幕闪烁的一般工作流程。

当需要使用屏幕闪烁功能拍摄图片时，请执行以下步骤。

1. 应用屏幕闪烁所需的界面更改，这可以为使用设备屏幕拍照提供充足的光线。对于一般用例，Google 建议进行以下界面更改（如我们的测试中所用）：
   • 应用界面上覆盖着一层白色。
   • 屏幕亮度已调至最高。
2. 将自动曝光 (AE) 模式设置为 CONTROL_AE_MODE_ON_EXTERNAL_FLASH（如果支持）。
3. 使用 CONTROL_AE_PRECAPTURE_TRIGGER 触发预拍测光序列。

4. 等待自动曝光 (AE) 和自动白平衡 (AWB) 收敛。

   注意 ：在检查 AE 和 AWB 是否收敛之前，请确保新的 AE 预捕获已开始，因为 AE 和 AWB 可能会与预捕获触发器之前的一些其他状态集收敛。

5. 收敛后，系统会使用应用的常规照片拍摄流程。

6. 向框架发送拍摄请求。

7. 等待接收拍摄结果。

8. 如果设置了 CONTROL_AE_MODE_ON_EXTERNAL_FLASH，则重置 AE 模式。

9. 清除屏幕闪烁的界面更改。

Camera2 示例代码

用白色叠加层覆盖应用界面

在应用的布局 XML 文件中添加一个 View。该视图具有足够的高度，可在屏幕闪烁捕获期间位于所有其他界面元素的顶部。默认情况下，该视图处于不可见状态，只有在应用屏幕闪烁界面更改时才会显示。

在以下代码示例中，白色 (#FFFFFF) 用作视图的示例。应用可以根据其要求选择颜色，或为用户提供多种颜色。

<View
    android:id="@+id/white_color_overlay"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#FFFFFF"
    android:visibility="invisible"
    android:elevation="8dp" />

将屏幕亮度调到最高

在 Android 应用中，有多种方法可以更改屏幕亮度。一种直接的方法是更改 Activity 窗口引用中的 screenBrightness WindowManager 参数。

Kotlin

private var previousBrightness: Float = -1.0f

private fun maximizeScreenBrightness() {
    activity?.window?.let { window ->
        window.attributes?.apply {
            previousBrightness = screenBrightness
            screenBrightness = 1f
            window.attributes = this
        }
    }
}

private fun restoreScreenBrightness() {
    activity?.window?.let { window ->
        window.attributes?.apply {
            screenBrightness = previousBrightness
            window.attributes = this
        }
    }
}

Java

private float mPreviousBrightness = -1.0f;

private void maximizeScreenBrightness() {
    if (getActivity() == null || getActivity().getWindow() == null) {
        return;
    }

    Window window = getActivity().getWindow();
    WindowManager.LayoutParams attributes = window.getAttributes();

    mPreviousBrightness = attributes.screenBrightness;
    attributes.screenBrightness = 1f;
    window.setAttributes(attributes);
}

private void restoreScreenBrightness() {
    if (getActivity() == null || getActivity().getWindow() == null) {
        return;
    }

    Window window = getActivity().getWindow();
    WindowManager.LayoutParams attributes = window.getAttributes();

    attributes.screenBrightness = mPreviousBrightness;
    window.setAttributes(attributes);
}

将 AE 模式设为 CONTROL_AE_MODE_ON_EXTERNAL_FLASH

CONTROL_AE_MODE_ON_EXTERNAL_FLASH 适用于 API 级别 28 或更高级别。 不过，并非所有设备都支持此 AE 模式，因此请检查该 AE 模式是否可用，并相应地设置值。如需检查可用性，请使用 CameraCharacteristics#CONTROL_AE_AVAILABLE_MODES。

Kotlin

private val characteristics: CameraCharacteristics by lazy {
    cameraManager.getCameraCharacteristics(cameraId)
}

@RequiresApi(Build.VERSION_CODES.P)
private fun isExternalFlashAeModeAvailable() =
    characteristics.get(CameraCharacteristics.CONTROL_AE_AVAILABLE_MODES)
        ?.contains(CaptureRequest.CONTROL_AE_MODE_ON_EXTERNAL_FLASH) ?: false

Java

try {
    mCharacteristics = mCameraManager.getCameraCharacteristics(mCameraId);
} catch (CameraAccessException e) {
    e.printStackTrace();
}

@RequiresApi(Build.VERSION_CODES.P)
private boolean isExternalFlashAeModeAvailable() {
    int[] availableAeModes = mCharacteristics.get(CameraCharacteristics.CONTROL_AE_AVAILABLE_MODES);

    for (int aeMode : availableAeModes) {
        if (aeMode == CaptureRequest.CONTROL_AE_MODE_ON_EXTERNAL_FLASH) {
            return true;
        }
    }
    return false;
}

如果应用设置了重复拍摄请求（预览需要），则需要为重复请求设置 AE 模式。否则，在下一次重复拍摄中，它可能会被默认或用户设置的其他 AE 模式覆盖。如果发生这种情况，相机可能没有足够的时间来执行其通常针对外接闪光灯 AE 模式执行的所有操作。

为确保相机完全处理 AE 模式更新请求，请在重复拍摄回调中检查拍摄结果，并等待结果中的 AE 模式更新。

可等待 AE 模式更新的拍摄回调

以下代码段展示了如何实现此目的。

Kotlin

private val repeatingCaptureCallback = object : CameraCaptureSession.CaptureCallback() {
    private var targetAeMode: Int? = null
    private var aeModeUpdateDeferred: CompletableDeferred? = null

    suspend fun awaitAeModeUpdate(targetAeMode: Int) {
        this.targetAeMode = targetAeMode
        aeModeUpdateDeferred = CompletableDeferred()
        // Makes the current coroutine wait until aeModeUpdateDeferred is completed. It is
        // completed once targetAeMode is found in the following capture callbacks
        aeModeUpdateDeferred?.await()
    }

    private fun process(result: CaptureResult) {
        // Checks if AE mode is updated and completes any awaiting Deferred
        aeModeUpdateDeferred?.let {
            val aeMode = result[CaptureResult.CONTROL_AE_MODE]
            if (aeMode == targetAeMode) {
                it.complete(Unit)
            }
        }
    }

    override fun onCaptureCompleted(
        session: CameraCaptureSession,
        request: CaptureRequest,
        result: TotalCaptureResult
    ) {
        super.onCaptureCompleted(session, request, result)
        process(result)
    }
}

Java

static class AwaitingCaptureCallback extends CameraCaptureSession.CaptureCallback {
    private int mTargetAeMode;
    private CountDownLatch mAeModeUpdateLatch = null;

    public void awaitAeModeUpdate(int targetAeMode) {
        mTargetAeMode = targetAeMode;
        mAeModeUpdateLatch = new CountDownLatch(1);
        // Makes the current thread wait until mAeModeUpdateLatch is released, it will be
        // released once targetAeMode is found in the capture callbacks below
        try {
            mAeModeUpdateLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void process(CaptureResult result) {
        // Checks if AE mode is updated and decrements the count of any awaiting latch
        if (mAeModeUpdateLatch != null) {
            int aeMode = result.get(CaptureResult.CONTROL_AE_MODE);
            if (aeMode == mTargetAeMode) {
                mAeModeUpdateLatch.countDown();
            }
        }
    }

    @Override
    public void onCaptureCompleted(@NonNull CameraCaptureSession session,
            @NonNull CaptureRequest request,
            @NonNull TotalCaptureResult result) {
        super.onCaptureCompleted(session, request, result);
        process(result);
    }
}

private final AwaitingCaptureCallback mRepeatingCaptureCallback = new AwaitingCaptureCallback();

设置重复请求以启用或停用 AE 模式

设置好捕获回调后，以下代码示例展示了如何设置重复请求。

Kotlin

/** [HandlerThread] where all camera operations run */
private val cameraThread = HandlerThread("CameraThread").apply { start() }

/** [Handler] corresponding to [cameraThread] */
private val cameraHandler = Handler(cameraThread.looper)

private suspend fun enableExternalFlashAeMode() {
    if (Build.VERSION.SDK_INT >= 28 && isExternalFlashAeModeAvailable()) {
        session.setRepeatingRequest(
            camera.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW).apply {
                addTarget(previewSurface)
                set(
                    CaptureRequest.CONTROL_AE_MODE,
                    CaptureRequest.CONTROL_AE_MODE_ON_EXTERNAL_FLASH
                )
            }.build(), repeatingCaptureCallback, cameraHandler
        )

        // Wait for the request to be processed by camera
        repeatingCaptureCallback.awaitAeModeUpdate(CaptureRequest.CONTROL_AE_MODE_ON_EXTERNAL_FLASH)
    }
}

private fun disableExternalFlashAeMode() {
    if (Build.VERSION.SDK_INT >= 28 && isExternalFlashAeModeAvailable()) {
        session.setRepeatingRequest(
            camera.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW).apply {
                addTarget(previewSurface)
            }.build(), repeatingCaptureCallback, cameraHandler
        )
    }
}

Java

private void setupCameraThread() {
    // HandlerThread where all camera operations run
    HandlerThread cameraThread = new HandlerThread("CameraThread");
    cameraThread.start();

    // Handler corresponding to cameraThread
    mCameraHandler = new Handler(cameraThread.getLooper());
}

private void enableExternalFlashAeMode() {
    if (Build.VERSION.SDK_INT >= 28 && isExternalFlashAeModeAvailable()) {
        try {
            CaptureRequest.Builder requestBuilder = mCamera.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
            requestBuilder.addTarget(mPreviewSurface);
            requestBuilder.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_ON_EXTERNAL_FLASH);
            mSession.setRepeatingRequest(requestBuilder.build(), mRepeatingCaptureCallback, mCameraHandler);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }

        // Wait for the request to be processed by camera
        mRepeatingCaptureCallback.awaitAeModeUpdate(CaptureRequest.CONTROL_AE_MODE_ON_EXTERNAL_FLASH);
    }
}

private void disableExternalFlashAeMode() {
    if (Build.VERSION.SDK_INT >= 28 && isExternalFlashAeModeAvailable()) {
        try {
            CaptureRequest.Builder requestBuilder = mCamera.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
            requestBuilder.addTarget(mPreviewSurface);
            mSession.setRepeatingRequest(requestBuilder.build(), mRepeatingCaptureCallback, mCameraHandler);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }
}

触发预拍摄序列

如需触发预捕获测光序列，您可以提交一个 CaptureRequest，并将 CONTROL_AE_PRECAPTURE_TRIGGER_START 值设置为请求。您需要等待请求处理完毕，然后等待 AE 和 AWB 收敛。

虽然预拍摄会通过单个拍摄请求触发，但等待 AE 和 AWB 收敛确实需要更复杂的处理。您可以使用设置为重复请求的拍摄回调来跟踪 AE 状态和 AWB 状态。

更新同一重复回调可让您简化代码。 应用通常需要一个预览，以便在设置相机时设置重复请求。因此，您可以将重复捕获回调设置为该初始重复请求一次，然后将其重复用于结果检查和等待。

更新了拍摄回调代码，以等待收敛

如需更新重复拍摄回调，请使用以下代码段。

Kotlin

private val repeatingCaptureCallback = object : CameraCaptureSession.CaptureCallback() {
    private var targetAeMode: Int? = null
    private var aeModeUpdateDeferred: CompletableDeferred? = null

    private var convergenceDeferred: CompletableDeferred? = null

    suspend fun awaitAeModeUpdate(targetAeMode: Int) {
        this.targetAeMode = targetAeMode
        aeModeUpdateDeferred = CompletableDeferred()
        // Makes the current coroutine wait until aeModeUpdateDeferred is completed. It is
        // completed once targetAeMode is found in the following capture callbacks
        aeModeUpdateDeferred?.await()
    }

    suspend fun awaitAeAwbConvergence() {
        convergenceDeferred = CompletableDeferred()
        // Makes the current coroutine wait until convergenceDeferred is completed, it will be
        // completed once both AE & AWB are reported as converged in the capture callbacks below
        convergenceDeferred?.await()
    }

    private fun process(result: CaptureResult) {
        // Checks if AE mode is updated and completes any awaiting Deferred
        aeModeUpdateDeferred?.let {
            val aeMode = result[CaptureResult.CONTROL_AE_MODE]
            if (aeMode == targetAeMode) {
                it.complete(Unit)
            }
        }

        // Checks for convergence and completes any awaiting Deferred
        convergenceDeferred?.let {
            val aeState = result[CaptureResult.CONTROL_AE_STATE]
            val awbState = result[CaptureResult.CONTROL_AWB_STATE]

            val isAeReady = (
                    aeState == null // May be null in some devices (e.g. legacy camera HW level)
                            || aeState == CaptureResult.CONTROL_AE_STATE_CONVERGED
                            || aeState == CaptureResult.CONTROL_AE_STATE_FLASH_REQUIRED
                    )

            val isAwbReady = (
                    awbState == null // May be null in some devices (e.g. legacy camera HW level)
                            || awbState == CaptureResult.CONTROL_AWB_STATE_CONVERGED
                    )

            if (isAeReady && isAwbReady) {
                // if any non-null convergenceDeferred is set, complete it
                it.complete(Unit)
            }
        }
    }

    override fun onCaptureCompleted(
        session: CameraCaptureSession,
        request: CaptureRequest,
        result: TotalCaptureResult
    ) {
        super.onCaptureCompleted(session, request, result)
        process(result)
    }
}

Java

static class AwaitingCaptureCallback extends CameraCaptureSession.CaptureCallback {
    private int mTargetAeMode;
    private CountDownLatch mAeModeUpdateLatch = null;

    private CountDownLatch mConvergenceLatch = null;

    public void awaitAeModeUpdate(int targetAeMode) {
        mTargetAeMode = targetAeMode;
        mAeModeUpdateLatch = new CountDownLatch(1);
        // Makes the current thread wait until mAeModeUpdateLatch is released, it will be
        // released once targetAeMode is found in the capture callbacks below
        try {
            mAeModeUpdateLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void awaitAeAwbConvergence() {
        mConvergenceLatch = new CountDownLatch(1);
        // Makes the current coroutine wait until mConvergenceLatch is released, it will be
        // released once both AE & AWB are reported as converged in the capture callbacks below
        try {
            mConvergenceLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void process(CaptureResult result) {
        // Checks if AE mode is updated and decrements the count of any awaiting latch
        if (mAeModeUpdateLatch != null) {
            int aeMode = result.get(CaptureResult.CONTROL_AE_MODE);
            if (aeMode == mTargetAeMode) {
                mAeModeUpdateLatch.countDown();
            }
        }

        // Checks for convergence and decrements the count of any awaiting latch
        if (mConvergenceLatch != null) {
            Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
            Integer awbState = result.get(CaptureResult.CONTROL_AWB_STATE);

            boolean isAeReady = (
                    aeState == null // May be null in some devices (e.g. legacy camera HW level)
                            || aeState == CaptureResult.CONTROL_AE_STATE_CONVERGED
                            || aeState == CaptureResult.CONTROL_AE_STATE_FLASH_REQUIRED
            );

            boolean isAwbReady = (
                    awbState == null // May be null in some devices (e.g. legacy camera HW level)
                            || awbState == CaptureResult.CONTROL_AWB_STATE_CONVERGED
            );

            if (isAeReady && isAwbReady) {
                mConvergenceLatch.countDown();
                mConvergenceLatch = null;
            }
        }
    }

    @Override
    public void onCaptureCompleted(@NonNull CameraCaptureSession session,
            @NonNull CaptureRequest request,
            @NonNull TotalCaptureResult result) {
        super.onCaptureCompleted(session, request, result);
        process(result);
    }
}

在相机设置期间将回调设置为重复请求

通过以下代码示例，您可以在初始化期间将回调设置为重复请求。

Kotlin

// Open the selected camera
camera = openCamera(cameraManager, cameraId, cameraHandler)

// Creates list of Surfaces where the camera will output frames
val targets = listOf(previewSurface, imageReaderSurface)

// Start a capture session using our open camera and list of Surfaces where frames will go
session = createCameraCaptureSession(camera, targets, cameraHandler)

val captureRequest = camera.createCaptureRequest(
        CameraDevice.TEMPLATE_PREVIEW).apply { addTarget(previewSurface) }

// This will keep sending the capture request as frequently as possible until the
// session is torn down or session.stopRepeating() is called
session.setRepeatingRequest(captureRequest.build(), repeatingCaptureCallback, cameraHandler)

Java

// Open the selected camera
mCamera = openCamera(mCameraManager, mCameraId, mCameraHandler);

// Creates list of Surfaces where the camera will output frames
List targets = new ArrayList<>(Arrays.asList(mPreviewSurface, mImageReaderSurface));

// Start a capture session using our open camera and list of Surfaces where frames will go
mSession = createCaptureSession(mCamera, targets, mCameraHandler);

try {
    CaptureRequest.Builder requestBuilder = mCamera.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
    requestBuilder.addTarget(mPreviewSurface);

    // This will keep sending the capture request as frequently as possible until the
    // session is torn down or session.stopRepeating() is called
    mSession.setRepeatingRequest(requestBuilder.build(), mRepeatingCaptureCallback, mCameraHandler);
} catch (CameraAccessException e) {
    e.printStackTrace();
}

预拍摄序列触发和等待

设置回调后，您可以使用以下代码示例触发和等待预拍摄序列。

Kotlin

private suspend fun runPrecaptureSequence() {
    // Creates a new capture request with CONTROL_AE_PRECAPTURE_TRIGGER_START
    val captureRequest = session.device.createCaptureRequest(
        CameraDevice.TEMPLATE_PREVIEW
    ).apply {
        addTarget(previewSurface)
        set(
            CaptureRequest.CONTROL_AE_PRECAPTURE_TRIGGER,
            CaptureRequest.CONTROL_AE_PRECAPTURE_TRIGGER_START
        )
    }

    val precaptureDeferred = CompletableDeferred()
    session.capture(captureRequest.build(), object: CameraCaptureSession.CaptureCallback() {
        override fun onCaptureCompleted(
            session: CameraCaptureSession,
            request: CaptureRequest,
            result: TotalCaptureResult
        ) {
            // Waiting for this callback ensures the precapture request has been processed
            precaptureDeferred.complete(Unit)
        }
    }, cameraHandler)

    precaptureDeferred.await()

    // Precapture trigger request has been processed, we can wait for AE & AWB convergence now
    repeatingCaptureCallback.awaitAeAwbConvergence()
}

Java

private void runPrecaptureSequence() {
    // Creates a new capture request with CONTROL_AE_PRECAPTURE_TRIGGER_START
    try {
        CaptureRequest.Builder requestBuilder =
                mSession.getDevice().createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        requestBuilder.addTarget(mPreviewSurface);
        requestBuilder.set(CaptureRequest.CONTROL_AE_PRECAPTURE_TRIGGER,
                CaptureRequest.CONTROL_AE_PRECAPTURE_TRIGGER_START);

        CountDownLatch precaptureLatch = new CountDownLatch(1);
        mSession.capture(requestBuilder.build(), new CameraCaptureSession.CaptureCallback() {
            @Override
            public void onCaptureCompleted(@NonNull CameraCaptureSession session,
                                           @NonNull CaptureRequest request,
                                           @NonNull TotalCaptureResult result) {
                Log.d(TAG, "CONTROL_AE_PRECAPTURE_TRIGGER_START processed");
                // Waiting for this callback ensures the precapture request has been processed
                precaptureLatch.countDown();
            }
        }, mCameraHandler);

        precaptureLatch.await();

        // Precapture trigger request has been processed, we can wait for AE & AWB convergence now
        mRepeatingCaptureCallback.awaitAeAwbConvergence();
    } catch (CameraAccessException | InterruptedException e) {
        e.printStackTrace();
    }
}

将所有内容拼接在一起

所有主要组件准备就绪后，每当需要拍摄照片时（例如当用户点击拍摄按钮拍摄照片时），所有步骤都可以按照前面的讨论和代码示例中提到的顺序执行。

Kotlin

// User clicks captureButton to take picture
captureButton.setOnClickListener { v ->
    // Apply the screen flash related UI changes
    whiteColorOverlayView.visibility = View.VISIBLE
    maximizeScreenBrightness()

    // Perform I/O heavy operations in a different scope
    lifecycleScope.launch(Dispatchers.IO) {
        // Enable external flash AE mode and wait for it to be processed
        enableExternalFlashAeMode()

        // Run precapture sequence and wait for it to complete
        runPrecaptureSequence()

        // Start taking picture and wait for it to complete
        takePhoto()

        disableExternalFlashAeMode()
        v.post {
            // Clear the screen flash related UI changes
            restoreScreenBrightness()
            whiteColorOverlayView.visibility = View.INVISIBLE
        }
    }
}

Java

// User clicks captureButton to take picture
mCaptureButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        // Apply the screen flash related UI changes
        mWhiteColorOverlayView.setVisibility(View.VISIBLE);
        maximizeScreenBrightness();

        // Perform heavy operations in a different thread
        Executors.newSingleThreadExecutor().execute(() -> {
            // Enable external flash AE mode and wait for it to be processed
            enableExternalFlashAeMode();

            // Run precapture sequence and wait for it to complete
            runPrecaptureSequence();

            // Start taking picture and wait for it to complete
            takePhoto();

            disableExternalFlashAeMode();

            v.post(() -> {
                // Clear the screen flash related UI changes
                restoreScreenBrightness();
                mWhiteColorOverlayView.setVisibility(View.INVISIBLE);
            });
        });
    }
});

示例图片

通过以下示例，您可以了解屏幕闪烁功能实现不正确和实现正确时的情况。

如果操作不当

如果屏幕闪烁功能未正确实现，您在多次拍摄、不同设备和不同光照条件下会得到不一致的结果。通常，拍摄的图片存在曝光不佳或色调问题。对于某些设备，这类 bug 在特定光照条件下（例如弱光环境，而非完全黑暗的环境）会变得更加明显。

下表显示了此类问题的示例。这些照片是在 CameraX 实验室基础架构中拍摄的，光源保持暖白色。通过这种暖白光源，您可以看出蓝色色调是一个实际问题，而不是光源的副作用。

环境	曝光不足	过度曝光	色调
昏暗环境（除手机外无其他光源）
弱光（额外 ~3 勒克斯光源）

如果操作得当

在相同设备和条件下使用标准实现时，您可以在下表中看到结果。

环境	曝光不足（已修正）	过度曝光（已修正）	色调问题（已修正）
昏暗环境（除手机外无其他光源）
弱光（额外 ~3 勒克斯光源）

如您所见，标准实现可显著提升图片质量。