屏幕闪光

屏幕闪光灯，也称为前置闪光灯或自拍闪光灯，利用手机屏幕亮度在低光照条件下使用前置摄像头拍摄图像时照亮拍摄对象。许多原生相机应用程序和社交媒体应用程序都提供此功能。由于大多数人在拍摄自拍照时会将手机拿得很近，因此这种方法非常有效。

但是，开发人员很难正确实现此功能并在各种设备上始终保持良好的拍摄质量。本指南演示了如何使用Camera2（低级别 Android 相机框架 API）正确实现此功能。

一般工作流程

要正确实现此功能，两个关键因素是预拍测光序列（自动曝光预拍）的使用和操作的时机。一般工作流程如图 1 所示。

Flow chart showing how a screen flash UI is used within Camera2.
图 1. 实现屏幕闪光灯的一般工作流程。

需要使用屏幕闪光灯功能拍摄图像时，将使用以下步骤。

  1. 应用屏幕闪光灯所需的 UI 更改，这可以为使用设备屏幕拍照提供足够的光线。对于一般用例，Google 建议使用以下 UI 更改，这些更改在我们测试中得到使用
    • 应用程序屏幕覆盖白色颜色叠加层。
    • 屏幕亮度最大化。
  2. 如果支持，则将自动曝光 (AE) 模式设置为CONTROL_AE_MODE_ON_EXTERNAL_FLASH。
  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。

  5. 收敛后，将使用应用程序的常规照片拍摄流程。

  6. 向框架发送捕获请求。

  7. 等待接收捕获结果。

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

  9. 清除屏幕闪光灯的 UI 更改。

Camera2 示例代码

用白色叠加层覆盖应用程序屏幕

在应用程序的布局 XML 文件中添加一个 View。该 View 具有足够的提升高度，可以在屏幕闪光灯捕获期间位于所有其他 UI 元素之上。默认情况下，它保持不可见，仅在应用屏幕闪光灯 UI 更改时才可见。

在以下代码示例中，白色 (#FFFFFF) 用作 View 的示例。应用程序可以根据其需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" />

最大化屏幕亮度

有多种方法可以在 Android 应用程序中更改屏幕亮度。一种直接的方法是更改screenBrightness Activity 窗口 参考中的 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 模式设置为CONTROL_AE_MODE_ON_EXTERNAL_FLASH

CONTROL_AE_MODE_ON_EXTERNAL_FLASH 可用于 API 级别 28 或更高版本。但是，并非所有设备都提供此 AE 模式，因此请检查 AE 模式是否可用并相应地设置值。要检查可用性，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;
}

如果应用程序设置了重复捕获请求（预览需要此请求），则需要将 AE 模式设置为重复请求。否则，它可能会在下一个重复捕获中被默认 AE 模式或其他用户设置的 AE 模式覆盖。如果发生这种情况，相机可能没有足够的时间执行其通常为外部闪光灯 AE 模式执行的所有操作。

为了确保相机完全处理 AE 模式更新请求，请检查重复捕获回调中的捕获结果，并等待结果中的 AE 模式更新。

可以等待 AE 模式更新的捕获回调

以下代码片段显示了如何实现这一点。

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禁用 AE 模式

有了捕获回调，以下代码示例显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();
        }
    }
}

触发预拍序列

要触发预拍测光序列，您可以提交一个CaptureRequest，并将CONTROL_AE_PRECAPTURE_TRIGGER_START 值设置为该请求。您需要等待处理该请求，然后等待 AE 和 AWB 收敛。

虽然预拍使用单个捕获请求触发，但等待 AE 和 AWB 收敛确实需要更高的复杂性。您可以使用设置为重复请求的捕获回调跟踪AE 状态 和AWB 状态。

更新相同的重复回调可以简化代码。应用程序通常需要预览，为此它们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初始重复请求一次，然后将其重复用于结果检查和等待。

捕获回调代码更新以等待收敛

要更新重复捕获回调，请使用以下代码片段。

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

以下代码示例允许您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();
}

预拍序列触发和等待

设置回调后，您可以使用以下代码示例进行预拍序列触发和等待。

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();
    }
}

将所有内容整合在一起

准备好所有主要组件后，每当需要拍摄照片时（例如，当用户点击捕获按钮拍摄照片时），所有步骤都将按照前面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

您可以从以下示例中看到不正确实现屏幕闪光灯以及正确实现屏幕闪光灯时会发生什么情况。

操作错误时

如果未正确实现屏幕闪光灯，则在多次捕获、不同设备和不同光照条件下会得到不一致的结果。通常，捕获的图像存在曝光不足或色偏问题。对于某些设备，这些类型的错误在特定光照条件下会更加明显，例如低光照环境而不是完全黑暗的环境。

下表显示了此类问题的示例。它们是在 CameraX 实验室基础设施中拍摄的，光源保持暖白色。这种暖白色光源可以让您看到蓝色色调是如何成为一个实际问题，而不是光源的副作用。

环境 曝光不足 曝光过度 色偏
黑暗环境（没有光源，只有手机） Nearly fully dark photo Overly lightened photo Photo with purplish tint
低光照（额外的约 3 lux 光源） Somewhat dark photo Overly lightened photo Photo with bluish tint

操作正确时

当对相同的设备和条件使用标准实现时，您可以在下表中看到结果。

环境 曝光不足（已修复） 曝光过度（已修复） 色偏（已修复）
黑暗环境（没有光源，只有手机） Clear photo Clear photo Clear photo without any tint
低光照（额外的约 3 lux 光源） Clear photo Clear photo A clear photo without tint

如观察到的那样，使用标准实现可以显著提高图像质量。