螢幕閃光燈

螢幕閃光燈又稱為前置閃光燈或自拍閃光燈，在光線不足的情況下使用前置鏡頭拍照時，會使用手機的螢幕亮度來照亮拍攝目標。許多原生相機應用程式和社群媒體應用程式都提供這項功能。多數人在取景自拍照時拿著手機，因此這個方法很有效。

不過，開發人員很難正確實作這項功能，並在所有裝置上維持良好的拍攝品質。本指南說明如何使用低階 Android 相機架構 API Camera2 正確實作這項功能。

一般工作流程

為了正確實作這項功能，兩項關鍵因素為使用預先擷取計量序列 (自動曝光前測)，以及運算的時間。圖 1 已看到一般工作流程。

圖 1.實作螢幕閃光燈的一般工作流程。

需要使用螢幕閃光燈功能擷取圖片時，請參考下列步驟。

1. 套用對螢幕閃光燈所需的 UI 變更，如此便能提供足夠光源，使用裝置螢幕拍照。針對一般用途，Google 建議在測試中變更下列 UI：
   • 應用程式畫面以白色重疊方式覆蓋。
   • 螢幕亮度已最大化。
2. 將自動曝光 (AE) 模式設為 CONTROL_AE_MODE_ON_EXTERNAL_FLASH (如果支援)。
3. 使用 CONTROL_AE_PRECAPTURE_TRIGGER 觸發預先擷取的計量序列。

4. 等待自動曝光 (AE) 和自動白平衡 (AWB) 完成收斂。

   注意： 在檢查 AE 和 AWB 之前，請確認新的 AE 預先擷取作業開始執行，且 AWB 已收斂，因為 AE 和 AWB 可以在預先擷取觸發前和之前的另一組狀態交匯。

5. 轉換完成後，就會使用應用程式常用的相片拍攝流程。

6. 將擷取要求傳送至架構。

7. 等待接收擷取結果。

8. 如果已設定 CONTROL_AE_MODE_ON_EXTERNAL_FLASH，請重設 AE 模式。

9. 清除螢幕閃光燈的使用者介面變更。

Camera2 程式碼範例

封面應用程式畫面顯示白色重疊元素

在應用程式的版面配置 XML 檔案中新增 View。檢視畫面的高度充足，可在螢幕閃光燈擷取期間顯示在所有其他 UI 元素上方。這個屬性預設為隱藏，只有在套用螢幕閃光燈 UI 變更時才會顯示。

以下程式碼範例會使用白色顏色 (#FFFFFF) 做為檢視區塊的範例。應用程式可以根據使用者的需求，選擇或提供多種顏色。

<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" />

將螢幕亮度調到最大

在 Android 應用程式中變更螢幕亮度的方法有很多種，一種直接方法是變更「活動視窗」參考資料中的 screenBrightness WindowManager 參數。

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);
}

將 AE 模式設為 CONTROL_AE_MODE_ON_EXTERNAL_FLASH

CONTROL_AE_MODE_ON_EXTERNAL_FLASH 適用於 API 級別 28 以上。不過，此 AE 模式僅適用於部分裝置，因此請檢查 AE 模式是否可用，並據此設定值。如要查看供應情形，請使用 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;
}

如果應用程式設有重複擷取要求集 (預先發布版的必要設定)，AE 模式必須設為重複要求。否則，在下一次的重複擷取中，可能會被預設或其他使用者設定的 AE 模式覆寫。如果發生這種情況，相機可能無法足夠執行在外部閃光燈 AE 模式下正常執行的所有作業。

為確保相機完全處理 AE 模式更新要求，請在重複擷取回呼中查看拍攝結果，並等待 AE 模式在結果中更新。

擷取可等待 AE 模式更新的回呼

下列程式碼片段說明如何完成這項操作。

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();

設定重複要求，啟用或停用 AE 模式

使用擷取回呼後，下列程式碼範例示範如何設定重複要求。

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();
        }
    }
}

觸發預先拍攝序列

如要觸發預先擷取的計量序列，您可以提交 CaptureRequest，並將 CONTROL_AE_PRECAPTURE_TRIGGER_START 值設為要求。您必須等待要求處理完畢，然後等待 AE 和 AWB 交涉。

雖然利用單一擷取要求預先擷取觸發條件，但在等待 AE 和 AWB 收斂需要更加複雜。您可以使用設為重複要求的擷取回呼集，追蹤 AE 狀態和 AWB 狀態。

更新相同的重複回呼，讓程式碼更簡單。應用程式通常需要預覽，以便在設定相機時設定重複要求。因此，您可以將重複擷取回呼設為該初始重複要求一次，然後重複使用該回呼進行結果檢查及等待。

擷取回呼代碼更新，以等待收斂

如要更新重複擷取回呼，請使用下列程式碼片段。

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);
    }
}

在設定相機時，將回呼設為重複要求

下列程式碼範例可讓您在初始化期間，為重複要求設定回呼。

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();
}

預先擷取序列觸發與等待

透過設定回呼，您可以使用以下的程式碼範例，觸發預先擷取序列並等待。

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();
    }
}

將所有內容拼接起來

備妥所有主要元件後，每當需要拍照時，就像使用者點選拍攝按鈕拍照時，所有步驟都可以按照上述討論和程式碼範例所述的順序執行。

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);
            });
        });
    }
});

範例圖片

請查看下列範例，瞭解系統未正確實作螢幕閃光燈及正確實作時會發生的情況。

完成後

如果未正確實作螢幕閃光燈，就會在多個擷取、裝置和光線條件下取得不一致的結果。通常拍攝的圖片會有低曝光度或色彩色調問題。對某些裝置而言，這類錯誤在特定照明條件下會更明顯 (例如低光源環境，而非完全黑暗的環境)。

下表列出這類問題的示例。這些影像是在 CameraX 研究室基礎架構中拍攝，且光源仍維持在暖白色色。這種溫暖白光來源可讓您瞭解藍色的色調是實際的問題，而非光源的副作用。

環境	曝光不足	過度曝光	色調
昏暗環境 (沒有光源，只有手機)
低光源 (額外的 3 勒克斯光源)

適當時機

如果相同的裝置和條件採用標準實作，可以在下表中查看結果。

環境	曝光不足 (固定)	過度曝光 (固定)	色調 (固定)
昏暗環境 (沒有光源，只有手機)
低光源 (額外的 3 勒克斯光源)

根據觀察到，使用標準實作可大幅改善圖片品質。