屏幕闪光灯也称为前置闪光灯或自拍闪光灯,用于在光线昏暗的环境下使用前置摄像头拍照时,利用手机屏幕亮度为拍摄对象提供照明。许多原生相机应用和社交媒体应用都支持该功能。由于大多数人在自拍自拍照时都会将手机保持在足够近的距离,因此这种方法非常有效。
但是,开发者很难正确实现该功能,并且在各种设备上一致地保持良好的拍摄质量。本指南介绍了如何使用低阶 Android 相机框架 API Camera2 正确实现此功能。
常规工作流程
为了正确实现该功能,两个关键因素是使用预拍摄测光序列(自动曝光预拍摄)和操作时间。一般工作流程如图 1 所示。
如果需要使用屏幕闪光灯功能拍摄图片,请按以下步骤操作。
- 应用屏幕闪烁所需的界面更改,以便提供足够的光源来使用设备屏幕拍照。对于一般用例,Google 建议对我们的测试进行以下界面更改:
- 应用屏幕被白色叠加层覆盖。
- 已最大化屏幕亮度。
- 将自动曝光 (AE) 模式设置为
CONTROL_AE_MODE_ON_EXTERNAL_FLASH
(如果支持)。 - 使用
CONTROL_AE_PRECAPTURE_TRIGGER
触发预拍测光序列。 等待自动曝光 (AE) 和自动白平衡 (AWB) 融合。
收敛后,系统将使用应用的常见照片拍摄流程。
向框架发送拍摄请求。
等待接收拍摄结果。
如果已设置
CONTROL_AE_MODE_ON_EXTERNAL_FLASH
,则重置 AE 模式。清除屏幕闪烁的界面更改。
Camera2 示例代码
带有白色叠加层的覆盖应用屏幕
在应用的布局 XML 文件中添加一个 View。在屏幕闪烁拍摄期间,视图有足够的高度来位于所有其他界面元素之上。默认情况下,它处于不可见状态,并且仅在应用屏幕闪烁界面时才可见。
以下代码示例使用白色 (#FFFFFF
) 作为视图的示例。应用可以根据需要选择颜色,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
在 Android 应用中,您可以通过多种方式更改屏幕亮度。一种直接方法是更改 Activity 窗口参考中的 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 模式设置为 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,此 AE 模式并非适用于所有设备,因此请检查 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; }
如果应用具有重复拍摄请求集(对于预览而言是必需的),则需要将 AE 模式设置为重复请求。否则,在下一次重复拍摄中,它可能会被默认或其他用户设置的 AE 模式覆盖。如果发生这种情况,相机可能无法获得足够的时间来执行其在外接闪光灯 AE 模式下正常执行的所有操作。
为了帮助确保相机完全处理 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 AE 模式在结果中更新。
拍摄可等待 AE 模式更新的回调
以下代码段展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停用 AE 模式
有了捕获回调后,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(); } } }
触发预拍序列
如需触发预拍测光序列,您可以提交 CaptureRequest
,并将 CONTROL_AE_PRECAPTURE_TRIGGER_START
值设置为请求。您需要等待系统处理请求,然后等待 AE 和 AWB 收敛。
虽然预拍触发器通过单个拍摄请求触发,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拍摄回调来跟踪 AE 状态和 AWB 状态。
通过更新同一重复回调,您可以简化代码。 应用通常需要在设置相机时为其设置重复请求的 Preview。因此,您可以将重复捕获回调对该初始重复请求设置一次,然后将其重复用于结果检查和等待目的。
捕获回调代码更新以等待收敛
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); } }
在摄像头设置期间针对重复请求设置回调
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 Listtargets = 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(); } }
拼接所有元素
准备好所有主要组件后,每当需要拍照时,当用户点击拍摄按钮拍照时,所有步骤都可以按照上文讨论和代码示例中所述的顺序执行。
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
您可以参考以下示例,了解未正确实现屏幕闪烁时会发生的情况,以及何时正确实现。
出错时
如果未正确实现屏幕闪烁,多次拍摄、设备和光照条件都会出现不一致的结果。通常,拍摄的照片会出现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(例如弱光环境而不是完全黑暗的环境)下会更明显。
下表显示了此类问题的示例。这些图像是在 CameraX 实验室基础架构拍摄的,其中的光源保持着暖白色。通过暖白光源,您可以看到蓝色色调成为实际问题,而不是光源的副作用。
环境 | 曝光不足 | 过度曝光 | 色调 |
---|---|---|---|
黑暗环境(没有光源,只有手机) | |||
弱光(额外提供约 3 勒克斯光源) |
正确做法
如果针对相同的设备和条件使用标准实现,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) | 色调调节(固定) |
---|---|---|---|
黑暗环境(没有光源,只有手机) | |||
弱光(额外提供约 3 勒克斯光源) |
如观察到,使用标准实现后,图片质量显著提高。