Android Camera2 每次拍照前触发闪光灯打闪的实现

如下内容方便有Camera2基础人查看，如不了解Camera2的人不建议查看。

拍照流程
先执行CaptureRequest.CONTROL_AF_TRIGGER_START锁定AF，然后根据回调不断判断AF状态，伪代码实现如下

Integer afState = result.get(CaptureResult.CONTROL_AF_STATE);
if (afState == null) {
    //--执行拍照`
} else if (CaptureResult.CONTROL_AF_STATE_FOCUSED_LOCKED == afState ||
CaptureResult.CONTROL_AF_STATE_NOT_FOCUSED_LOCKED == afState) {
    //--判断AE状态
}

判断AF状态是lock或者unlock之后还需要在进一步判断AE状态是否收敛，然后在执行拍照。AF状态lock或unlock后判断AE状态判断伪代码如下

if (afState == null) {
    `//--执行拍照`
} else if (CaptureResult.CONTROL_AF_STATE_FOCUSED_LOCKED == afState ||
CaptureResult.CONTROL_AF_STATE_NOT_FOCUSED_LOCKED == afState) {
    if (aeState == null ||
        aeState == CaptureResult.CONTROL_AE_STATE_CONVERGED) {
        //--执行拍照
    } else {
        //--AE未收敛，主动执行AE_trigger_start
    }
}

如果AE未收敛则主动触发一次AE操作执行CONTROL_AE_PRECAPTURE_TRIGGER_START，然后根据Capture回调判断AE状态，伪代码如下：

Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
if (aeState == null ||
    aeState == CaptureResult.CONTROL_AE_STATE_PRECAPTURE ||
    aeState == CaptureRequest.CONTROL_AE_STATE_FLASH_REQUIRED) {
    //--锁定AE
}

AE收敛完成，为了确保场景亮度变化自动触发AE，这里需要锁定AE，伪代码如下

final CaptureRequest.Builder captureBuilder;
try {
    captureBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
    captureBuilder.addTarget(mPreviewSurface);
    setAutoFlash(captureBuilder);
    //--锁定AE
    captureBuilder.set(CaptureRequest.CONTROL_AE_LOCK, Boolean.TRUE);
    try {
        mCaptureSession.setRepeatingRequest(captureBuilder.build(), mCaptureCallback, mBackgroundHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
        Log.d("ArMn", "--------STATE_WATTING_LOCK_AE--------Excepture" );
    }
} catch (CameraAccessException e) {
    e.printStackTrace();
        }

//---监听AE LOCK状态
if (aeState == null || aeState == CaptureResult.CONTROL_AE_STATE_LOCKED) {
    //--------执行拍照
}

在AF和AE都满足拍照条件后，执行拍照。如上是拍照中AF和AE状态处理流程，也是确保图片质量的关键点。

现在我们讨论下如何在每次拍照前触发一次闪光灯
在每次拍照前触发闪光灯需要遵循多个条件

条件一
AE_MODE需要设置成CONTROL_AE_MODE_ON_ALWAYS_FLASH
条件二
FLASH_MODE需要设置成FLASH_MODE_SINGLE

完整代码实现如下：

按下拍照按钮

private void takePicture() {
    lockFocus();
}

锁AF（如果在锁定AF之前设置了CONTROL_AE_REGIONS，则AF LOCK会触发打闪）

private void lockFocus() {
    try {
        // This is how to tell the camera to lock focus.
        mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_TRIGGER,
                CaptureRequest.CONTROL_AF_TRIGGER_START);
        // Tell #mCaptureCallback to wait for the lock.
        mState = STATE_WAITING_LOCK;
        mCaptureSession.capture(mPreviewRequestBuilder.build(), mCaptureCallback,
                mBackgroundHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

判断AF状态

case STATE_WAITING_LOCK: {
    Integer afState = result.get(CaptureResult.CONTROL_AF_STATE);
    Log.d("ArMn", "--------- STATE_WAITING_LOCK-------afState: " + afState);
    if (afState == null) {
        captureStillPicture();
    } else if (CaptureResult.CONTROL_AF_STATE_FOCUSED_LOCKED == afState ||
            CaptureResult.CONTROL_AF_STATE_NOT_FOCUSED_LOCKED == afState) {
        // CONTROL_AE_STATE can be null on some devices
        Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
        if (aeState == null ||
                aeState == CaptureResult.CONTROL_AE_STATE_CONVERGED) {
            mState = STATE_PICTURE_TAKEN;
            Log.d("ArMn", "--------captureStillPicture--------");
            captureStillPicture();
        } else {
            Log.d("ArMn", "--------runPrecaptureSequence--------");
              runPrecaptureSequence();
        }
    }
    break;
}

执行AE打闪（如果AF没有触发打闪，则此处会触发打闪）

private void runPrecaptureSequence() {
    try {
        final CaptureRequest.Builder captureBuilder =
                mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW)
        captureBuilder.addTarget(mImageReader.getSurface());
        setAutoFlash(captureBuilder);
        captureBuilder.set(CaptureRequest.CONTROL_AE_PRECAPTURE_TRIGGER,
                CaptureRequest.CONTROL_AE_PRECAPTURE_TRIGGER_START);
        // This is how to tell the camera to trigger.
        // Tell #mCaptureCallback to wait for the precapture sequence to be set.
        mState = STATE_WAITING_PRECAPTURE;
        mCaptureSession.capture(captureBuilder.build(), mCaptureCallback,
                mBackgroundHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }

判断AE打闪状态

case STATE_WAITING_PRECAPTURE: {
    // CONTROL_AE_STATE can be null on some devices
    Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
    Log.d("ArMn", "--------STATE_WAITING_PRECAPTURE--------aeState: " + aeState);
    if (aeState == null ||
            aeState == CaptureResult.CONTROL_AE_STATE_PRECAPTURE ||
            aeState == CaptureRequest.CONTROL_AE_STATE_FLASH_REQUIRED) {
        mState = STATE_WATTING_LOCK_AE;
    }
    break;

锁定AE状态

    private void lockAe() {
        mState = STATE_WAITING_NON_PRECAPTURE;
        final CaptureRequest.Builder captureBuilder;
        try {
            captureBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
            captureBuilder.addTarget(mPreviewSurface);
            setAutoFlash(captureBuilder);
            captureBuilder.set(CaptureRequest.CONTROL_AE_LOCK, Boolean.TRUE);
            try {
                mCaptureSession.setRepeatingRequest(captureBuilder.build(), mCaptureCallback, mBackgroundHandler);
            } catch (CameraAccessException e) {
                e.printStackTrace();
                Log.d("ArMn", "--------STATE_WATTING_LOCK_AE--------Excepture" );
            }
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }

判断AE锁定状态

case STATE_WAITING_NON_PRECAPTURE: {
    // CONTROL_AE_STATE can be null on some devices
    Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
    Log.d("ArMn", "--------captureStillPicture-- STATE_WAITING_NON_PRECAPTURE ------state: " + aeState);
    if (aeState == null || aeState == CaptureResult.CONTROL_AE_STATE_LOCKED) {
        mState = STATE_PICTURE_TAKEN;
        captureStillPicture();
    }
    break;
}

执行拍照

private void captureStillPicture() {
    try {
        final Activity activity = getActivity();
        if (null == activity || null == mCameraDevice) {
            return;
        }
        // This is the CaptureRequest.Builder that we use to take a picture.
        final CaptureRequest.Builder captureBuilder =
                mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_STILL_CAPTURE);
        captureBuilder.addTarget(mImageReader.getSurface());
        // Use the same AE and AF modes as the preview.
        captureBuilder.set(CaptureRequest.CONTROL_AF_MODE,
                CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
        setAutoFlash(captureBuilder);
        mCaptureSession.capture(captureBuilder.build(), CaptureCallback, null);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

注意: 不要在锁定AE的时候下发CONTROL_AE_PRECAPTURE_TRIGGER，不然会导致闪光灯常亮。建议每触发一次capture都重新创建一次CapureRequest对象。如果在触发CONTROL_AE_PRECAPTURE_TRIGGER成功之后不锁定AE，闪光灯会很快关闭。如果执行了锁定AE会在AE状态lock后关闭

Flash常用的如下设置(摘抄高通Camera源码)

    private void applyFlash(CaptureRequest.Builder request, String value) {

        switch (value) {
            case "on":
                if (mLongshotActive) {
                    request.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_ON);
                    request.set(CaptureRequest.FLASH_MODE, CaptureRequest.FLASH_MODE_TORCH);
                } else {
                    request.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_ON_ALWAYS_FLASH);
                    request.set(CaptureRequest.FLASH_MODE, CaptureRequest.FLASH_MODE_SINGLE);
                }
                break;
            case "auto":
                if (mLongshotActive) {
                    // When long shot is active, turn off the flash in auto mode
                    request.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_ON);
                    request.set(CaptureRequest.FLASH_MODE, CaptureRequest.FLASH_MODE_OFF);
                } else {
                    request.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_ON_AUTO_FLASH);
                    request.set(CaptureRequest.FLASH_MODE, CaptureRequest.FLASH_MODE_SINGLE);
                }
                break;
            case "off":
                request.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_ON);
                request.set(CaptureRequest.FLASH_MODE, CaptureRequest.FLASH_MODE_OFF);
                break;
        }
    }