如何在没有预览的情况下将焦点锁定在服务上的 Camera2API 应用程序上

Question

I am building an app which uses Camera2API to take pictures. The thing is I need the Camera to take a picture without needing a preview. So far, I managed to do it by dumping (and adapting) the code from an activity into a service and it works like a charm, except for the fact that it is not focusing. On previous versions I had a state machine in charge of that focusing on the preview by means of a separate CaptureRequest.Builder, but I can't make it work without creating a new CaptureRequest.Builder on the service.

I followed this topic on the following stackoverflow discussion How to lock focus in camera2 api, android? but I did not manage to make it work.

My code does the following: First I create a camera session once the camera has been opened.

public void createCameraSession() {
    try {
        // Here, we create a CameraCaptureSession for camera preview.
        cameraDevice.createCaptureSession(Arrays.asList(imageReader.getSurface()),
                new CameraCaptureSession.StateCallback() {

                    @Override
                    public void onConfigured(@NonNull CameraCaptureSession cameraCaptureSession) {
                        // The camera is already closed
                        if (null == cameraDevice) {
                            return;
                        }

                        // When the session is ready, we start displaying the preview.
                        mCaptureSession = cameraCaptureSession;

                        camera2TakePicture();
                    }

                    @Override
                    public void onConfigureFailed(
                            @NonNull CameraCaptureSession cameraCaptureSession) {
                    }
                }, null
        );
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

Then on that camera session I call my method "camera2TakePicture()":

protected void camera2TakePicture() {
    if (null == cameraDevice) {
        return;
    }
    try {
        Surface readerSurface = imageReader.getSurface();
        List<Surface> outputSurfaces = new ArrayList<Surface>(2);
        outputSurfaces.add(readerSurface);
        final CaptureRequest.Builder captureBuilder = cameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_STILL_CAPTURE);
        captureBuilder.addTarget(readerSurface);
        captureBuilder.set(CaptureRequest.CONTROL_MODE, CameraMetadata.CONTROL_MODE_AUTO);
        captureBuilder.set(CaptureRequest.CONTROL_AF_MODE, CameraMetadata.CONTROL_AF_MODE_AUTO);
        captureBuilder.set(CaptureRequest.CONTROL_AF_TRIGGER, CameraMetadata.CONTROL_AF_TRIGGER_START);
        //MeteringRectangle meteringRectangle = getAFRegion();
        //captureBuilder.set(CaptureRequest.CONTROL_AF_REGIONS, new MeteringRectangle[] {meteringRectangle});
        /**** TO BE USED ONCE SAMSUNG TABLETS HAVE BEEN REPLACED ****/
        boolean samsungReplaced = false;
        if(Boolean.parseBoolean(getPreferenceValue(this, "manualCamSettings"))) {
            int exposureCompensation = Integer.parseInt(getPreferenceValue(this, "exposureCompensation"));
            captureBuilder.set(CaptureRequest.CONTROL_AE_EXPOSURE_COMPENSATION, exposureCompensation);
            if(samsungReplaced) {
                //Exposure
                captureBuilder.set(CaptureRequest.CONTROL_AE_MODE, CameraMetadata.CONTROL_AE_MODE_OFF);
                Float shutterSpeed = 1 / Float.parseFloat(getPreferenceValue(this, "camSSpeed"));
                Long exposureTimeInNanoSec = new Long(Math.round(shutterSpeed * Math.pow(10, 9)));
                captureBuilder.set(CaptureRequest.SENSOR_EXPOSURE_TIME, exposureTimeInNanoSec);
                captureBuilder.set(CaptureRequest.SENSOR_FRAME_DURATION, 10 * exposureTimeInNanoSec);
                //ISO
                int ISO = Integer.parseInt(getPreferenceValue(this, "camISO"));
                captureBuilder.set(CaptureRequest.SENSOR_SENSITIVITY, ISO);
                //Aperture
                Float aperture = Float.parseFloat(getPreferenceValue(this, "camAperture"));
                captureBuilder.set(CaptureRequest.LENS_APERTURE, aperture);
            }
        }
        // Orientation
        WindowManager window = (WindowManager) getSystemService(Context.WINDOW_SERVICE);
        Display display = window.getDefaultDisplay();
        int rotation = display.getRotation();
        captureBuilder.set(CaptureRequest.JPEG_ORIENTATION, ORIENTATIONS.get(rotation));

        CameraCaptureSession.CaptureCallback CaptureCallback
                = new CameraCaptureSession.CaptureCallback() {

            @Override
            public void onCaptureCompleted(@NonNull CameraCaptureSession session,
                                           @NonNull CaptureRequest request,
                                           @NonNull TotalCaptureResult result) {
                super.onCaptureCompleted(session, request, result);
                while(result.get(CaptureResult.CONTROL_AF_STATE) != CaptureResult.CONTROL_AF_STATE_FOCUSED_LOCKED){
                    System.out.println("Not focused");
                }
                System.out.println("Focused");
            }
        };

        mCaptureSession.stopRepeating();
        mCaptureSession.abortCaptures();
        mCaptureSession.capture(captureBuilder.build(), CaptureCallback, null);
        captureBuilder.set(CaptureRequest.CONTROL_AF_TRIGGER, CameraMetadata.CONTROL_AF_TRIGGER_IDLE);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

As you can see, I set the CONTROL_AF_MODE to AUTO then start the AF_TRIGGER and launch the capture. I add a check on onCaptureCompleted() but the AF_STATE never seems to be on FOCUSED_LOCKED. It stays on ACTIVE_SCAN.

What am I doing wrong?

Answer 1

In your code snippet, you've stopped the repeating request, and issue one capture request for the still image, but just one.

Do you then go on to restart the repeating request? If you don't, there are no frames flowing through the camera, and AF cannot make progress.

So if you want to lock AF before you take a picture, you want to

1. Set AF_TRIGGER to START for a single capture only
2. Run preview until you get AE_STATE out of ACTIVE_SCAN
3. Issue single capture for still image.

Being in the background or foreground doesn't really change any of this.