QCameraImageCapture captures an overexposed image

fama_lab3d

Hello,
my application uses a camera to both capture photos and display a real-time view like a video.
In particular, I'm using QGraphicsVideoItem as a viewfinder for the QCamera.
The video is rendered perfectly, but whenever I try to capture a single frame via QCameraImageCapture, the image is overexposed, so it's unusable.

I'll give you the code I use:

void image_acquisition_service::set_camera(QCameraInfo const &info)
{
    if(camera)
        camera->stop();

    camera = std::make_unique<QCamera>(info);
}

void image_acquisition_service::acquire_video(QGraphicsVideoItem *viewfinder, QCameraViewfinderSettings const &settings)
{
    if(camera)
    {
        camera->stop();
        camera->setCaptureMode(QCamera::CaptureVideo);
        camera->setViewfinder(viewfinder);
        camera->setViewfinderSettings(settings);
        camera->start();
    }
}

QImage image_acquisition_service::acquire_image()
{
    if(!camera)
        return {};

    camera->stop();

    QCameraImageCapture imagecapture(camera.get());
    imagecapture.setCaptureDestination(QCameraImageCapture::CaptureToBuffer);
    // Uses the highest resolution
    QList<QSize> resolutions = imagecapture.supportedResolutions();
    std::sort(resolutions.begin(), resolutions.end(), [](QSize const &a, QSize const &b) {
        return a.width() < b.width() && a.height() < b.height();
    });

    QImageEncoderSettings imageSettings;
    imageSettings.setResolution(resolutions.back());
    imageSettings.setQuality(QMultimedia::VeryHighQuality);
    imagecapture.setEncodingSettings(imageSettings);

    QImage img; // the image we'll capture

    // https://stackoverflow.com/questions/3556421/blocked-waiting-for-a-asynchronous-qt-signal
    QEventLoop loop;
    loop.connect(&imagecapture, &QCameraImageCapture::imageAvailable, &loop, [&img, &loop](int, QVideoFrame const &frame) { img = frame.image(); loop.quit(); });
    QObject::connect(camera.get(), &QCamera::errorOccurred, [&loop](QCamera::Error e) { qCritical() << "Error while acquiring the image: " << e; loop.quit(); });

    camera->setViewfinder(new QCameraViewfinder()); // use a fake viewfinder or QCamera::unbind will fail when re-acquiring the video
    camera->setCaptureMode(QCamera::CaptureStillImage);
    camera->start();
    camera->searchAndLock();
    imagecapture.capture();
    camera->unlock();

    loop.exec();

    camera->stop();

    return img;
}

So, my application chooses a camera and calls acquire_video(viewfinder, settings).
Then, when the users asks for a particular operation, I acquire a frame by calling acquire_image(), giving me an overexposed:
image.

SGaist

Hi and welcome to devnet,

I am not convinced about your code logique to acquire a still image. Do you have the same issue if using the official QCamera example ?

fama_lab3d

@SGaist thanks for the welcome :)
The logic I wrote is based on the code I've found on the docs regarding QCamera/QCameraImageCapture.
I'll do a test tomorrow morning at work with the code from the QCamera example.

fama_lab3d

@SGaist looking at the QCamera example gave me a lot of info to fix my issue.
I was using the QCamera wrongly.
Too many start/stop followed by lock/unlock etc.
Now the image is captured correctly.