QCamera + QCameraViewfinder: how to mirror the the image displayed in the viewfinder?

Violet Giraffe

I have a car PC, and I've written a program to display the image from the rear view camera (connected through a USB video capture card). But the image from the camera has to be mirrored, hence my question.

SGaist

Ok good.

Did you consider QML ?

Violet Giraffe

This post is deleted!

Violet Giraffe

@SGaist, no, I never used QML and don't intend to ever use it. How would that help?

SGaist

You can do some post-processing more easily with QML.

Violet Giraffe

Okay then, would it be possible to write my own viewfinder widget that mirrors the image without actually implementing the whole viewfinder by hand myself?

I suppose it's hypothetically possible to make the viewfinder invisible, grab()it on timer (or render it to QPixmap, or whatever), manipulate the image and then draw that image in another, visible widget. But I have serious doubts about performance of this solution. It must work on an Intel Atom PC, and should produce minimal CPU load, too.

SGaist

One thing you can try:

Get the video surface format of your QCameraViewFinder
Call videoSurfaceProperty.setProperty("mirrored", true)
Then set it on again.

Violet Giraffe

Sorry, but I can't find anything related to video surface format in the QCameraViewFinder class. What's videoSurfaceProperty in your answer?
I can get QCameraViewFinderSettings from QCamera, but that doesn't appear to be what you're referring to. I've looked at the QCameraViewFinderSettings source file and there's no mention of the "mirrored" property.

SGaist

No, it's the QVideoRendererControl that you must first retrieve first.

The mirrored property I stumbled upon while checking something unrelated in QtMultimedia's sources.

Andy314

Hello @SGaist ,

how can I get the QVideoRenderControl?
From the QViewFinder, which function, I find no.

Violet Giraffe

@SGaist, thanks. I, too, can't find a way to get it from QViewFinder (which is a QVideoWidget). Looks like I need QMediaService to get QVideoRenderControl from it.

SGaist

QMediaService *mediaService = viewFinder->mediaObject()->service();
QVideoRendererControl *rendererControl = mediaService->requestControl<QVideoRendererControl *>();
rendererControl->setSurface(myVideoSurface);

Don't forget to add the necessary checks.

Violet Giraffe

@SGaist
No such luck :( viewFinder->mediaObject() is nullptr.
IS there a restriction on when I should call mediaObject() relative to, say, show() or any other lifecycle-related method of the view finder?

SGaist

Did you set the viewFinder on the camera before calling mediaObject ?

Violet Giraffe

@SGaist
Nope, i was doing it just before that. So, now I put this code right after setViewfinder, and mediaService is not null, but mediaService->requestControl<QVideoRendererControl *>() returns nullptr.

SGaist

What OS are you running ? It might be that the backend doesn't provide that control.

Violet Giraffe

Windows 10.

nicesw123

how to mirror the the image displayed in the viewfinder?

+1

Actually
+1000

This is a very good question!
I also want to mirror the image (flip it left to right)!

PS:
I'm using Debian Testing, which requires these packages, to get QCamera working: qt5-default, libqt5multimedia5, libqt5multimedia5-plugins, libqt5multimediaquick-p5, libqt5multimediawidgets5, qml-module-qtmultimedia, qtmultimedia5-dbg, qtmultimedia5-dev

SGaist

Just a quick update:

    QVideoRendererControl *rendererControl = camera.service()->requestControl<QVideoRendererControl *>();
    if (rendererControl) {
        QAbstractVideoSurface *surface = rendererControl->surface();
        QVideoSurfaceFormat format = surface->surfaceFormat();
        format.setProperty("mirrored", true);

        surface->stop();
        surface->start(format);
    } else {
        qDebug() << "Backend doesn't provide a video renderer control";
    }

Violet Giraffe

Thanks, but still no. rendererControl is null, and I tried executing your snippet both before and after camera.start() call.