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

[Violet Giraffe]

I need the live view image from the camera being displayed in the QCameraViewfinder to be mirrored (flipped horizontally). What are my options for doing this?

[Violet Giraffe]

Please don't tell me it's not possible...

[SGaist]

Hi,

I don't remember that option but that doesn't mean it doesn't exists. What is your use case ?

[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:

1. Get the video surface format of your QCameraViewFinder
2. Call videoSurfaceProperty.setProperty("mirrored", true)
3. 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.