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How to modify Camera Example to be able to process the video frames?



  • I am trying to modify the Camera Example so I can access each frame and do some processing (face detection, draw a rectangle around the face) before displaying it. I read that I have to subclass the QAbstractVideoSurface class and pass it as viewfinder but the viewfinder used in the example is from QCameraViewFinder class which is a subclass of QVideoWidget. I can't simply replace it as the ui compiler will complain about a couple of methods not being available. what needs to be exactly done? do I have to subclass both of the classes QAbstractVideoSurface and QVideoWidget? there's no documentation or example available on this. Any help and suggestion is greatly appreciated.


  • Lifetime Qt Champion

    Hi,

    What do you want to use for face detection ?
    If OpenCV then you should do the capture and processing with it and then display the result with Qt.



  • Actually the problem is how to get the frames from the camera. As you can see in that example a widget is promoted to QCameraViewFinder and it's used in designer. I tried to replace it with a class say FaceDetector which I subclassed from QAbstractVideoSurface but the ui compiler could not place it in a stacked layout complaining it's not a QWidget. I tried modifying the QCameraViewFinder which is subclassed from QVideoWidget but there's no way to get the frames from a QVideoWidget as I understand it's only for displaying the frames. So I want to know what should I replace that promoted widget with so I can achieve my objective. do I have to sublass both QVideoWidget and QAbstractVideoSurface together? I have already successfully done this that is detection of the face using OpenCV and Dlib in another application but now I want to make a GUI application with that ability. The closest example I could find was the Camera Example supplied with Qt. So all I need to do is get the frames, do my processing and display it. I read in a couple of examples online and here in this forum that I need to subclass QAbstractVideoSurface and implement the present() method. But I don't know how to replace the viewFinder of the said example by my class. I hope I've been able to explain clearly. Thank you for reading my question.



  • @morkia I think the way to go is subclassing QAbstractVideoFilter, check the details.
    One thing though, all the examples I saw so far use QML as the fornt end (i.e. to camera viewfinder)



  • Just a dumb WAG, but are you sure that you are reimplementing all the pure virtuals in the QAbstract classes you are using? Shouldn't even compile if not.



  • @Pablo-J-Rogina is there a way to do it in pure C++? I also can see more examples in QML than C++ but this shouldn't be impossible. Also the examples supplied by Qt are all in QML but one which I have mentioned in my earlier post.



  • @morkia said in How to modify Camera Example to be able to process the video frames?:

    is there a way to do it in pure C++?

    I really don't know. It's a big question for me why the QAbstractVideoFilter is implemented in C++ but all the examples I know come from QML side.

    You may want to take a look at QAbstractVideoFilter source code , where you'll find references to QVideoFrame and QAbstractVideoSurface as well.



  • @morkia One of the gotchas you may run into is that QAbstractVideoSurface can potentially represent either a buffer in GPU memory, or a buffer in CPU memory, depending on the platform and circumstances. So there may not actually be an image you can copy out of it and into your face detection code.


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