About Qt QImage::Format and OpenCV



    • Environment : Lenovo notepad-PC Y430P, Win10 64bit, Qt5.10.0, Visual Studio2017, OpenCV3.4.0 / Python3.6, opencv_python3.3.1

    • Scenario: Programming in Python, face recognition in image and video, everything is OK, using notepad-PC's own camera. However programming in Qt, recognition in image is OK, problem occurs in video.

    • Questions: #1 can = cvCreateCameraCapture(-1);return a address pointer, pretend it return right. pFrame = cvQueryFrame(cam); but this function return NULL! Why can not get frame from video steam? It is a question about opencv, I hope anyone can answer in Qt forum, lucky enough. ^_^
      #2 I think it maybe something wrong with the notepad-PC own camera, so I use USB camera, in this way pFrame = cvQueryFrame(cam); return non-NULL, I display the image in graphics view, but all gray. Code:

    pFrame = cvQueryFrame(cam);
    QImage image((const uchar *)pFrame->imageData, pFrame->width, pFrame->height, QImage::Format_RGB888);
    QGraphicsScene *scene = new QGraphicsScene;
    scene->addPixmap(QPixmap::fromImage(image));
    ui.graphicsView->setScene(scene);
    ui.graphicsView->show();
    

    Maybe the format is wrong, but I have trid all the variable QImage::Format_*** no one is right, How to distinguish format of image, in another way, when I use QImage::Format_RGB888, when I use other? I hope someone can tell me or provide some information link.

    Thanks.


  • Lifetime Qt Champion

    Hi,

    Did you check the format of the camera video stream ? You might have to convert it first before doing something with it with Qt.



  • Sorry, last week I was busy with work, have no time to learn Qt, so even though I saw your comment, I did not respond to you.

    Today I have tried the example in Qt install path, the program can find two camera: notepad-PC own camera and USB camera, and successfully show the video when I choose each one.

    The USB camera is an old one, I can not find its instructions, how can I get the format of the video stream? Is there any function in Qt can get the video format?

    I use the software in Windows 10 - Camera to record, the files of two camera are all .mp4. Is mp4 is the video format? And is right to use QImage::Format_RGB888?

    I have so much questions, and hope you can answer patiently, thanks very very very much! ^_^


  • Lifetime Qt Champion

    Ok, so now what are you using to get the image from your camera ?

    You can operate a colourspace conversion with OpenCV so that you ensure you are using RGB 8bit and then transformed into a QImage/QPixmap as needed.


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