QAbstractVideoSurface present silent crash on image copy

  • Hi,

    I try to copy the frame passed to the function present of my QAbstractVideoSurface class. When I place a breakpoint at the start of the function, and hit play manually to pause the application flow at each present calls, there is no crash. If I remove the breakpoint and let the application run normally, the application will crash at the second call to the function.

    Here is my function

    bool VideoSurface::present(const QVideoFrame &frame)
    if (frame.isValid())
    QVideoFrame videoFrame(frame);
    if( )
    m_lastFrame = QImage(videoFrame.width(), videoFrame.height(), QImage::Format_ARGB32);
    memcpy(m_lastFrame.bits(), videoFrame.bits(), videoFrame.mappedBytes());

            return true;
    return true;


    Does any one have an idea why?

    Thank you!

  • Lifetime Qt Champion


    Are you sure that frame is mapped ?

  • Well that was a good observation, but I beleive it is not mapped. I changed

    @if( )@


    @if( videoFrame.isMapped() && )@

    And the application still crash.

    I added a qDebug() call before the memcpy and just after it, and it seems the application crashes on the first call of memcpy, not at the second as I thought.

    If I add a breakpoint at the start of the function, start debugging and hit play as soon as the breakpoint is reached, I got the second qDebug() call and the memcpy succeed.

    I also add a videoFrame.isMapped() condition just before the memcpy, to be safe. And the application still crashed at the first call of memcpy.

  • Well my QImage bytes space is lower than the frame...Here is my debug :

    @qDebug() << "frame buffer bits address is " << videoFrame.bits() << " and is composed of " << videoFrame.mappedBytes() << "bytes with format " << videoFrame.pixelFormat();@
    frame buffer bits address is 0x112000000 and is composed of 3686432 bytes with format Format_RGB32

    @qDebug() << "image buffer bits address is " << m_lastFrame.bits() << " and is composed of " << m_lastFrame.byteCount() << "bytes with color model " << m_lastFrame.pixelFormat().colorModel() << " and TypeInterpretation " << m_lastFrame.pixelFormat().typeInterpretation();@
    image buffer bits address is 0x112385000 and is composed of 3686400 bytes with color model 0 and TypeInterpretation 0

    So this is why the memcpy failed. But I do not understand why they do not have the same size in bytes...

    The color model 0 is QPixelFormat::RGB and the typeInterpretation 0 is QPixelFormat::UnsignedInteger

  • Lifetime Qt Champion

    Is it me or does it looks like there's one pixel messing around ?

  • Isn't 32 bytes missing? With 32 bits per pixels, there are likely 4 pixels messing around.

    Still, this is strange !

    For now, I used m_lastFrame.byteCount() for the memcpy and it is working fine. Do you think this could be a bug from the Qt side?

    Thanks !

  • Lifetime Qt Champion

    Yes, you're right, that makes it 4, late night math :D

    Pretty good question, I'd recommend asking on the interest mailing list and you should also check the "bug report system":

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