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Force QMediaPlayer to update position accurately for video scrubbing application?

  • I am writing an application which will allow the user to scrub through an open video. Developing on Windows 7/8 with Qt 5.3, I have been using QMediaPlayer and QVideoWidget following the qvideowidget example project. The result has been pretty good, except that the QVideoWidget seems only to update during idle time. Still, it's a good start and it's usable.

    However, when I build on Mac OS 10.10 (again with Qt 5.3), scrubbing behaves as though there were only one frame per second in the video. As I drag the "position" slider, the video jumps from one frame to the frame one second later, then one second after that, even though I am calling QMediaPlayer::setPosition several times with positions between those two frames.

    The problem can be reproduced using the videowidget example that ships with Qt 5.3 here: Qt\Examples\Qt-5.3\multimediawidgets\videowidget. When the slider is dragged on a Windows machine, the QVideoWidget moves between frames that are spaced fairly close together. When the slider is dragged on a Mac (at least on mine), the QVideoWidget jumps between frames spaced about one second apart. No matter how long I wait for an "in between" frame to render, it won't happen unless I hit the "play" button.

    I've tried calling QMediaPlayer::play() and QMediaPlayer::pause() one after the other to force an update, but this doesn't seem to work--QMediaPlayer works asynchronously, so the update doesn't have time to take effect.

    If I check the value of QMediaPlayer::position, I find that it actually doesn't change between these jumps. It appears that when I call QMediaPlayer::setPosition, it is actually rounding the position to one second increments on a Mac and finer increments on a Windows machine.

    Ideally, I would like to jump to a particular position in the video and render that frame immediately on the QVideoWidget. Is there any way to force QMediaPlayer to set the position accurately and update the associated QVideoWidget? Is there a better way to implement smooth scrubbing in a video?

    Thanks for your help!

  • In case anyone else has a similar problem...

    My best guess is that the issue stems from limitations in the codec used by QMediaPlayer, since this seems to be the main difference between the two platforms. Rather than deal with the codec issues directly, I looked around for other options.

    MLT (http://www.mltframework.org/) seemed promising, but it is a major pain to compile and the primary author seems to have settled on offering SDK support to commercial users only.

    libVLC (https://wiki.videolan.org/LibVLC/) looks a lot better. In particular, I've been using vlc-qt (https://github.com/ntadej/vlc-qt). The latter has an interface that will look quite familiar to users of QMediaPlayer and QVideoWidget. It was an easy replacement in my own application, and the result was much smoother video scrubbing on both Windows and Mac.

    Hope this helps someone else!


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