Use QtAV for video thumbnailing - qt

Question

I'm using the wonderful [QtAV](https://github.com/wang-bin/QtAV/) package to perform video decoding. What I want to achieve is to obtain a thumbnail for a video. Here is what I have done so far.
bool saveThumb( QString videoFile ) {
AVDemuxer *demux = new AVDemuxer();
demux->setSeekUnit( SeekByFrame );
demux->setSeekType( KeyFrameSeek );
VideoDecoder *vdec = VideoDecoder::create( VideoDecoderId_FFmpeg );
demux->setMedia( videoFile );
qDebug() << "Loading file:" << demux->load();
qDebug() << "Seeking to 50%" << demux->seek( 0.5 );
qDebug() << "Reading frame:" << demux->readFrame();
vdec->setCodecContext( demux->videoCodecContext() );
vdec->open();
Packet pkt = demux->packet();
qDebug() << "Packet valid:" << pkt.isValid();
qDebug() << "Decoding packet:" << vdec->decode( pkt );
VideoFrame frame = vdec->frame();
qDebug() << "Valid frame:" << frame.isValid();
QImage img = frame.toImage();
qDebug() << "Valid image:" << ( not img.isNull() );
bool saved = img.save( videoFile + ".jpg" );
return saved;
}
My problem is that frame.isValid() always returns false, no matter to where I seek, or which video I play. All the checks above return true.
I would like to add that if I use AVPlayer and play the video, the video renders properly, however there is no audio playing.
Also I am able to capture snapshots using AVPlayer::videoCapture()->capture()
For the record, I have tried this using both Qt4 and Qt5

Answer 1

For getting thumbnails of videos you can simply use QtAV VideoFrameExtractor class like:
#include "QtAV/VideoFrameExtractor.h"
...
bool saveThumb(const QString& videoFile) {
auto extractor = new QtAV::VideoFrameExtractor;
connect(
extractor,
&QtAV::VideoFrameExtractor::frameExtracted,
extractor,
[this, extractor, videoFile](const QtAV::VideoFrame& frame) {
const auto& img = frame.toImage();
auto saved = img.save(videoFile + ".jpg" );
extractor->deleteLater();
});
connect(
extractor,
&QtAV::VideoFrameExtractor::error,
extractor,
[this, extractor](const QString& errorStr) {
qDebug() << errorStr;
extractor->deleteLater();
});
extractor->setAsync(true);
extractor->setSource(videoFile);
extractor->setPosition(1);
}