How can I get a video meta data using Qt phonon? - qt

Question

I want to get the bitrate of a video. And I use Qt Phonon to achieve this goal.
Since there is a class called Phonon::MediaObject and provide a method called metaData(),I use that method but the result shows zero. Here is my code, I wonder why and how can I get the metadata such as bitrate in qt with Phonon
QString source="E:\\sh.mp4";
Phonon::MediaObject media;
media.setCurrentSource(Phonon::MediaSource(source));
QMap <QString, QString> metaData = media.metaData();
int trackBitrate = metaData.value("bitrate").toInt();
qDebug()<<trackBitrate;
The result is 0 all the time

Answer 1

I just figured this out myself.
Meta data in video files do not contain bitrate. It only contains extra information about the media that don't have any effect on playback. So even if Phonon::MediaObject::metaData() worked without loading the video, it will not help you.
I ended up using libformat, part of ffmpeg library to get the bitrate. Here's the code.
If you copy and paste this, it should work.
Download FFMpeg here: http://dranger.com/ffmpeg/tutorial01.html
This first tutorial will tell you how to link: http://dranger.com/ffmpeg/tutorial01.html
#include <QString>
#include <QMultiMap>
#include <stdio.h>
#include <libavformat/avformat.h>
#include <libavutil/dict.h>
void processMedia(const char* mediaFile)
{
AVFormatContext *pFormatCtx = NULL;
AVDictionaryEntry *tag = NULL;
// Register all formats and codecs
av_register_all();
// Open video file
if(avformat_open_input(&pFormatCtx, mediaFile, NULL, NULL)!=0)
return;
// Retrieve stream information
if(av_find_stream_info(pFormatCtx)<0)
return;
//Get Bitrate
float bitRate = pFormatCtx->bit_rate;
//Get Meta
QMultiMap<QString, QString> metaData;
while ((tag = av_dict_get(pFormatCtx->metadata, "", tag,
AV_DICT_IGNORE_SUFFIX)))
{
QString keyString(tag->key);
QString valueString(tag->value);
metaData.insert(keyString, valueString);
printf("%s=%s\n", tag->key, tag->value);
}
// Close the video file
av_close_input_file(pFormatCtx);
}

Answer 2

When you set the data source, the MediaObject enters the LoadingState. At that point, metadata might not yet be available.
The object emits a metaDataChanged signal when the metadata is ready. You should react to that signal and only attempt accessing the metadata once it has been emitted.