Retrieving video size using QtGStreamer returns ZERO - c++

Question

I'm playing with QtGStreamer 0.10.0 and I'm trying to retrieve the video size but it's returning ZERO for height and width values.
However, I am able to play the video with no problems on a QImage.
QGst::init();
pipeline = QGst::Pipeline::create();
filesrc = QGst::ElementFactory::make("filesrc");
filesrc->setProperty("location", "sample.avi");
pipeline->add(filesrc);
decodebin = QGst::ElementFactory::make("decodebin2").dynamicCast<QGst::Bin>();
pipeline->add(decodebin);
QGlib::connect(decodebin, "pad-added", this, &MyMultimedia::onNewDecodedPad);
QGlib::connect(decodebin, "pad-removed", this, &MyMultimedia::onRemoveDecodedPad);
filesrc->link(decodebin);
// more code ...
The code above shows the begining of the pipeline setup. By connecting my method MyMultimedia::onNewDecodedPad on the signal "pad-added" I have access to the data of the video. At least that's what I think.
void MyMultimedia::onNewDecodedPad(QGst::PadPtr pad)
{
QGst::CapsPtr caps = pad->caps();
QGst::StructurePtr structure = caps->internalStructure(0);
if (structure->name().contains("video/x-raw"))
{
// Trying to print width and height using a couple of different ways,
// but all of them returns 0 for width/height.
qDebug() << "#1 Size: " << structure->value("width").get<int>() << "x" << structure->value("height").get<int>();
qDebug() << "#2 Size: " << structure->value("width").toInt() << "x" << structure->value("height").toInt();
qDebug() << "#3 Size: " << structure.data()->value("width").get<int>() << "x" << structure.data()->value("height").get<int>();
// numberOfFields also returns 0, which is very wierd.
qDebug() << "numberOfFields:" << structure->numberOfFields();
}
// some other code
}
I wonder what I might be doing wrong. Any tips? I was unable to find a relevant example on the web using this API.

Answer 1

Solved it. At onNewDecodedPad() you still don't have access to information about the video frames.
The class MyMultimedia inherits from QGst::Utils::ApplicationSink, so I had to implement a method named QGst::FlowReturn MyMultimedia::newBuffer() that is called by QtGstreamer whenever a new frame is ready.
In other words, use this method to copy the frame of the video to a QImage. What I didn't know is that pullBuffer() returns a QGst::BufferPtr, which has a QGst::CapsPtr. It's an internal structure from this var that holds the information I was looking for:
QGst::FlowReturn MyMultimedia::newBuffer()
{
QGst::BufferPtr buf_ptr = pullBuffer();
QGst::CapsPtr caps_ptr = buf_ptr->caps();
QGst::StructurePtr struct_ptr = caps_ptr->internalStructure(0);
qDebug() << struct_ptr->value("width").get<int>() <<
"x" <<
struct_ptr->value("height").get<int>();
// ...
}