7.6 KiB
Using appsink/appsrc in Qt
This page last changed on May 24, 2013 by xartigas.
Goal
For those times when you need to stream data into or out of GStreamer through your application, GStreamer includes two helpful elements:
appsink
- Allows applications to easily extract data from a GStreamer pipelineappsrc
- Allows applications to easily stream data into a GStreamer pipeline
This tutorial will demonstrate how to use both of them by constructing a pipeline to decode an audio file, stream it into an application's code, then stream it back into your audio output device. All this, using QtGStreamer.
Steps
First, the files. These are also available in the
examples/appsink-src
directory of the QGstreamer SDK.
CMakeLists.txt
project(qtgst-example-appsink-src)
find_package(QtGStreamer REQUIRED)
find_package(Qt4 REQUIRED)
include_directories(${QTGSTREAMER_INCLUDES} ${QT_QTCORE_INCLUDE_DIRS})
add_definitions(${QTGSTREAMER_DEFINITIONS})
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${QTGSTREAMER_FLAGS}")
add_executable(appsink-src main.cpp)
target_link_libraries(appsink-src ${QTGSTREAMER_UTILS_LIBRARIES} ${QT_QTCORE_LIBRARIES})
main.cpp
#include <iostream>
#include <QtCore/QCoreApplication>
#include <QGlib/Error>
#include <QGlib/Connect>
#include <QGst/Init>
#include <QGst/Bus>
#include <QGst/Pipeline>
#include <QGst/Parse>
#include <QGst/Message>
#include <QGst/Utils/ApplicationSink>
#include <QGst/Utils/ApplicationSource>
class MySink : public QGst::Utils::ApplicationSink
{
public:
MySink(QGst::Utils::ApplicationSource *src)
: QGst::Utils::ApplicationSink(), m_src(src) {}
protected:
virtual void eos()
{
m_src->endOfStream();
}
virtual QGst::FlowReturn newBuffer()
{
m_src->pushBuffer(pullBuffer());
return QGst::FlowOk;
}
private:
QGst::Utils::ApplicationSource *m_src;
};
class Player : public QCoreApplication
{
public:
Player(int argc, char **argv);
~Player();
private:
void onBusMessage(const QGst::MessagePtr & message);
private:
QGst::Utils::ApplicationSource m_src;
MySink m_sink;
QGst::PipelinePtr pipeline1;
QGst::PipelinePtr pipeline2;
};
Player::Player(int argc, char **argv)
: QCoreApplication(argc, argv), m_sink(&m_src)
{
QGst::init(&argc, &argv);
if (argc <= 1) {
std::cerr << "Usage: " << argv[0] << " <audio_file>" << std::endl;
std::exit(1);
}
const char *caps = "audio/x-raw-int,channels=1,rate=8000,"
"signed=(boolean)true,width=16,depth=16,endianness=1234";
/* source pipeline */
QString pipe1Descr = QString("filesrc location=\"%1\" ! "
"decodebin2 ! "
"audioconvert ! "
"audioresample ! "
"appsink name=\"mysink\" caps=\"%2\"").arg(argv[1], caps);
pipeline1 = QGst::Parse::launch(pipe1Descr).dynamicCast<QGst::Pipeline>();
m_sink.setElement(pipeline1->getElementByName("mysink"));
QGlib::connect(pipeline1->bus(), "message::error", this, &Player::onBusMessage);
pipeline1->bus()->addSignalWatch();
/* sink pipeline */
QString pipe2Descr = QString("appsrc name=\"mysrc\" caps=\"%1\" ! autoaudiosink").arg(caps);
pipeline2 = QGst::Parse::launch(pipe2Descr).dynamicCast<QGst::Pipeline>();
m_src.setElement(pipeline2->getElementByName("mysrc"));
QGlib::connect(pipeline2->bus(), "message", this, &Player::onBusMessage);
pipeline2->bus()->addSignalWatch();
/* start playing */
pipeline1->setState(QGst::StatePlaying);
pipeline2->setState(QGst::StatePlaying);
}
Player::~Player()
{
pipeline1->setState(QGst::StateNull);
pipeline2->setState(QGst::StateNull);
}
void Player::onBusMessage(const QGst::MessagePtr & message)
{
switch (message->type()) {
case QGst::MessageEos:
quit();
break;
case QGst::MessageError:
qCritical() << message.staticCast<QGst::ErrorMessage>()->error();
break;
default:
break;
}
}
int main(int argc, char **argv)
{
Player p(argc, argv);
return p.exec();
}
Walkthrough
As this is a very simple example, most of the action happens in the
Player
's constructor. First, GStreamer is initialized through
QGst::init()
:
GStreamer Initialization
QGst::init(&argc, &argv);
Now we can construct the first half of the pipeline:
Pipeline Setup
const char *caps = "audio/x-raw-int,channels=1,rate=8000,"
"signed=(boolean)true,width=16,depth=16,endianness=1234";
/* source pipeline */
QString pipe1Descr = QString("filesrc location=\"%1\" ! "
"decodebin2 ! "
"audioconvert ! "
"audioresample ! "
"appsink name=\"mysink\" caps=\"%2\"").arg(argv[1], caps);
pipeline1 = QGst::Parse::launch(pipe1Descr).dynamicCast<QGst::Pipeline>();
m_sink.setElement(pipeline1->getElementByName("mysink"));
QGlib::connect(pipeline1->bus(), "message::error", this, &Player::onBusMessage);
pipeline1->bus()->addSignalWatch();
QGst::Parse::launch()
parses the text description of a pipeline and
returns a QGst::PipelinePtr
. In this case, the pipeline is composed
of:
- A
filesrc
element to read the file decodebin2
to automatically examine the stream and pick the right decoder(s)audioconvert
andaudioresample
to convert the output of thedecodebin2
into the caps specified for theappsink
- An
appsink
element with specific caps
Next, we tell our MySink
class (which is a subclass
of QGst::Utils::ApplicationSink
) what appsink
element to use.
The second half of the pipeline is created similarly:
Second Pipeline
/* sink pipeline */
QString pipe2Descr = QString("appsrc name=\"mysrc\" caps=\"%1\" ! autoaudiosink").arg(caps);
pipeline2 = QGst::Parse::launch(pipe2Descr).dynamicCast<QGst::Pipeline>();
m_src.setElement(pipeline2->getElementByName("mysrc"));
QGlib::connect(pipeline2->bus(), "message", this, &Player::onBusMessage);
pipeline2->bus()->addSignalWatch();
Finally, the pipeline is started:
Starting the pipeline
/* start playing */
pipeline1->setState(QGst::StatePlaying);
pipeline2->setState(QGst::StatePlaying);
Once the pipelines are started, the first one begins pushing buffers
into the appsink
element. Our MySink
class implements the
newBuffer()
method, which is called by QGStreamer when a new buffer is
ready for processing:
MySink::newBuffer()
virtual QGst::FlowReturn newBuffer()
{
m_src->pushBuffer(pullBuffer());
return QGst::FlowOk;
}
Our implementation takes the new buffer and pushes it into the
appsrc
element, which got assigned in the Player
constructor:
Player::Player()
Player::Player(int argc, char **argv)
: QCoreApplication(argc, argv), m_sink(&m_src)
From there, buffers flow into the autoaudiosink
element, which
automatically figures out a way to send it to your speakers.
Conclusion
You should now have an understanding of how to push and pull arbitrary data into and out of a GStreamer pipeline.
It has been a pleasure having you here, and see you soon!
Document generated by Confluence on Oct 08, 2015 10:27