Gstreamer Basic tutorial 1: Hello world!

Goal

Nothing better to get a first impression about a software library than to print “Hello World” on the screen!

But since we are dealing with multimedia frameworks, we are going to play a video instead.

Do not be scared by the amount of code below: there are only 4 lines which do real work. The rest is cleanup code, and, in C, this is always a bit verbose.

Without further ado, get ready for your first GStreamer application...

Hello world

Copy this code into a text file named 

basic-tutorial-1.c

 (or find it in the SDK installation).

basic-tutorial-1.c

Compile it as described in Installing on Linux, Installing
on Mac OS X or Installing on Windows. If you get compilation errors, double-check the instructions given in those sections.

If everything built fine, fire up the executable! You should see a window pop up, containing a video being played straight from the Internet, along with audio. Congratulations!

Need help? (Click to expand)

Walkthrough

Let's review these lines of code and see what they do:

This must always be your first GStreamer command. Among other things, 

gst_init()

:

Initializes all internal structures

Checks what plug-ins are available

Executes any command-line option intended for GStreamer

If you always pass your command-line parameters 

argc

 and 

argv

 to 

gst_init()

,
your application will automatically benefit from the GStreamer standard command-line options (more on this in Basic
tutorial 10: GStreamer tools)

This line is the heart of this tutorial, and exemplifies two key points: 

gst_parse_launch()

 and 

playbin2

.

gst_parse_launch

GStreamer is a framework designed to handle multimedia flows. Media travels from the “source” elements (the producers), down to the “sink” elements (the consumers), passing through a series of intermediate elements performing all kinds of tasks. The set of
all the interconnected elements is called a “pipeline”.

In GStreamer you usually build the pipeline by manually assembling the individual elements, but, when the pipeline is easy enough, and you do not need any advanced features, you can take the shortcut: 

gst_parse_launch()

.

This function takes a textual representation of a pipeline and turns it into an actual pipeline, which is very handy. In fact, this function is so handy there is a tool built completely around it which you will get very acquainted with (see Basic
tutorial 10: GStreamer tools to learn about 

gst-launch

 and the 

gst-launch

 syntax).

playbin2

So, what kind of pipeline are we asking 

gst_parse_launch()

to
build for us? Here enters the second key point: We are building a pipeline composed of a single element called 

playbin2

.

playbin2

 is
a special element which acts as a source and as a sink, and is capable of implementing a whole pipeline. Internally, it creates and connects all the necessary elements to play your media, so you do not have to worry about it.

It does not allow the control granularity that a manual pipeline does, but, still, it permits enough customization to suffice for a wide range of applications. Including this tutorial.

In this example, we are only passing one parameter to 

playbin2

,
which is the URI of the media we want to play. Try changing it to something else! Whether it is an

http://

 or 

file://

 URI, 

playbin2

 will
instantiate the appropriate GStreamer source transparently!

If you mistype the URI, or the file does not exist, or you are missing a plug-in, GStreamer provides several notification mechanisms, but the only thing we are doing in this example is exiting on error, so do not expect much feedback.

This line highlights another interesting concept: the state. Every GStreamer element has an associated state, which you can more or less think of as the Play/Pause button in your regular DVD player. For now, suffice to say that playback will not start unless
you set the pipeline to the PLAYING state.

In this line, 

gst_element_set_state()

 is
setting 

pipeline

 (our only element, remember) to the PLAYING state, thus initiating playback.

These lines will wait until an error occurs or the end of the stream is found. 

gst_element_get_bus()

 retrieves
the pipeline's bus, and 

gst_bus_timed_pop_filtered()

 will
block until you receive either an ERROR or an EOS (End-Of-Stream) through that bus. Do not worry much about this line, the GStreamer bus is explained in Basic
tutorial 2: GStreamer concepts.

And that's it! From this point onwards, GStreamer takes care of everything. Execution will end when the media reaches its end (EOS) or an error is encountered (try closing the video window, or unplugging the network cable). The application can always be stopped
by pressing control-C in the console.

Cleanup

Before terminating the application, though, there is a couple of things we need to do to tidy up correctly after ourselves.

Always read the documentation of the functions you use, to know if you should free the objects they return after using them.

In this case, 

gst_bus_timed_pop_filtered()

 returned
a message which needs to be freed with 

gst_message_unref()

 (more
about messages in Basic tutorial 2: GStreamer concepts).

gst_element_get_bus()

 added
a reference to the bus that must be freed with 

gst_object_unref()

.
Setting the pipeline to the NULL state will make sure it frees any resources it has allocated (More about states in Basic
tutorial 3: Dynamic pipelines). Finally, unreferencing the pipeline will destroy it, and all its contents.

Conclusion

And so ends your first tutorial with GStreamer. We hope its brevity serves as an example of how powerful this framework is!

Let's recap a bit. Today we have learned:

How to initialize GStreamer using 

gst_init()

.

How to quickly build a pipeline from a textual description using 

gst_parse_launch()

.

How to create an automatic playback pipeline using 

playbin2

.

How to signal GStreamer to start playback using 

gst_element_set_state()

.

How to sit back and relax, while GStreamer takes care of everything, using 

gst_element_get_bus()

 and 

gst_bus_timed_pop_filtered()

.

The next tutorial will keep introducing more basic GStreamer elements, and show you how to build a pipeline manually.

It has been a pleasure having you here, and see you soon!

None 

 Edit
Labels