Wednesday, January 25, 2012

Notes about writing a gstreamer plugin in vala.

Median/max/sobel filters/OpenCL kernels applied to 
Sintel using Intels OCL SDK and a gstreamer plugin.

Vala, as some of you may know, is a "compiler for the GObject type system". This means, vala is providing a high-level language with e.g. literals for (nearly) all features of gobject and glib. Because vala get's translated to C, it easily integrates with existing C code. Vala code can also be used from C (and other languages, most easily using gobject intropsection) too.

Gstreamer is a nice multimedia framework - and component of the GNOME ecosystem, now more related to - and is also based on gobject/glib. So it is quite obvious to use vala to build gstreamer plugins and elements.

A gstreamer plugin is organized as a plugin, which wraps one or more elements, which provide the actual functionality. Detailed informations on how to write plugins can be found in the plugin writers guide.

But what is needed to write a plugin and elements in vala? And if it's good to do so.

gst_plugin_desc and plugin_init ()
There are two special structures that need to appear in a Gstreamer plugin: A plugin description (namely the Gst.PluginDesc [vala class]) and the plugin_init () function.
The identifiers of these structures (struct and function) need to match those expected by gstreamer. The plugin description identifier needs to be called gst_plugin_desc and the plugin_init() function also needs to have this name.
In vala it is important to place these methods on the toplevel of your file, and not within a namespace, otherwise vala will prefix the resulting C functions with prefix dervied from the namespace. An alternative is to use a CCode attribute for those structures, but why do this if it can be done easier (as described before).
An working example with correctly named plugin description and init function can be found here.

Because of the naming you should also put your elements of the plugin within a Gst namespace e.g. namespace Gst.Myplugin. Because this translates to C structures (generated by vala) prefixed by gst_myplugin_… and the gst (or Gst or GST) prefix is needed, because the buildsystem is looking for those symbols and exports them.
Look in the example above or below to see where to put the element and what kind of namespace to use.

Elements: Gst.BaseTransform
It's quite easy to implement a simple filter. This can be done by subclassing Gst.BaseTransform and overriding (so providing an implementation for) a couple of functions. Have a look here to find out what you need to implement. Or here to see an actual working implementation.

gst-template and buildsystem
You can use the plugin template provided by gstreamer but you obviously need to modify the buildsystem to generate c code from the vala files.
Have a look at this

Packages & Testing
The basic requirements can be met with the following packages:

$ sudo yum install gstreamer-devel \
gstreamer-plugins-base-devel gstreamer-plugins-base-devel-docs \
vala devhelp

Have you finally compiled your sources try the plugin with gst-launch, remember to add the hidden .libs dir to your path:
$GST_PLUGIN_PATH=src/.libs/ gst-launch  videotestsrc ! myelem ! autovideosink

So yes - you can write gstreamer plugins in vala - so what element are you going to write today?

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