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230 lines
11 KiB
Text
230 lines
11 KiB
Text
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GStreamer Internals
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Benjamin Otte
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Universität Hamburg, Germany
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otte@gnome.org
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Abstract
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GStreamer is a multimedia streaming framework. It aims to provide developers
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with an abstracted view on media files, while still allowing him to do all
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modifications necessary for the different types ofmedia handling applications.
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The framework is utilizes as a graph-based streaming architecture, which is
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implemented in the GStreamer core. Being media agnostic, the Gstreamer core
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uses a plugin-based architecture to provide the building blocks for streams
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processing. The GStreamer plugins collection provides these building blocks
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for multimedia processing.
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Introduction
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While there is a good number of high-quality applications available today for
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audio and video playback such as MPlayer[1] and Xine[2], none of these provide
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a generic media processing backend and a lot of code duplication has been
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going on when trying to provide a generic playback backend. On top of this,
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most application backends are limited in their abilities and only provide
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solutions for the problem space of the application. GStreamer tries to
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overcome these issues by providing a generic infrastructure that allows
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creating all sorts of applications. The applications show the flexibility of
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this approach. Examples are Rhythmbox[3], an audio player, Totem[4], a video
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player, Marlin[5], an audio editor, /*FIXME*/[6], an audio synthesis
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application and fluendo[7], a video streaming server.
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Although the GStreamer framework's focus is multimedia processing, the core
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has been used outside the real of multimedia, for example in the gst-sci
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package[8] that provides statistical data analysis.
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This paper focusses on the GStreamer core and explains the goals of the
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framework and how the core tries to achieve these by following a simple
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mp3 playback example.
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Plugins
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To allow easy extensibility, the complete media processing functionality
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inside GStreamer is provided via plugins. Upon initialization a plugin
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registeres its different capabilities with the GStreamer library. These
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capabilities are schedulers, typefind functions or - most common - elements.
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The capabilities of plugins are recorded inside the registry.
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The registry
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The registry is a cache file that is used to inspect certain plugin capabilities
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without the need to load the plugin. As an example those stored capabilites
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enabled automatically determining which plugins must be loaded in order to
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decode a certain media file.
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The gst-register(1) command updates the registry file. The gst-inspect(1)
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command allows querying plugins and their capabilities.
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[Add: output of gst-inspect gstelements]
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Elements
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Elements are at the main building block inside GStreamer. An element takes
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data from 0 to n input sink pads, processes it and produces data for 0 to n
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output source pads. Pads are used to enable data flow between elements in
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GStreamer. A pad can be viewed as a "place" or "port" on an element where
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links may be made with other elements, and through which data can flow to or
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from those elements. For the most part, all data in GStreamer flows one way
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through a link between elements. Data flows out of one element through one or
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more source pads, and elements accept incoming data through one or more sink
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pads. Elements are ordered in a tree structure by putting them inside
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container elements, called bins. This allows to operate only on the toplevel
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element (called the pipeline element) which propagates these operations to the
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contained elements.
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[Add: gst-editor with a simple mp3 decoder]
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There exists a simple command line tool to quickly construct media pipelines,
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called gst-launch. It helps to get comfortable with using it when developing
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code for or with GStreamer. /* FIXME: write more? */
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As an example the pipeline above would be presented by the command gst-launch
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filesrc ! mad ! alsasink
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The sample program
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/* note that the sample program does not do error checking for simplicities
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* sake */
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int
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main (int argc, char **argv)
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{
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GstElement *pipeline;
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gst_init (&argc, &argv);
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pipeline = gst_parse_launch ("filesrc location=./music.mp3 ! mad ! osssink",
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NULL);
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gst_element_set_state (pipeline, GST_STATE_PLAYING);
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while (gst_bin_iterate (GST_BIN (pipeline)));
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gst_object_unref (GST_OBJECT (pipeline));
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}
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Step 1: gst_init
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gst_init initializes the GStreamer library. The most important part here is
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loading information from the registry. The other important part is preparing
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the subsystems that need it. It also processes the command line options and
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environment variables specific to GStreamer, like the debugging options.
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The debugging subsystem
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GStreamer includes a powerful debugging subsystem. The need for such a system
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becomes apparent when looking at the way GStreamer works. It is built out of
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little independet elements that process unknown data for long times with or
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without user intervention, realtime requirements or other factors that can
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affect processing. Debugging messages are divided into named categories and 5
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levels for importance, ranging from "log" to "error". Desired debugging output
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can be specified either on the command line or as environment variable
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GST_DEBUG.
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Step 2: creating the pipeline
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A pipeline is set up by creating an element tree, setting options on these
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elements and finally linking their pads.
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Elements are created from their element factories. Element factories contain
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information about elements loaded from the registry. Getting the right element
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factory is done by either knowing its name (in the example "filesrc" is such a
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name) or querying the features of element factories and then deciding which
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one to use. Upon requesting an element, the element factory automatically
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loads the required plugin and returns a newly created element.
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Setting options on elements can be achieved in 2 ways. Either by knowing the
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GObject properties of the element and setting the property directly (the
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location property of filesrc is set in the example pipeline) or by using
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interfaces. Interfaces are used when there is a set of elements that allows the
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same features in a different context. For example there is an interface for
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setting tags that is implemented by different encoding plugins that support
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tag writing as well as tag changing plugins. Another example would be the
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mixer interface that allows changing the volume of an audio element. It is
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implemented by different audio sinks (oss, alsa) as well as the generic volume
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changing element. There is a set of interfaces included in the GStreamer
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Plugins plugin set, but people are of course free to add interfaces and
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elements implementing them.
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The last step in creating a pipeline is linking pads. When attempting to link
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two pads, GStreamer checks that a link is possible and if so, links them. After
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they are linked data may pass through this link. Most of the time (just like
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in this example) convenience funtions are used that automatically select the
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right elements to connect inside a GStreamer pipeline.
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Step 3: setting the state
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GStreamer elements know of four different states: NULL, READY, PAUSED and
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PLAYING. Each state and more important each state transition allows and
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requires different things from elements. State transitions are done step by
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step internally. (Note that in the following description state transitions in
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opposite directions can be assumed to do exactly the reverse thing.) Every
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transition may fail to succeed. In that case the gst_element_set_state
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function will return an error.
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GST_STATE_NULL to GST_STATE_READY
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GST_STATE_NULL is the 'uninitialized' state. Since it is always possible to
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create an element, nothing that might require interaction or can fail is done
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while creating the element. During the state transition elements are supposed
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to initialize external ressources. A file source opens its file, X elements
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open connections to the X server etc. This ensures that all elements can
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provide the best possible information about their capabilities during future
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interactions. The GStreamer core essentially does nothing. After this
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transition all external dependencies are initialized and supposed to work and
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the element is ready to start.
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GST_STATE_READY to GST_STATE_PAUSED
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During this state change all internal dependencies are resolved. The GStreamer
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core tries to resolve links between pads by negotiating capabilites of pads.
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(See below for an explanation.) The schedulers will prepare the elements for
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playback and the elements will prepare their internal data structures. After
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this state change is successful, nearly all elements are done with their setup.
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GST_STATE_PAUSED to GST_STATE_PLAYING
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The major difference between these two states is that in the playing state
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data is processed. Therefore the two major things happening here are the
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schedulers finishing their setup and readying their elements to run and the
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clocking subsystem starting the clocks. After this state change succeeded
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elements' processing function may finally be called.
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capabilities (or short: caps) and negotiation
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"Caps" are the format descriptions of the data passed. A caps is a list of
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structures. Each structure describes one "mime type" by n properties and its
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name. Note that "mime type" is used escaped because there is no 1:1 mapping
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between GStreamer caps names and real world mime types though GStreamer tries
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to orient itself at those. Properties are either fixed values, ranges or lists
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of values. There's also two special caps: any and empty.
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The mathematicians reading this should think of caps as a mathematical set of
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formats that is a union of the formats described in every structure. The
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GStreamer core provides functions to union, intersect and subtract caps or
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test them for various conditions (subsets, equality, etc).
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The negotiation phase works as follows: Both pads figure out all possible caps
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for themselves - most of the time depending on caps on other pads in the
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element. The core then takes those, intersects them and if the intersection
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isn't empty fixates them. Fixation is a process that selects the best possible
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fixed caps from a caps. A fixed caps is a caps that describes only one format
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and cannot be reduced further. After both pads acdepted the fixed caps, its
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format is then used to describe the contents of the buffers that are passed on
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this link. Caps can be serialized and deserialized to a string representation.
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[Add: a medium complex caps description (audioconvert?)]
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1. Mplayer media player - http://www.mplayerhq.hu
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2. Xine media player - http://xine.sourceforge.net
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3. Rhythmbox audio player - http://web.rhythmbox.org
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4. Totem video player - http://hadess.net/totem /* FIXME? */
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5. Marlin sample editor - http://marlin.sourceforge.net
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6. /* FIXME */
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7. Fluendo - http://www.fluendo.com
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8. gst-sci - /*FIXME */
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