mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
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729 lines
24 KiB
Text
729 lines
24 KiB
Text
<!DOCTYPE book PUBLIC "-//GNOME//DTD DocBook PNG Variant V1.0//EN" "" [
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<!ENTITY TITLEPAGE SYSTEM "titlepage.sgml">
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<!ENTITY INTRO SYSTEM "intro.sgml">
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<!ENTITY TESTAPP SYSTEM "testapp.sgml">
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<!ENTITY LOOPBASED SYSTEM "loopbased.sgml">
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<!ENTITY BUFFERS SYSTEM ".sgml">
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<!ENTITY SRCNSINK SYSTEM ".sgml">
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<!ENTITY STATEMANAGE SYSTEM ".sgml">
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<!ENTITY CHECKLIST SYSTEM ".sgml">
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<!ENTITY GStreamer "<application>GStreamer</application>">
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]>
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<book id="index">
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&TITLEPAGE;
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<!-- ############# part ############### -->
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<part id="introduction"><title>Introduction</title>
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<partintro>
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<para>
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&GStreamer; is a framework for creating
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streaming media applications. It is extremely powerful and versatile,
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and this versatility stems in part from its modularity, and its ability
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to incorporate new modules seamlessly into its framework.
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This document describes how to extend the capabilities of
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&GStreamer; by creating new plugins.
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</para>
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<para>
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It first describes the concepts required and the ways in which
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&GStreamer; can be extended. It then goes
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through a worked example of how to write a simple filter (for data
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processing), and how to test and debug it. More advanced concepts are
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then introduced, with worked examples of each. Next, writing source
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and sink elements (for performing input and output) is discussed.
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Finally, checklists of things to be sure to do when extending
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&GStreamer; are presented.
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</para>
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</partintro>
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&INTRO;
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</part>
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<!-- ############ part ############# -->
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<part id="basic-concepts"><title>Basic concepts</title>
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<partintro>
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<para>
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This section introduces the basic concepts required to understand the
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issues involved in extending &GStreamer;
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</para>
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<para>
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Many of these concepts are explained in greater detail in the
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GStreamer Application Development Manual, and are merely mentioned
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here to refresh your memory.
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</para>
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</partintro>
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<chapter id="cha-plugins">
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<title>Plugins</title>
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<para>
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Extensions to &GStreamer; can be made using a plugin mechanism. This is
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used extensively in &GStreamer; even if only the standard package is
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being used: a few very basic functions reside in the core library, and
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all others are implemented in plugins.
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</para>
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<para>
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Plugins are only loaded when needed: a plugin registry is used to
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store the details of the plugins so that it is not neccessary to load
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all plugins to determine which are needed.
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This registry needs to be updated whenever a new plugin is added to the
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system: see the <emphasis>gstreamer-register</emphasis> utility and the
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documentation in the <emphasis>GStreamer Application Development
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Manual</emphasis> for more details.
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</para>
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<para>
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User extensions to &GStreamer; can be installed in the main plugin
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directory, and will immediately be available for use in applications.
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<emphasis>gstreamer-register</emphasis> should be run to update
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the repository: but the system should work correctly even if it hasn't
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been - it will just take longer to load the correct plugin.
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</para>
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<para>
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User specific plugin directories and registries will be available
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in future versions of &GStreamer;.
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</para>
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</chapter>
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<chapter id="cha-elements">
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<title>Elements</title>
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<para>
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Elements are at the core of &GStreamer;. Without elements, &GStreamer;
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is just
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a bunch of pipe fittings with nothing to connect. A large number of
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elements (filters, sources and sinks) ship with &GStreamer;, but extra
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elements can also be written.
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</para>
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<para>
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An element may be constructed in several different ways, but all must
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conform to the same basic rules. A simple filter may be built with the
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FilterFactory, where the only code that need be written is the actual
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filter code. A more complex filter, or a source or sink, will need to
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be written out fully for complete access to the features and
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performance possible with &GStreamer;.
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</para>
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<para>
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The implementation of a new element will be contained in a plugin:
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a single plugin may contain the implementation of several elements, or
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just a single one.
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</para>
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</chapter>
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<chapter id="cha-buffers">
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<title>Buffers</title>
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<para>
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Buffers are structures used to pass data between elements. All streams
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of data are chopped up into chunks which are stored in buffers.
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Buffers can be of any size, and also contain metadata indicating the
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type of data contained in them. Buffers can be allocated by various
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different schemes, and may either be passed on by elements or
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unreferenced (and the memory used by the buffer freed).
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</para>
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</chapter>
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<chapter id="cha-typing">
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<title>Typing and Properties</title>
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<para>
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A type system is used to ensure that the data passed between elements
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is in a recognised format, and that the various parameters required
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to fully specify that format match up correctly. Each connection
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that is made between elements has a specified type. This is related,
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but different, to the metadata in buffers which describes the type
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of data in that particular buffer. See later in this document for
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details of the available types.
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</para>
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</chapter>
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<chapter id="cha-metadata">
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<title>Metadata</title>
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<para>
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</para>
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</chapter>
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<chapter id="cha-scheduling">
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<title>Scheduling</title>
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<para>
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</para>
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</chapter>
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<chapter id="cha-chainloop">
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<title>Chain vs Loop Elements</title>
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<para>
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</para>
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</chapter>
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<chapter id="cha-chainloop">
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<title>Autopluggers</title>
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<para>
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&GStreamer; has an autoplugging mechanism, which enables application
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writers to simply specify start and end elements for a path, and
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the system will then create a path which links these elements,
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in accordance with the type information provided by the elements.
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</para>
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<para>
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It is possible to devise many different schemes for generating such
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pathways, perhaps to optimise based on special criteria, or with
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some specific constraints. It is thus possible to define new
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autoplugging systems, using the plugin system.
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</para>
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</chapter>
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</part>
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<!-- ############ part ############# -->
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<part id="typesnprops"><title>Types and Properties</title>
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<partintro>
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<para>
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There is a very large set of possible types that may be used to
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pass data between elements. Indeed, each new element that is defined
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may use a new data format (though unless at least one other element
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recognises that format, it will be most likely be useless since
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nothing will be able to link with it).
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</para>
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<para>
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In order for types to be useful, and for systems like autopluggers to
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work, it is neccessary that all elements
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agree on the type definitions, and which properties are required
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for each type. The &GStreamer; framework itself
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simply provides the ability to define types and parameters, but does
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not fix the meaning of types and parameters, and does not enforce
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standards on the creation of new types. This is a matter for
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a policy to decide, not technical systems to enforce.
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</para>
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<para>
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For now, the policy is simple:
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<itemizedlist>
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<listitem>
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<para>
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Do not create a new type if you could use one which already
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exists.
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</para>
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</listitem>
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<listitem>
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<para>
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If creating a new type, discuss it first with the other
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&GStreamer; developers, on at least one of: IRC, mailing lists,
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the &GStreamer; wiki.
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</para>
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</listitem>
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<listitem>
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<para>
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Try to ensure that the name for a new format is as unlikely to
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conflict with anything else created already, and is not a more
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generalised name than it should be. For example:
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"audio/compressed" would be too generalised a name to represent
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audio data compressed with an mp3 codec. Instead "audio/mp3"
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might be an appropriate name, or "audio/compressed" could exist
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and have a property indicating the type of compression used.
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</para>
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</listitem>
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<listitem>
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<para>
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Ensure that, when you do create a new type, you specify it
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clearly, and get it added to the list of known types so that
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other developers can use the type correctly when writing their
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elements.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</partintro>
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<chapter id="cha-basic-types">
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<title>The basic types</title>
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<para>
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This is a list of the basic types used for buffers. For each type, we
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give the name ("mime type") of the type, the list of properties which
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are associated with the type, the meaning of each property, and the
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purpose of the type.
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</para>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>audio/raw</emphasis>
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- Unstructured and uncompressed raw audio data.
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</para><para>
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<emphasis>law</emphasis>
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- The law used to describe the data. This is an integer for which
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there are three valid values: 0 for linear, 1 for
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mu law, 2 for A law.
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</para><para>
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<emphasis>endianness</emphasis>
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- The order of bytes in a sample. This is a boolean:
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0 means little-endian (ie, bytes are least significant first),
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1 means big-endian (ie, most significant byte first).
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</para><para>
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<emphasis>signed</emphasis>
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- Whether the samples are signed or not. This is a boolean:
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0 means unsigned, 1 means signed.
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</para><para>
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<emphasis>width</emphasis>
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- The number of bits per sample. This is extremely likely to be
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a multiple of 8, but as ever this is up to each element supporting
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this format to specify.
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</para><para>
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<emphasis>depth</emphasis>
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- The number of bits used per sample. This must be less than or
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equal to the width: if less than the width, the low bits are
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assumed to be the ones used. For example, width=32, depth=24
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means that each sample is stored in a 32 bit word, but only the
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low 24 bits are actually used.
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</para><para>
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<emphasis>rate</emphasis>
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- The sample rate of the data, in samples per second.
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</para><para>
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<emphasis>channels</emphasis>
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- The number of channels of audio data.
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</para>
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<para>
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For example: 16 bit, unsigned, linear, monophonic, big-endian,
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44100KHz audio would be represented by
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"law=0,endianness=1,signed=0,width=16,depth=16,rate=44100,channels=1"
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</para>
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</listitem>
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</itemizedlist>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>audio/mp3</emphasis>
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- Audio data compressed using the mp3 encoding scheme.
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</para><para>
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<emphasis>framed</emphasis>
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- This is a boolean. If true (1), each buffer contains exactly
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one frame. If false (0), frames and buffers do not (necessarily)
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match up. If the data is not framed, the values of some of the
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properties will not be available, but others will be assumed to
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be constant throughout the file, or may be found in other ways.
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</para><para>
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<emphasis>layer</emphasis>
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- The compression scheme layer used to compress the data.
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This is an integer, and can currently have the value 1, 2
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or 3.
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</para><para>
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<emphasis>bitrate</emphasis>
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- The bitrate, in kilobits per second.
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For VBR (variable bitrate) mp3 data, this is the average bitrate.
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</para><para>
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<emphasis>channels</emphasis>
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- The number of channels of audio data present. This could
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theoretically be any integer greater than 0, but in practice will
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be either 1 or 2.
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</para><para>
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<emphasis>joint-stereo</emphasis>
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- Boolean. If true, channels must not be zero. If true, this
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implies that stereo data is stored as a combined signal and
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the difference between the signals, rather than as two entirely
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separate signals.
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</para><para>
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There are many other properties relevant for
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<emphasis>audio/mp3</emphasis> data: these may be added to this
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specification at a later date.
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</para>
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</listitem>
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</itemizedlist>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>audio/x-ogg</emphasis>
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- Audio data compressed using the Ogg Vorbis encoding scheme.
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There are currently no parameters defined for this type. FIXME.
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</para>
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</listitem>
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</itemizedlist>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>video/raw</emphasis>
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- Raw video data.
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</para><para>
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<emphasis>fourcc</emphasis>
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- A FOURCC code identifying the format in which this data is
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stored. FOURCC (Four Character Code) is a simple system to
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allow unambiguous identification of a video datastream format.
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See <ulink
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url="http://www.webartz.com/fourcc/"
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type="http">http://www.webartz.com/fourcc/</ulink>
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</para><para>
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<emphasis>width</emphasis>
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- The number of pixels wide that each video frame is.
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</para><para>
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<emphasis>height</emphasis>
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- The number of pixels high that each video frame is.
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</para>
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</listitem>
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</itemizedlist>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>video/mpeg</emphasis>
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- Video data compressed using an mpeg encoding scheme.
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</para><para>
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<emphasis>mpegversion</emphasis>
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</para><para>
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<emphasis>systemstream</emphasis>
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</para>
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</listitem>
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</itemizedlist>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>video/avi</emphasis>
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- Video data compressed using the AVI encoding scheme.
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There are currently no parameters defined for this type. FIXME.
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</para>
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</listitem>
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</itemizedlist>
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</chapter>
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<chapter id="cha-types-test">
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<title>Building a simple format for testing</title>
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<para>
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</para>
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</chapter>
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<chapter id="cha-types-simplemime">
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<title>A simple MIME type</title>
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<para>
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</para>
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</chapter>
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<chapter id="cha-types-props">
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<title>Type properties</title>
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<para>
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</para>
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</chapter>
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<chapter id="cha-types-typefind">
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<title>Typefind functions and autoplugging</title>
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<para>
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</para>
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</chapter>
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</part>
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|
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<!-- ############ part ############# -->
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<part id="first-plugin"><title>Building our first plugin</title>
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<partintro>
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<para>
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We are now have the neccessary concepts to build our first plugin.
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We are going to build an element which has a single input pad and
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a single output pad, and simply passes anything it reads on
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the input pad through and out on the output pad. We will also
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see where we could add code to convert this plugin into something
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more useful.
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</para>
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<para>
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The example code used in this section can be found in
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<filename>examples/plugins/</filename>
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</para>
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</partintro>
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<chapter id="cha-boilerplate">
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<title>Constructing the boilerplate</title>
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<para>
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The first thing to do when making a new element is to specify some basic
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details about it: what its name is, who wrote it, what version number it
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is, etc. We also need to define an object to represent the element and to
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store the data the element needs. I shall refer to these details
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collectively as the <emphasis>boilerplate</emphasis>.
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</para>
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<sect1 id="sect-boilerplate-gobject">
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<title>Doing it the hard way with GstObject</title>
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<para>
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The standard way of defining the boilerplate is simply to write some
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code, and fill in some structures. The easiest way to do this is to
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copy an example and modify according to your needs.
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</para>
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<para>
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First we will examine the code you would be likely to place in a header
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file (although since the interface to the code is entirely defined
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by the pluging system, and doesn't depend on reading a header file,
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this is not crucial.)
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The code here can be found in
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<filename>examples/plugins/example.h</filename>
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</para>
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<programlisting>
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/* Definition of structure storing data for this element. */
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typedef struct _GstExample GstExample;
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struct _GstExample {
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GstElement element;
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GstPad *sinkpad,*srcpad;
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gint8 active;
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};
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/* Standard definition defining a class for this element. */
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typedef struct _GstExampleClass GstExampleClass;
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struct _GstExampleClass {
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GstElementClass parent_class;
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};
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/* Standard macros for defining types for this element. */
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#define GST_TYPE_EXAMPLE \
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(gst_example_get_type())
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#define GST_EXAMPLE(obj) \
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(GTK_CHECK_CAST((obj),GST_TYPE_EXAMPLE,GstExample))
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#define GST_EXAMPLE_CLASS(klass) \
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(GTK_CHECK_CLASS_CAST((klass),GST_TYPE_EXAMPLE,GstExample))
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#define GST_IS_EXAMPLE(obj) \
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(GTK_CHECK_TYPE((obj),GST_TYPE_EXAMPLE))
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#define GST_IS_EXAMPLE_CLASS(obj) \
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(GTK_CHECK_CLASS_TYPE((klass),GST_TYPE_EXAMPLE))
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/* Standard function returning type information. */
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GtkType gst_example_get_type(void);
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</programlisting>
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</sect1>
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|
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<sect1 id="sect-boilerplate-filterfactory">
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<title>Doing it the easy way with FilterFactory</title>
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|
<para>
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|
A plan for the future is to create a FilterFactory, to make the
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|
process of making a new filter a simple process of specifying a few
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|
details, and writing a small amount of code to perform the actual
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|
data processing.
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|
</para>
|
|
<para>
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|
Unfortunately, this hasn't yet been implemented. It is also likely
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|
that when it is, it will not be possible to cover all the possibilities
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|
available by writing the boilerplate yourself, so some plugins will
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always need to be manually registered.
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|
</para>
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<para>
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|
As a rough outline of what is planned: the FilterFactory will take a
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|
list of appropriate function pointers, and data structures to define
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|
a filter. With a reasonable measure of preprocessor magic, the
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|
plugin writer will then simply need to provide definitions of the
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|
functions and data structures desired, and a name for the filter, and
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|
then call a macro from within plugin_init() which will register the
|
|
new filter. All the fluff that goes into the definition of a filter
|
|
will thus be hidden from view.
|
|
</para>
|
|
<para>
|
|
Ideally, we will come up with a way for various FilterFactory-provided
|
|
functions to be overridden, to the point where you can construct
|
|
almost the most complex stuff with it, it just saves typing.
|
|
</para>
|
|
<para>
|
|
Of course, the filter factory can be used to create sources and sinks
|
|
too: simply create a filter with only source or sink pads.
|
|
</para>
|
|
<para>
|
|
You may be thinking that this should really be called an
|
|
ElementFactory. Well, we agree, but there is already something else
|
|
justifiably ealled an ElementFactory (this is the thing which actually
|
|
makes instances of elements). There is also already something called
|
|
a PluginFactory. We just have too many factories and not enough words.
|
|
And since this isn't yet written, it doesn't get priority for claiming
|
|
a name.
|
|
</para>
|
|
</sect1>
|
|
</chapter>
|
|
|
|
<chapter id="cha-defineelt">
|
|
<title>Defining an element</title>
|
|
<para>
|
|
</para>
|
|
|
|
<sect1 id="sect-defineelt-pads">
|
|
<title>Specifying the pads</title>
|
|
<para>
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="sect-defineelt-fns">
|
|
<title>Attaching functions</title>
|
|
<para>
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="sect-defineelt-chainfn">
|
|
<title>The chain function</title>
|
|
<para>
|
|
</para>
|
|
</sect1>
|
|
</chapter>
|
|
|
|
<chapter id="cha-plugininit">
|
|
<title>The plugin_init function</title>
|
|
<para>
|
|
Once we have written code defining all the parts of the plugin,
|
|
we need to write the plugin_init() function. This is a special
|
|
function, which is called as soon as the plugin is loaded, and
|
|
must return a pointer to a newly allocated GstPlugin structure.
|
|
This structure contains the details of all the facilities provided
|
|
by the plugin. Helper functions are provided to help fill the
|
|
structure: for future compatability it is recommended that these
|
|
functions are used, as documented below.
|
|
</para>
|
|
<para>
|
|
Note that the information returned by the plugin_init() function
|
|
will be cached in a central registry. For this reason, it is
|
|
important that the same information is always returned by
|
|
the function: for example, it must not make element factories
|
|
available based on runtime conditions. If an element can only
|
|
work in certain conditions (for example, if the soundcard is not
|
|
being used by some other process) this must be reflected by the
|
|
element being unable to enter the READY state if unavailable, rather
|
|
than the plugin attempting to deny existence of the plugin.
|
|
</para>
|
|
|
|
<sect1 id="sect-plugininit-types">
|
|
<title>Registering new types</title>
|
|
<para>
|
|
</para>
|
|
<programlisting>
|
|
void gst_plugin_add_type(GstPlugin *plugin,
|
|
GstTypeFactory *factory);
|
|
</programlisting>
|
|
<para>
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="sect-plugininit-filter">
|
|
<title>Registering new element factories</title>
|
|
<para>
|
|
</para>
|
|
<programlisting>
|
|
void gst_plugin_add_factory(GstPlugin *plugin,
|
|
GstElementFactory *factory);
|
|
</programlisting>
|
|
<para>
|
|
Multiple element factories can be provided by a single plugin:
|
|
all it needs to do is call gst_plugin_add_factory() for each
|
|
element factory it wishes to provide.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="sect-plugininit-autopluggers">
|
|
<title>Registering new autopluggers</title>
|
|
<para>
|
|
</para>
|
|
<programlisting>
|
|
void gst_plugin_add_autoplugger(GstPlugin *plugin,
|
|
GstAutoplugFactory *factory);
|
|
</programlisting>
|
|
</sect1>
|
|
|
|
</chapter>
|
|
|
|
</part>
|
|
|
|
<!-- ############ part ############# -->
|
|
|
|
<part id="test-app"><title>Building a simple test application</title>
|
|
<partintro>
|
|
<para>
|
|
</para>
|
|
</partintro>
|
|
|
|
&TESTAPP;
|
|
</part>
|
|
|
|
<!-- ############ part ############# -->
|
|
|
|
<part id="loopbased"><title>Loop-based Elements</title>
|
|
<partintro>
|
|
<para>
|
|
</para>
|
|
</partintro>
|
|
|
|
&LOOPBASED;
|
|
</part>
|
|
|
|
<!-- ############ part ############# -->
|
|
|
|
<part id="buffersnmeta"><title>Buffers and Metadata</title>
|
|
<partintro>
|
|
<para>
|
|
</para>
|
|
</partintro>
|
|
|
|
&BUFFERS;
|
|
Anatomy of a Buffer
|
|
Refcounts and mutability
|
|
Metadata
|
|
How Properties work efficiently
|
|
Metadata mutability
|
|
(FIXME: this is an unsolved problem)
|
|
</part>
|
|
|
|
<!-- ############ part ############# -->
|
|
|
|
<part id="srcnsink"><title>Sources and Sinks</title>
|
|
<partintro>
|
|
<para>
|
|
</para>
|
|
</partintro>
|
|
|
|
&SRCNSINK;
|
|
Writing a source
|
|
Pull vs loop based
|
|
Region pulling
|
|
(NOTE: somewhere explain how filters use this)
|
|
Writing a sink
|
|
Gee, that was easy
|
|
</part>
|
|
|
|
<!-- ############ part ############# -->
|
|
|
|
<part id="statemanage"><title>State management</title>
|
|
<partintro>
|
|
<para>
|
|
</para>
|
|
</partintro>
|
|
|
|
&STATEMANAGE;
|
|
What are states?
|
|
Mangaging filter state
|
|
</part>
|
|
|
|
<!-- ############ part ############# -->
|
|
|
|
<part id="checklist"><title>Checklist</title>
|
|
<partintro>
|
|
<para>
|
|
</para>
|
|
</partintro>
|
|
|
|
&CHECKLIST;
|
|
Things to check when writing a filter
|
|
Things to check when writing a source or sink
|
|
</part>
|
|
|
|
</book>
|
|
|
|
|
|
|
|
|
|
|
|
=====
|
|
|
|
Omega: a chain-based element has chain functions on each sink pad, the
|
|
connected source pad may directly call (i.e. on the stack) the chain
|
|
function
|
|
Omega: each chain function is responsible for doing something useful,
|
|
generally processing the buffer and pushing out the other end
|
|
Omega: a loop-based element has a single function attatched to the element
|
|
(not tha pads) that spins in a loop calling gst_pad_pull(sinkpad),
|
|
do stuff, gst_pad_push(srcpad)
|
|
|