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Added the type system design.
Original commit message from CVS: Added the type system design.
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Wim Taymans
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docs/random/types2
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docs/random/types2
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1. Introduction
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---------------
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The type system is used to attach meaning to the bytes in a GstBuffer.
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A plugin can decide to add metadata to the GstBuffer, this metadata
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will carry an associated typeid.
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Types are also used by the plugins to expose the type of their pads to
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the type system.
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Types are essential for autoplugging.
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We will explain the inner workings of the type system in this document, as
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well as the API used in the plugins.
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2. Type properties
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------------------
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a) identification of the type
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Types are identified by one or more MIME types.
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b) detection of types
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The type of any given GstBuffer can be detected using
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- a typefind function: a function that will inspect the bytes
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in the buffer and return a gboolean indicating the
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buffer is of the given type.
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- a template for the bytes/bits in the data that must be
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satisfied in order for the GstBuffer to be of the given
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type.
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- other properties that act more like a hint like:
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the extension of the source filename.
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the URI of the source.
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3. Type registration
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--------------------
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a) the core libraries will create to types:
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video/raw image/raw
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audio/raw
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b) all other types will be provided by the plugins
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before a type can become available to other plugins, the plugin
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has to register the type.
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The system will keep a directed graph of the types and the plugins
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that operate on them.
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example:
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video/mpeg
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!
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mpeg1parse
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/ \
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video/mpeg1 audio/mp3 -----\
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! ! \ !
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mpeg_play mpeg2dec mpg123 xing ...
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\ / \ /
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video/raw audio/raw-----\
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! ! ! !
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videosink SDLsink audiosink alsasink ...
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The system can find the needed plugins to convert video/mpeg to
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audio/raw using this graph.
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4. type equivalence
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-------------------
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some types can have the same meaning for example:
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audio/mp3 and audio/mpeg
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or
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video/raw and image/raw
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a plugin that exposes its output type as audio/mpeg and another plugins
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with input type audio/mp3 can be connected. The system has to know about
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the equivalence of both types, even it they have a different mime type.
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5. type hierarchy
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-----------------
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some plugins can ouput a specific subset of an allready existing type.
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example:
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mp3parse inputs audio/mp3 and packs the stream into mp3 audio frames
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with mime type: audio/mp3-frame
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mpg123 only accepts audio/mp3-frame but not audio/mp3.
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another mp3 decoder (libmpg123) can accept audio/mp3 (and thus also
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audio/mp3-frame)
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it must be possible to connect both libmpg123 and mpg123 to the mp3parse
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element.
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it must not be possible to connect mpg123 to an element that outputs only
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audio/mp3 but not audio/mp3-frame.
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We say that audio/mp3-frame is a more specific subset of type audio/mp3.
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we can create a type hierarchy:
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audio/mp3
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/ \
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audio/mp3-frame audio/mp3-layer12
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/ \
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audio/mp3-layer1 audio/mp3-layer2
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6. multi-type pads
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------------------
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certain plugins might accept multiple non equivalent types in one of their
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input pads. Consequently a plugin might output non equivalent types in
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its output pad.
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It must therefore be possible to specify multiple types for a pad.
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example:
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mpegdemux may be able to demux both MPEG1 and MPEG2 system streams.
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we show the type hierarchy of the video/mpeg as follows:
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video/mpeg
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/ \
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video/mpeg1 video/mpeg2 ---------------\
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/ \ / \ !
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mpeg1-system* mpeg1-video mpeg2-ts mpeg2-ps* mpeg2-video
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the mpegdemux element might specify the type of the input pad as
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one of video/mpeg1-system and video/mpeg2-ts
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7. definition of types
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----------------------
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A plugin will provide the following information to the type system:
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- a mime type string describing the hierarchy and where the types
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they provide are located in that hierarchy.
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- typefind functions for some of the types.
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- extensions for some of the types
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We will propose a syntax to define the type hierarchy
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a) equivalent types :
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separated with a | sign
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( audio/mp3 | audio/mpeg )
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b) type hierarchy :
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in braces:
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( audio/mp3 ( audio/mp3-frame))
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c) multi-type pads
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( mpegdemux ( video/mpeg1-system + video/mpeg2-ps) )
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the pad will have type mpegdemux which is the parent for
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type video/mpeg1-system or video/mpeg2-ps
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mpegdemux
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/ \
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video/mpeg1-system video/mpeg2-ps
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once the type hierarchy has been registered, the typeid of each
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element can be obtained with:
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guint16 gst_type_find_by_mime (gchar *mime)
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extra typefind functions and/or extensions can be added to the
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typeid afterwards.
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8. type matching
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----------------
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The more specific typefind functions, the functions associated with
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types in the leaf nodes, are tried first.
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when a specific type has been found ex. video/mpeg1-system elements
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that can handle this type or one of its parents are selected:
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mpegdemux: mpeg1parse: supermpegdecoder:
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video/mpeg video/mpeg video/mpeg
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! !
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mpegdemux video/mpeg1-system
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!
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video/mpeg1-system
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In the above case, also the super mpeg element is selected because it stated
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that it can handle all sorts of video/mpeg data.
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example 2:
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suppose the typefind functions finds an mp3 stream, fillowing elments
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are selected:
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libmpg123 mp3parse:
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audio/mp3 audio/mp3
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mpg123 is not selected because its pad type is too specific (audio/mp3-frame):
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mpg123
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audio/mp3
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!
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audio/mp3-frame
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if the typefind would find a mp3-frame type, all three objects would be selected.
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8. consideration
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----------------
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It is clear that clear indications have to be given to the type hierarchy,
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especially for the top nodes.
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The more specific an element is in its mime type specification, the more flexible
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and extendible the plugin system can be.
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