docs/random/types - mtk-gstreamer - Git at Google

 GstTypes exist to try to make sure data eveyrone is talking about the
 right kind of data.  They aid quite a bit in autoplugging, in fact make it
 possible.  Each well-formed type includes a function (typefind) that will
 take one or more buffers and determine whether or not it is indeed a
 stream of that type, and possible a metadata to go with it.  It may
 provide related metadata structure IDs (and must if it provides metadata
 from the typefind function).

 Because multiple elements and plugins are very likely to be using the same
 types, the process of creating/finding types is designed to be done with a
 single function call.  All operations on GstTypes occur via their guint16
 ID numbers, with the GstType structure "private" to the GST library.  A
 plugin wishing to use a give type would contain a static struct of type
 GstTypeFactory, which lists the MIME type, possible extensions (which may
 overlap the mime-types file), a typefind function, and any other cruft I
 decide to add.

 A plugin init function would take this typefactory and hand it to the
 gst_type_new() (FIXME: badly named) function, which would first search for
 that same MIME type in the current list.  If it found one, it would
 compare the two to see if the new one is "better".  Better is defined as
 having more extentions (to be merged) or a typefind function verses none.

 The point of returning an existing MIME type is a result of the goal of
 unifying types enough to guarantee that, for instance, all MP3 decoders
 will work interchangably.  If MP3 decoder A says "MIME type 'audio/mpeg'
 with extensions 'mpeg3'" and decoder B says "MIME type 'audio/mpeg' with
 extensions 'mp3'", we don't want to have two types defined, possibly with
 two typefind functions. If we did, it's not obvious which of the two would
 be tried first (luck) and if both would really identify streams as mp3
 correctly in all cases.  And whichever wins, we're stuck using the
 associated decoder to play that stream.  We lose the choice between any
 valid mp3 decoder, and thus the whole point of the type system.
	GstTypes exist to try to make sure data eveyrone is talking about the
	right kind of data. They aid quite a bit in autoplugging, in fact make it
	possible. Each well-formed type includes a function (typefind) that will
	take one or more buffers and determine whether or not it is indeed a
	stream of that type, and possible a metadata to go with it. It may
	provide related metadata structure IDs (and must if it provides metadata
	from the typefind function).

	Because multiple elements and plugins are very likely to be using the same
	types, the process of creating/finding types is designed to be done with a
	single function call. All operations on GstTypes occur via their guint16
	ID numbers, with the GstType structure "private" to the GST library. A
	plugin wishing to use a give type would contain a static struct of type
	GstTypeFactory, which lists the MIME type, possible extensions (which may
	overlap the mime-types file), a typefind function, and any other cruft I
	decide to add.

	A plugin init function would take this typefactory and hand it to the
	gst_type_new() (FIXME: badly named) function, which would first search for
	that same MIME type in the current list. If it found one, it would
	compare the two to see if the new one is "better". Better is defined as
	having more extentions (to be merged) or a typefind function verses none.

	The point of returning an existing MIME type is a result of the goal of
	unifying types enough to guarantee that, for instance, all MP3 decoders
	will work interchangably. If MP3 decoder A says "MIME type 'audio/mpeg'
	with extensions 'mpeg3'" and decoder B says "MIME type 'audio/mpeg' with
	extensions 'mp3'", we don't want to have two types defined, possibly with
	two typefind functions. If we did, it's not obvious which of the two would
	be tried first (luck) and if both would really identify streams as mp3
	correctly in all cases. And whichever wins, we're stuck using the
	associated decoder to play that stream. We lose the choice between any
	valid mp3 decoder, and thus the whole point of the type system.