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Another type system proposal.
Original commit message from CVS: Another type system proposal.
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Wim Taymans
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173
docs/random/types3
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docs/random/types3
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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. major types
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--------------
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major types are identified with mime types and are used to denote a
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family of types.
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More specific information about the type is given using properties. This
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will allow us to be very specific without creating a lot of mime types.
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3. API
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------
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Both a simple array based specification and a real API will be
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provided to build the capabilities.
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In the array based approach, we basically build an array of pointers.
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Some macros will be available to specify ranges, boolean values, lists
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and id's. (not sure if this can be done)
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#define GST_TYPE_INT_RANGE(a, b) GST_TYPE_RANGE,(a),(b)
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#define GST_TYPE_BOOLEAN(a) GST_TYPE_BOOLEAN,(a)
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#define GST_TYPE_LIST(a...) GST_TYPE_LIST,(##a),NULL
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for example:
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static GstTypeCapsFactory mpg123_sink_caps[] = {
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"audio/mp3",
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"layer", GST_TYPE_INT_RANGE (1, 3),
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"bitrate", GST_TYPE_INT_RANGE (8, 320),
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"framed", GST_TYPE_BOOLEAN (true),
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NULL
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};
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will expand to the array:
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static GstTypeCapsFactory mpg123_sink_caps[] = {
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"audio/mp3",
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"layer", GST_TYPE_RANGE,1,3,
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"bitrate", GST_TYPE_RANGE,8,320,
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"famed", GST_TYPE_BOOLEAN,true,
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NULL,
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};
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when we register the caps factory, the strings will be converted
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into GQuarks and be stored into a GData Keyed Data List.
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This will result in a GstTypeCaps structure:
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struct _GstTypeCaps {
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guint16 id; // if of the major type
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GData *properties;
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}
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4. example using arrays
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-----------------------
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mpg123: an mpeg audio decoder.
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// a factory for the major type we use
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static GstTypeFactory mp3factory = {
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"audio/mp3", // major type
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".mp3 .mp2 .mp1 .mpga", // extenstions
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NULL, // typefind function
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};
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// capabilities of the sink pad
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static GstTypeCapsFactory mpg123_sink_caps[] = {
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"audio/mp3",
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"layer", GST_TYPE_INT_RANGE (1, 3),
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"bitrate", GST_TYPE_INT_RANGE (8, 320),
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"framed", GST_TYPE_BOOLEAN (true),
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NULL
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};
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// capabilities of the source pad
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static GstTypeCapsFactory mpg123_src_caps[] = {
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"audio/raw",
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"rate", GST_TYPE_INT_RANGE (8000, 48000),
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"channels", GST_TYPE_INT_RANGE (1, 2),
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NULL
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};
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static GstTypeCaps *sinkcaps = NULL, *rawcaps = NULL;
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static gst_mpg123_init (GstMpg123 *mpg123)
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{
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mpg123->sinpad = gst_pad_new ("sink", GST_PAD_SINK);
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gst_element_add_pad (GST_ELEMENT (mpg123), mpg123->sinkpad);
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gst_pad_set_caps (mpg123->sinkpad, sinkcaps);
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...
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}
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GstPlugin *plugin_init (GModule *module)
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{
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...
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plugin = gst_plugin_new ("mpg123");
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gst_plugin_add_type_factory (plugin, mp3factory);
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...
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sinkcaps = gst_type_register_caps (mpg123_sink_caps, NULL);
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rawcaps = gst_type_register_caps (mpg123_src_caps, NULL);
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...
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}
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mpeg2dec: an mpeg video decoder that can do mpeg1 and mpeg2.
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static GstTypeFactory mpegfactory = {
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"video/mpeg", // major type
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".mpg .mpeg", // extenstions
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NULL, // typefind function
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};
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static GstTypeCapsFactory mpeg2dec_sink_caps[] = {
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"video/mpeg",
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"mpegtype", GST_TYPE_LIST (
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GST_TYPE_INT(1),
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GST_TYPE_INT(2),
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),
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NULL
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};
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static GstTypeCapsFactory mpeg2dec_src_caps[] = {
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"video/raw",
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"fourcc", GST_TYPE_LIST (
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GST_TYPE_INT32 (0x32315659),
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GST_TYPE_INT32 (0x32...),
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),
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"width", GST_TYPE_INT_RANGE (16, 4096),
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"height", GST_TYPE_INT_RANGE (16, 4096),
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NULL
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};
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static GstTypeCaps *sinkcaps = NULL, *rawcaps = NULL;
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GstPlugin *plugin_init (GModule *module)
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{
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...
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plugin = gst_plugin_new ("mpeg2dec");
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...
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sinkcaps = gst_type_register_caps (mpeg2dec_sink_caps, NULL);
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rawcaps = gst_type_register_caps (mpeg2dec_src_caps, NULL);
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...
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}
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5. capabilty compatibility
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--------------------------
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Two pads are compatible if:
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- The major types are equal
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- range of the sink pad contains the range of the src pad
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