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205 lines
7.3 KiB
Text
205 lines
7.3 KiB
Text
During the course of a discussion on IRC, it turned out
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that there are many possible ways to handle the capabilities.
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A capabilitiy is bascially a set of properties attached to a
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mimetype in order to more closely describe the mimetype.
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Capabiltities are supposed to be attached to pads so that the
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autoplugging algorithm has more specific information to connect
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compatible pads.
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We present 3 possible implementation for the capabilities. we need
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to pick one of them.
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1. static capabilities
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----------------------
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When an element is created, it creates its pads like:
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mpg123->sinkpad = gst_pad_new ("sink", GST_PAD_SINK);
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gst_element_add_pad (GST_ELEMENT (mpg123), mpg123->sinkpad);
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mpg123->srcpad = gst_pad_new ("src", GST_PAD_SRC);
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gst_element_add_pad (GST_ELEMENT (mpg123), mpg123->srcpad);
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In the static capabilities case, it will attach a GstCaps* structure
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to the pad. The GstCaps structure in the above example might look like:
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static GstCapsFactory mpg123_sink_caps = {
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"audio/mp3",
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"layer", GST_CAPS_INT_RANGE (1, 3),
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"bitrate", GST_CAPS_INT_RANGE (8, 320),
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NULL
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};
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with
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mpg123sinkcaps = gst_caps_register (mpg123_sink_caps);
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the factory can be converted into a GstCaps* structure. The
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GstCaps* structure is attached to the pad with:
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gst_pad_add_caps (mpg123->sinkpad, mpg123sinkcaps);
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The GstElement would then have a sinkpad with the given
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mimetype (audio/mp3) and with the capabilitities of accepting
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mpeg layer 1 to 3 and a bitrate from 8 up to 320 Kbps.
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Likewise, the src pad could be set up in the same way. An
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example capability factory could look like:
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static GstCapsFactory mpg123_src_caps = {
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"audio/raw",
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"format", GST_CAPS_BITFIELD (...),
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"depth", GST_CAPS_INT (16),
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"rate", GST_CAPS_INT_RANGE (4000, 96000),
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"channels", GST_CAPS_INT_RANGE (1, 2),
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NULL
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};
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All GstElements would present their pads with the appropriate
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capabilities structure.
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The autoplugger would then proceed (once the source media type
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is known with a typefind function) in finding all the elements
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with compatible pads and connecting them into a pipeline.
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All elements of the complete pipeline could then be constructed
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with one single pass. No new elements should be added to the
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pipeline because we can figure out all the possibilities using the
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pad capabilities.
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We call this the static case because the capabilities of the pads
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are supposed to stay the same after creating the element.
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While the ability to completly setup the pipeline before actually
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starting playback is an advantage regarding performance, one obvious
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problem with this setup is that the static case may be too static in
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some cases. We can illustrate this with the following setup:
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----------) (------------
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mpg123 ! ! audiosink
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src sink
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! !
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----------) (------------
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The mpg123 element has its src capabilities set up as mpg123_src_caps
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in the above example.
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The audio renderer has its capabilities set up with the following
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factory:
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static GstCapsFactory audio_sink_caps = {
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"audio/raw",
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"format", GST_CAPS_BITFIELD (...),
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"depth", GST_CAPS_INT (16),
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"rate", GST_CAPS_INT_RANGE (22000, 44000),
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"channels", GST_CAPS_INT_RANGE (1, 2),
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NULL
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};
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The static autoplugger has to be carefull when connecting the mpg123
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element with the audiosink because it is theoretically possible that
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the mpg123 element outputs raw audio with a rate that cannot be
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handled by the audiosink (ex. 4000KHz). In the absense of another
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audiosink with more capabilities, the autoplugging of this simple
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pipeline will not be possible and would fail.
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the autoplugging algorithm would probably select another element to
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insert between the mpg123 element and the audiosink in order to handle
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the (uncommon) case of a rate conversion (audioscaler).
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It is clear that this static setup might even fail or work suboptimal
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for even the common case and should therefore be considered as too
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restrictive.
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2. dynamic capabilities
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-----------------------
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The idea of dynamic capabilities is that the capabilities are not set
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at element create time but rather while the pipeline is running.
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An element would still list its mime type using:
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gst_pad_add_type_id(mpg123->sinkpad, mp3type);
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The idea would then be that a rough draft of the pipeline would be
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built afer the media type of the stream has been detected with the
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typefind functions. The rough draft would consist of laying out a
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global plan to reach the renderer(s). this plan would basically list
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the set of conversions that have to be performed. (mime-type to
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mime-type conversion).
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Elements that accept the src mime-type are tried by giving it a buffer.
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If the element accepts the buffer, it will set its capabilities for
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both the sink pad and the src pad. At that time other elements can be
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tried and added to the src pad, until we reach the renderer. As usual
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one has to be carefull to add just the minimum amount of elements to
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reach the renderer. The global plan will help with that.
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Since we basically do not use the capabilities of the sink pad one has
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to question the need for sink pad capabilities in the first place.
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We might also have a hard time trying different elements until we find
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a compatible one that does not cause a dead end at some point.
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3. combined setup
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-----------------
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This combined setup will minimise the effort needed to try different
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elements encountered by option 2 while still allowing a more dynamic
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setup based on the actual media stream we are handling.
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The combined setup will list/add the sink capabilities at create time.
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It will only set the mime-type of its src pads.
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As with option2, a global plan will be built. At runtime the src pads
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will actually specify the capabilities they need for any element that
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wants to be connected to its source pads.
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In this case we specifiy the capabilities for all the sink pads of an
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element at create time. The capabilities of the src pads would only
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become available when data has been processed by the element.
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The autoplugger would then be able to choose an element that can handle
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the capability listed by the src pad.
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in our previous example:
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----------) (------------
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mpg123 ! ! audiosink
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src sink
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! !
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----------) (------------
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the audiosink element would specify its sink pad capabilities at create
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time, while the mpg123 elements src pad would not yet have any capabilities
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set.
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When data is handled by the mpg123 element, a capability would be added to
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the mpg123 src pad. This capability might be:
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static GstCapsFactory mpg123_src_caps = {
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"audio/raw",
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"format", GST_CAPS_INT (S16),
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"depth", GST_CAPS_INT (16),
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"rate", GST_CAPS_INT (44000),
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"channels", GST_CAPS_INT (2),
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NULL
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};
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This capability would be compatible with the audiosinks sinkpad capabilities
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and the autoplugger would therefore be able to connect the two elements.
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While allowing a more flexible setup with option3, compared to option1, we
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introduce a slightly higher overhead when we need to dynamically connect
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elements. This overhead will not be as big as option2 because we don't
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have to 'try' elements.
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so:
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src caps: added at runtime to list the caps needed for an element that
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wants to connect to this pad.
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sink caps: the (static) capabilities that this sinkpad has.
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