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289 lines
11 KiB
Text
289 lines
11 KiB
Text
Decodebin design
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GstDecodeBin
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------------
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Description:
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Autoplug and decode to raw media
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Input : single pad with ANY caps Output : Dynamic pads
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* Contents
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_ a GstTypeFindElement connected to the single sink pad
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_ optionally a demuxer/parser
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_ optionally one or more DecodeGroup
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* Autoplugging
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The goal is to reach 'target' caps (by default raw media).
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This is done by using the GstCaps of a source pad and finding the available
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demuxers/decoders GstElement that can be linked to that pad.
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The process starts with the source pad of typefind and stops when no more
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non-target caps are left. It is commonly done while pre-rolling, but can also
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happen whenever a new pad appears on any element.
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Once a target caps has been found, that pad is ghosted and the
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'pad-added' signal is emitted.
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If no compatible elements can be found for a GstCaps, the pad is ghosted and
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the 'unknown-type' signal is emitted.
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* Assisted auto-plugging
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When starting the auto-plugging process for a given GstCaps, two signals are
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emitted in the following way in order to allow the application/user to assist or
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fine-tune the process.
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_ 'autoplug-continue' :
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gboolean user_function (GstElement * decodebin, GstPad *pad, GstCaps * caps)
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This signal is fired at the very beginning with the source pad GstCaps. If
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the callback returns TRUE, the process continues normally. If the callback
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returns FALSE, then the GstCaps are considered as a target caps and the
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autoplugging process stops.
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- 'autoplug-factories' :
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GValueArray user_function (GstElement* decodebin, GstPad* pad,
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GstCaps* caps);
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Get a list of elementfactories for @pad with @caps. This function is used to
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instruct decodebin2 of the elements it should try to autoplug. The default
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behaviour when this function is not overridern is to get all elements that
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can handle @caps from the registry sorted by rank.
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- 'autoplug-select' :
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gint user_function (GstElement* decodebin, GstPad* pad, GstCaps* caps,
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GValueArray* factories);
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This signal is fired once autoplugging has got a list of compatible
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GstElementFactory. The signal is emitted with the GstCaps of the source pad
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and a pointer on the GValueArray of compatible factories.
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The callback should return the index of the elementfactory in @factories
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that should be tried next.
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If the callback returns -1, the autoplugging process will stop as if no
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compatible factories were found.
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The default implementation of this function will try to autoplug the first
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factory of the list.
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* Target Caps
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The target caps are a read/write GObject property of decodebin.
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By default the target caps are:
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_ Raw audio : audio/x-raw-int, audio/x-raw-float
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_ and raw video : video/x-raw-rgb, video/x-raw-yuv
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_ and Text : text/plain, text/x-pango-markup
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* media chain/group handling
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When autoplugging, all streams coming out of a demuxer will be grouped in a
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DecodeGroup.
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All new source pads created on that demuxer after it has emitted the
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'no-more-pads' signal will be put in another DecodeGroup.
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Only one decodegroup can be active at any given time. If a new decodegroup is
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created while another one exists, that decodegroup will be set as blocking until
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the existing one has drained.
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DecodeGroup
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-----------
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Description:
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Streams belonging to the same group/chain of a media file.
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* Contents
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The DecodeGroup contains:
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_ a GstMultiQueue to which all streams of a the media group are connected.
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_ the eventual decoders which are autoplugged in order to produce the
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requested target pads.
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* Proper group draining
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The DecodeGroup takes care that all the streams in the group are completely
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drained (EOS has come through all source ghost pads).
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* Pre-roll and block
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The DecodeGroup has a global blocking feature. If enabled, all the ghosted
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source pads for that group will be blocked.
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A method is available to unblock all blocked pads for that group.
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GstMultiQueue
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-------------
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Description:
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Multiple input-output data queue
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The GstMultiQueue achieves the same functionnality as GstQueue, with a few
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differences:
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* Multiple streams handling.
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The element handles queueing data on more than one stream at once. To
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achieve such a feature it has request sink pads (sink_%d) and 'sometimes' src
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pads (src_%d).
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When requesting a given sinkpad, the associated srcpad for that stream will
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be created. Ex: requesting sink_1 will generate src_1.
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* Non-starvation on multiple streams.
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If more than one stream is used with the element, the streams' queues will
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be dynamically grown (up to a limit), in order to ensure that no stream is
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risking data starvation. This guarantees that at any given time there are at
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least N bytes queued and available for each individual stream.
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If an EOS event comes through a srcpad, the associated queue should be
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considered as 'not-empty' in the queue-size-growing algorithm.
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* Non-linked srcpads graceful handling.
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A GstTask is started for all srcpads when going to GST_STATE_PAUSED.
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The task are blocking against a GCondition which will be fired in two
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different cases:
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_ When the associated queue has received a buffer.
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_ When the associated queue was previously declared as 'not-linked' and the
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first buffer of the queue is scheduled to be pushed synchronously in
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relation to the order in which it arrived globally in the element (see
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'Synchronous data pushing' below).
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When woken up by the GCondition, the GstTask will try to push the next
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GstBuffer/GstEvent on the queue. If pushing the GstBuffer/GstEvent returns
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GST_FLOW_NOT_LINKED, then the associated queue is marked as 'not-linked'. If
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pushing the GstBuffer/GstEvent succeeded the queue will no longer be marked as
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'not-linked'.
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If pushing on all srcpads returns GstFlowReturn different from GST_FLOW_OK,
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then all the srcpads' tasks are stopped and subsequent pushes on sinkpads will
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return GST_FLOW_NOT_LINKED.
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* Synchronous data pushing for non-linked pads.
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In order to better support dynamic switching between streams, the multiqueue
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(unlike the current GStreamer queue) continues to push buffers on non-linked
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pads rather than shutting down.
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In addition, to prevent a non-linked stream from very quickly consuming all
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available buffers and thus 'racing ahead' of the other streams, the element
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must ensure that buffers and inlined events for a non-linked stream are pushed
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in the same order as they were received, relative to the other streams
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controlled by the element. This means that a buffer cannot be pushed to a
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non-linked pad any sooner than buffers in any other stream which were received
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before it.
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=====================================
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Parsers, decoders and auto-plugging
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=====================================
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This section has DRAFT status.
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Some media formats come in different "flavours" or "stream formats". These
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formats differ in the way the setup data and media data is signalled and/or
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packaged. An example for this is H.264 video, where there is a bytestream
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format (with codec setup data signalled inline and units prefixed by a sync
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code and packet length information) and a "raw" format where codec setup
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data is signalled out of band (via the caps) and the chunking is implicit
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in the way the buffers were muxed into a container, to mention just two of
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the possible variants.
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Especially on embedded platforms it is common that decoders can only
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handle one particular stream format, and not all of them.
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Where there are multiple stream formats, parsers are usually expected
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to be able to convert between the different formats. This will, if
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implemented correctly, work as expected in a static pipeline such as
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... ! parser ! decoder ! sink
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where the parser can query the decoder's capabilities even before
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processing the first piece of data, and configure itself to convert
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accordingly, if conversion is needed at all.
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In an auto-plugging context this is not so straight-forward though,
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because elements are plugged incrementally and not before the previous
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element has processes some data and decided what it will output exactly
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(unless the template caps are completely fixed, then it can continue
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right away, this is not always the case here though, see below). A
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parser will thus have to decide on *some* output format so auto-plugging
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can continue. It doesn't know anything about the available decoders and
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their capabilities though, so it's possible that it will choose a format
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that is not supported by any of the available decoders, or by the preferred
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decoder.
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If the parser had sufficiently concise but fixed source pad template caps,
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decodebin could continue to plug a decoder right away, allowing the
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parser to configure itself in the same way as it would with a static
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pipeline. This is not an option, unfortunately, because often the
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parser needs to process some data to determine e.g. the format's profile or
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other stream properties (resolution, sample rate, channel configuration, etc.),
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and there may be different decoders for different profiles (e.g. DSP codec
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for baseline profile, and software fallback for main/high profile; or a DSP
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codec only supporting certain resolutions, with a software fallback for
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unusual resolutions). So if decodebin just plugged the most highest-ranking
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decoder, that decoder might not be be able to handle the actual stream later
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on, which would yield in an error (this is a data flow error then which would
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be hard to intercept and avoid in decodebin). In other words, we can't solve
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this issue by plugging a decoder right away with the parser.
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So decodebin need to communicate to the parser the set of available decoder
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caps (which would contain the relevant capabilities/restrictions such as
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supported profiles, resolutions, etc.), after the usual "autoplug-*" signal
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filtering/sorting of course.
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This could be done in multiple ways, e.g.
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- plug a capsfilter element right after the parser, and construct
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a set of filter caps from the list of available decoders (one
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could append at the end just the name(s) of the caps structures
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from the parser pad template caps to function as an 'ANY other'
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caps equivalent). This would let the parser negotiate to a
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supported stream format in the same way as with the static
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pipeline mentioned above, but of course incur some overhead
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through the additional capsfilter element.
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- one could add a filter-caps equivalent property to the parsers
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(and/or GstBaseParse class) (e.g. "prefered-caps" or so).
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- one could add some kind of "fixate-caps" or "fixate-format"
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signal to such parsers
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Alternatively, one could simply make all decoders incorporate parsers, so
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that always all formats are supported. This is problematic for other reasons
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though (e.g. we would not be able to detect the profile in all cases then
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before plugging a decoder, which would make it hard to just play the audio
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part of a stream and not the video if a suitable decoder was missing, for
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example).
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