Commit eacaff27 authored by Edward Hervey's avatar Edward Hervey
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docs/design/design-decodebin.txt: Added design document for new decodebin

Original commit message from CVS:
* docs/design/design-decodebin.txt:
Added design document for new decodebin
parent 747d560f
2006-03-22 Edward Hervey <>
* docs/design/design-decodebin.txt:
Added design document for new decodebin
2006-03-22 Wim Taymans <>
* gst-libs/gst/audio/gstbaseaudiosink.c:
Decodebin design
Autoplug and decode to raw media
Input : single pad with ANY caps Output : Dynamic pads
* Contents
_ a GstTypeFindElement connected to the single sink pad
_ optionnaly a demuxer/parser
_ optionnaly one or more DecodeGroup
* Autoplugging
The goal is to reach 'target' caps (by default raw media).
This is done by using the GstCaps of a source pad and finding the available
demuxers/decoders GstElement that can be linked to that pad.
The process starts with the source pad of typefind and stops when no more
non-target caps are left. It is commonly done while pre-rolling, but can also
happen whenever a new pad appears on any element.
Once a target caps has been found, that pad is ghosted and the
'new-decoded-pad' signal is emitted.
If no compatible elements can be found for a GstCaps, the pad is ghosted and
the 'unknown-type' signal is emitted.
* Assisted auto-plugging
When starting the auto-plugging process for a given GstCaps, two signals are
emitted in the following way in order to allow the application/user to assist or
fine-tune the process.
_ 'autoplug-continue' :
gboolean user_function (GstElement * decodebin, GstCaps * caps)
This signal is fired at the very beginning with the source pad GstCaps. If
the callback returns TRUE, the process continues normally. If the callback
returns FALSE, then the GstCaps are considered as a target caps and the
autoplugging process stops.
- 'autoplug-sort' :
gboolean user_function (GstElement * decodebin, GstCaps * caps, GList
This signal is fired once autoplugging has got a list of compatible
GstElementFactory. The signal is emitted with the GstCaps of the source pad
and a pointer on the GList* of compatible factories.
The callback can re-order the given list of factories in order to put the
preferred factory at the beginning of the GList *.
If the callback returns TRUE, then the autoplugging process will create an
element to link using the first GstElementFactory of that list. If the
callback returns FALSE, then the autoplugging process will stop as if no
compatible factories was found.
* Target Caps
The target caps are a read/write GObject property of decodebin.
By default the target caps are:
_ Raw audio : audio/x-raw-int, audio/x-raw-float
_ and raw video : video/x-raw-rgb, video/x-raw-yuv
_ and Text : text/plain
* media chain/group handling
When autoplugging, all streams coming out of a demuxer will be grouped in a
All new source pads created on that demuxer after it has emitted the
'no-more-pads' signal will be put in another DecodeGroup.
Only one decodegroup can be active at any given time. If a new decodegroup is
created while another one exists, that decodegroup will be set as blocking until
the existing one has drained.
Streams belonging to the same group/chain of a media file.
* Contents
The DecodeGroup contains:
_ a GstMultiQueue to which all streams of a the media group are connected.
_ the eventual decoders which are autoplugged in order to produce the
requested target pads.
* Proper group draining
The DecodeGroup takes care that all the streams in the group are completely
drained (EOS has come through all source ghost pads).
* Pre-roll and block
The DecodeGroup has a global blocking feature. If enabled, all the ghosted
source pads for that group will be blocked.
A method is available to unblock all blocked pads for that group.
Multiple input-output data queue
The GstMultiQueue achieves the same functionnality as GstQueue, with a few
* Multiple streams handling.
The element handles queueing data on more than one stream at once. To
achieve such a feature it has request sink pads (sink_%d) and 'sometimes' src
pads (src_%d).
When requesting a given sinkpad, the associated srcpad for that stream will
be created. Ex: requesting sink_1 will generate src_1.
* Non-starvation on multiple streams.
If more than one stream is used with the element, the streams' queues will
be dynamically grown (up to a limit), in order to ensure that no stream is
risking data starvation. This guarantees that at any given time there are at
least N bytes queued and available for each individual stream.
If an EOS event comes through a srcpad, the associated queue should be
considered as 'not-empty' in the queue-size-growing algorithm.
* Non-linked srcpads graceful handling.
A GstTask is started for all srcpads when going to GST_STATE_PAUSED.
The task are blocking against a GCondition which will be fired in two
different cases:
_ When the associated queue has received a buffer.
_ When the associated queue was previously declared as 'not-linked' and the
first buffer of the queue is scheduled to be pushed synchronously in
relation to the order in which it arrived globally in the element (see
'Synchronous data pushing' below).
When woken up by the GCondition, the GstTask will try to push the next
GstBuffer/GstEvent on the queue. If pushing the GstBuffer/GstEvent returns
GST_FLOW_NOT_LINKED, then the associated queue is marked as 'not-linked'. If
pushing the GstBuffer/GstEvent succeeded the queue will no longer be marked as
If pushing on all srcpads returns GstFlowReturn different from GST_FLOW_OK,
then all the srcpads' tasks are stopped and subsequent pushes on sinkpads will
* Synchronous data pushing for non-linked pads.
In order to better support dynamic switching between streams, the multiqueue
(unlike the current GStreamer queue) continues to push buffers on non-linked
pads rather than shutting down.
In addition, to prevent a non-linked stream from very quickly consuming all
available buffers and thus 'racing ahead' of the other streams, the element
must ensure that buffers and inlined events for a non-linked stream are pushed
in the same order as they were received, relative to the other streams
controlled by the element. This means that a buffer cannot be pushed to a
non-linked pad any sooner than buffers in any other stream which were received
before it.
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