mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-30 20:20:38 +00:00
8063481f3f
Original commit message from CVS: * docs/design/design-decodebin.txt: Added design document for new decodebin (Target Caps): text/x-pango-markup is also a default target caps.
194 lines
6.3 KiB
Text
194 lines
6.3 KiB
Text
Decodebin design
|
|
|
|
GstDecodeBin
|
|
------------
|
|
|
|
Description:
|
|
|
|
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
|
|
**list)
|
|
|
|
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, text/x-pango-markup
|
|
|
|
|
|
* media chain/group handling
|
|
|
|
When autoplugging, all streams coming out of a demuxer will be grouped in a
|
|
DecodeGroup.
|
|
|
|
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.
|
|
|
|
|
|
|
|
DecodeGroup
|
|
-----------
|
|
|
|
Description:
|
|
|
|
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.
|
|
|
|
|
|
|
|
GstMultiQueue
|
|
-------------
|
|
|
|
Description:
|
|
|
|
Multiple input-output data queue
|
|
|
|
The GstMultiQueue achieves the same functionnality as GstQueue, with a few
|
|
differences:
|
|
|
|
* 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
|
|
'not-linked'.
|
|
|
|
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
|
|
return GST_FLOW_NOT_LINKED.
|
|
|
|
* 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.
|