Decodebin design



  • Autoplug and decode to raw media

  • Input: single pad with ANY caps

  • Output: Dynamic pads


  • a GstTypeFindElement connected to the single sink pad

  • optionally a demuxer/parser

  • optionally one or more DecodeGroup


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 'pad-added' 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, GstPad *pad, 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-factories':
    GValueArray user_function (GstElement* decodebin, GstPad* pad, GstCaps* caps);
Get a list of elementfactories for `@pad` with `@caps`. This function is
used to instruct decodebin2 of the elements it should try to
autoplug. The default behaviour when this function is not overriden
is to get all elements that can handle @caps from the registry
sorted by rank.
  • 'autoplug-select':
    gint user_function (GstElement* decodebin, GstPad* pad, GstCaps*caps, GValueArray* factories);
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 GValueArray of compatible factories.

The callback should return the index of the elementfactory in
@factories that should be tried next.

If the callback returns -1, the autoplugging process will stop as if
no compatible factories were found.

The default implementation of this function will try to autoplug the first factory of the list.

Target Caps

The target caps are a read/write GObject property of decodebin.

By default the target caps are:

  • Raw audio: audio/x-raw

  • Raw video: video/x-raw

  • Raw text: text/x-raw, format={utf8,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.



Streams belonging to the same group/chain of a media file.


The DecodeGroup contains:

  • a GstMultiQueue to which all streams of 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.

multiqueue achieves the same functionality as queue, 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_%u) and 'sometimes' src pads (src_%u).

    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 returns GST_FLOW_NOT_LINKED, the associated queue is marked as not-linked. If pushing succeeds, the queue will no longer be marked as not-linked.

    If pushing on all srcpads returns a 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.

Parsers, decoders and auto-plugging

This section has DRAFT status.

Some media formats come in different "flavours" or "stream formats". These formats differ in the way the setup data and media data is signalled and/or packaged. An example for this is H.264 video, where there is a bytestream format (with codec setup data signalled inline and units prefixed by a sync code and packet length information) and a "raw" format where codec setup data is signalled out of band (via the caps) and the chunking is implicit in the way the buffers were muxed into a container, to mention just two of the possible variants.

Especially on embedded platforms it is common that decoders can only handle one particular stream format, and not all of them.

Where there are multiple stream formats, parsers are usually expected to be able to convert between the different formats. This will, if implemented correctly, work as expected in a static pipeline such as

... ! parser ! decoder ! sink

where the parser can query the decoder's capabilities even before processing the first piece of data, and configure itself to convert accordingly, if conversion is needed at all.

In an auto-plugging context this is not so straight-forward though, because elements are plugged incrementally and not before the previous element has processed some data and decided what it will output exactly (unless the template caps are completely fixed, then it can continue right away, this is not always the case here though, see below). A parser will thus have to decide on some output format so auto-plugging can continue. It doesn't know anything about the available decoders and their capabilities though, so it's possible that it will choose a format that is not supported by any of the available decoders, or by the preferred decoder.

If the parser had sufficiently concise but fixed source pad template caps, decodebin could continue to plug a decoder right away, allowing the parser to configure itself in the same way as it would with a static pipeline. This is not an option, unfortunately, because often the parser needs to process some data to determine e.g. the format's profile or other stream properties (resolution, sample rate, channel configuration, etc.), and there may be different decoders for different profiles (e.g. DSP codec for baseline profile, and software fallback for main/high profile; or a DSP codec only supporting certain resolutions, with a software fallback for unusual resolutions). So if decodebin just plugged the most highest-ranking decoder, that decoder might not be be able to handle the actual stream later on, which would yield an error (this is a data flow error then which would be hard to intercept and avoid in decodebin). In other words, we can't solve this issue by plugging a decoder right away with the parser.

So decodebin needs to communicate to the parser the set of available decoder caps (which would contain the relevant capabilities/restrictions such as supported profiles, resolutions, etc.), after the usual "autoplug-*" signal filtering/sorting of course.

This is done by plugging a capsfilter element right after the parser, and constructing set of filter caps from the list of available decoders (one appends at the end just the name(s) of the caps structures from the parser pad template caps to function as an 'ANY other' caps equivalent). This let the parser negotiate to a supported stream format in the same way as with the static pipeline mentioned above, but of course incur some overhead through the additional capsfilter element.

The results of the search are