Capabilities negotiation is the process of deciding on an adequate format for dataflow within a GStreamer pipeline. Ideally, negotiation (also known as "capsnego") transfers information from those parts of the pipeline that have information to those parts of the pipeline that are flexible, constrained by those parts of the pipeline that are not flexible.

Basic rules

These simple rules must be followed:

  1. downstream suggests formats
  2. upstream decides on format

There are 4 queries/events used in caps negotiation:

  1. GST_QUERY_CAPS: get possible formats
  2. GST_QUERY_ACCEPT_CAPS: check if format is possible
  3. GST_EVENT_CAPS: configure format (downstream)
  4. GST_EVENT_RECONFIGURE: inform upstream of possibly new caps


A pad can ask the peer pad for its supported GstCaps. It does this with the CAPS query. The list of supported caps can be used to choose an appropriate GstCaps for the data transfer. The CAPS query works recursively, elements should take their peers into consideration when constructing the possible caps. Because the result caps can be very large, a filter can be used to restrict the caps. Only the caps that match the filter will be returned as the result caps. The order of the filter caps gives the order of preference of the caller and should be taken into account for the returned caps.

  • filter (in) GST_TYPE_CAPS (default NULL): - a GstCaps to filter the results against
  • caps (out) GST_TYPE_CAPS (default NULL): - the result caps

A pad can ask the peer pad if it supports a given caps. It does this with the ACCEPT_CAPS query. The caps must be fixed. The ACCEPT_CAPS query is not required to work recursively, it can simply return TRUE if a subsequent CAPS event with those caps would return success.

  • caps (in) GST_TYPE_CAPS: - a GstCaps to check, must be fixed
  • result (out) G_TYPE_BOOLEAN (default FALSE): - TRUE if the caps are accepted


When a media format is negotiated, peer elements are notified of the GstCaps with the CAPS event. The caps must be fixed.

  • caps GST_TYPE_CAPS: - the negotiated GstCaps, must be fixed


GStreamer’s two scheduling modes, push mode and pull mode, lend themselves to different mechanisms to achieve this goal. As it is more common we describe push mode negotiation first.

Push-mode negotiation

Push-mode negotiation happens when elements want to push buffers and need to decide on the format. This is called downstream negotiation because the upstream element decides the format for the downstream element. This is the most common case.

Negotiation can also happen when a downstream element wants to receive another data format from an upstream element. This is called upstream negotiation.

The basics of negotiation are as follows:

  • GstCaps (see caps) are refcounted before they are pushed as an event to describe the contents of the following buffer.

  • An element should reconfigure itself to the new format received as a CAPS event before processing the following buffers. If the data type in the caps event is not acceptable, the element should refuse the event. The element should also refuse the next buffers by returning an appropriate GST_FLOW_NOT_NEGOTIATED return value from the chain function.

  • Downstream elements can request a format change of the stream by sending a RECONFIGURE event upstream. Upstream elements will renegotiate a new format when they receive a RECONFIGURE event.

The general flow for a source pad starting the negotiation.

            src              sink
             |                 |
             |  querycaps?     |
             |     caps        |
select caps  |< - - - - - - - -|
from the     |                 |
candidates   |                 |
             |                 |-.
             |  accepts?       | |
 type A      |---------------->| | optional
             |      yes        | |
             |< - - - - - - - -| |
             |                 |-'
             |  send_event()   |
send CAPS    |---------------->| Receive type A, reconfigure to
event A      |                 | process type A.
             |                 |
             |  push           |
push buffer  |---------------->| Process buffer of type A
             |                 |

One possible implementation in pseudo code:

    [element wants to create a buffer]
    if not format
      # see what we can do
      ourcaps = gst_pad_query_caps (srcpad)
      # see what the peer can do filtered against our caps
      candidates = gst_pad_peer_query_caps (srcpad, ourcaps)

    foreach candidate in candidates
      # make sure the caps is fixed
      fixedcaps = gst_pad_fixate_caps (srcpad, candidate)

    # see if the peer accepts it
    if gst_pad_peer_accept_caps (srcpad, fixedcaps)
      # store the caps as the negotiated caps, this will
      # call the setcaps function on the pad
      gst_pad_push_event (srcpad, gst_event_new_caps (fixedcaps))

Negotiate allocator/bufferpool with the ALLOCATION query

    buffer = gst_buffer_new_allocate (NULL, size, 0);
    # fill buffer and push

The general flow for a sink pad starting a renegotiation.

            src              sink
             |                 |
             |  accepts?       |
             |<----------------| type B
             |      yes        |
             |- - - - - - - - >|-.
             |                 | | suggest B caps next
             |                 |<'
             |                 |
             |   push_event()  |
 mark      .-|<----------------| send RECONFIGURE event
renegotiate| |                 |
           '>|                 |
             |  querycaps()    |
renegotiate  |---------------->|
             |  suggest B      |
             |< - - - - - - - -|
             |                 |
             |  send_event()   |
send CAPS    |---------------->| Receive type B, reconfigure to
event B      |                 | process type B.
             |                 |
             |  push           |
push buffer  |---------------->| Process buffer of type B
             |                 |

Use cases:

videotestsrc ! xvimagesink
  • Who decides what format to use?

    • src pad always decides, by convention. sinkpad can suggest a format by putting it high in the caps query result GstCaps.
    • since the src decides, it can always choose something that it can do, so this step can only fail if the sinkpad stated it could accept something while later on it couldn't.
  • When does negotiation happen?

    • before srcpad does a push, it figures out a type as stated in 1), then it pushes a caps event with the type. The sink checks the media type and configures itself for this type.
    • the source then usually does an ALLOCATION query to negotiate a bufferpool with the sink. It then allocates a buffer from the pool and pushes it to the sink. Since the sink accepted the caps, it can create a pool for the format.
    • since the sink stated in 1) it could accept the type, it will be able to handle it.
  • How can sink request another format?

    • sink asks if new format is possible for the source.
    • sink pushes RECONFIGURE event upstream
    • src receives the RECONFIGURE event and marks renegotiation
    • On the next buffer push, the source renegotiates the caps and the bufferpool. The sink will put the new new preferred format high in the list of caps it returns from its caps query.
videotestsrc ! queue ! xvimagesink
  • queue proxies all accept and caps queries to the other peer pad.
  • queue proxies the bufferpool
  • queue proxies the RECONFIGURE event
  • queue stores CAPS event in the queue. This means that the queue can contain buffers with different types.

Pull-mode negotiation

A pipeline in pull mode has different negotiation needs than one activated in push mode. Push mode is optimized for two use cases:

  • Playback of media files, in which the demuxers and the decoders are the points from which format information should disseminate to the rest of the pipeline; and

  • Recording from live sources, in which users are accustomed to putting a capsfilter directly after the source element; thus the caps information flow proceeds from the user, through the potential caps of the source, to the sinks of the pipeline.

In contrast, pull mode has other typical use cases:

  • Playback from a lossy source, such as RTP, in which more knowledge about the latency of the pipeline can increase quality; or

  • Audio synthesis, in which audio APIs are tuned to produce only the necessary number of samples, typically driven by a hardware interrupt to fill a DMA buffer or a Jack[0] port buffer.

  • Low-latency effects processing, whereby filters should be applied as data is transferred from a ring buffer to a sink instead of beforehand. For example, instead of using the internal alsasink ringbuffer thread in push-mode wavsrc ! volume ! alsasink, placing the volume inside the sound card writer thread via wavsrc ! audioringbuffer ! volume ! alsasink.


The problem with pull mode is that the sink has to know the format in order to know how many bytes to pull via gst_pad_pull_range(). This means that before pulling, the sink must initiate negotation to decide on a format.

Recalling the principles of capsnego, whereby information must flow from those that have it to those that do not, we see that the three named use cases have different negotiation requirements:

  • RTP and low-latency playback are both like the normal playback case, in which information flows downstream.

  • In audio synthesis, the part of the pipeline that has the most information is the sink, constrained by the capabilities of the graph that feeds it. However the caps are not completely specified; at some point the user has to intervene to choose the sample rate, at least. This can be done externally to gstreamer, as in the jack elements, or internally via a capsfilter, as is customary with live sources.

Given that sinks potentially need the input of sources, as in the RTP case and at least as a filter in the synthesis case, there must be a negotiation phase before the pull thread is activated. Also, given the low latency offered by pull mode, we want to avoid capsnego from within the pulling thread, in case it causes us to miss our scheduling deadlines.

The pull thread is usually started in the PAUSED→PLAYING state change. We must be able to complete the negotiation before this state change happens.

The time to do capsnego, then, is after the SCHEDULING query has succeeded, but before the sink has spawned the pulling thread.


The sink determines that the upstream elements support pull based scheduling by doing a SCHEDULING query.

The sink initiates the negotiation process by intersecting the results of gst_pad_query_caps() on its sink pad and its peer src pad. This is the operation performed by gst_pad_get_allowed_caps() In the simple passthrough case, the peer pad’s caps query should return the intersection of calling get_allowed_caps() on all of its sink pads. In this way the sink element knows the capabilities of the entire pipeline.

The sink element then fixates the resulting caps, if necessary, resulting in the flow caps. From now on, the caps query of the sinkpad will only return these fixed caps meaning that upstream elements will only be able to produce this format.

If the sink element could not set caps on its sink pad, it should post an error message on the bus indicating that negotiation was not possible.

When negotiation succeeded, the sinkpad and all upstream internally linked pads are activated in pull mode. Typically, this operation will trigger negotiation on the downstream elements, which will now be forced to negotiate to the final fixed desired caps of the sinkpad.

After these steps, the sink element returns ASYNC from the state change function. The state will commit to PAUSED when the first buffer is received in the sink. This is needed to provide a consistent API to the applications that expect ASYNC return values from sinks but it also allows us to perform the remainder of the negotiation outside of the context of the pulling thread.


We can identify 3 patterns in negotiation:

  • Fixed : Can't choose the output format

    • Caps encoded in the stream
    • A video/audio decoder
    • usually uses gst_pad_use_fixed_caps()
  • Transform

    • Caps not modified (passthrough)
    • can do caps transform based on element property
    • fixed caps get transformed into fixed caps
    • videobox
  • Dynamic : can choose output format

    • A converter element
    • depends on downstream caps, needs to do a CAPS query to find transform.
    • usually prefers to use the identity transform
    • fixed caps can be transformed into unfixed caps.

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