Caps negotiation use cases

In what follows we will look at some use cases for push-mode scheduling. The pull-mode scheduling negotiation phase is discussed in the section called “Pull-mode Caps negotiation” and is actually similar as we will see.

Since the sink pads only suggest formats and the source pads need to decide, the most complicated work is done in the source pads. We can identify 3 caps negotiation use cases for the source pads:

Fixed negotiation

In this case, the source pad can only produce a fixed format. Usually this format is encoded inside the media. No downstream element can ask for a different format, the only way that the source pad will renegotiate is when the element decides to change the caps itself.

Elements that could implement fixed caps (on their source pads) are, in general, all elements that are not renegotiable. Examples include:

  • A typefinder, since the type found is part of the actual data stream and can thus not be re-negotiated. The typefinder will look at the stream of bytes, figure out the type, send a CAPS event with the caps and then push buffers of the type.

  • Pretty much all demuxers, since the contained elementary data streams are defined in the file headers, and thus not renegotiable.

  • Some decoders, where the format is embedded in the data stream and not part of the peercaps and where the decoder itself is not reconfigurable, too.

  • Some sources that produce a fixed format.

gst_pad_use_fixed_caps() is used on the source pad with fixed caps. As long as the pad is not negotiated, the default CAPS query will return the caps presented in the padtemplate. As soon as the pad is negotiated, the CAPS query will return the negotiated caps (and nothing else). These are the relevant code snippets for fixed caps source pads.


[..]
  pad = gst_pad_new_from_static_template (..);
  gst_pad_use_fixed_caps (pad);
[..]

      

The fixed caps can then be set on the pad by calling gst_pad_set_caps ().


[..]
    caps = gst_caps_new_simple ("audio/x-raw",
        "format", G_TYPE_STRING, GST_AUDIO_NE(F32),
        "rate", G_TYPE_INT, <samplerate>,
        "channels", G_TYPE_INT, <num-channels>, NULL);
    if (!gst_pad_set_caps (pad, caps)) {
      GST_ELEMENT_ERROR (element, CORE, NEGOTIATION, (NULL),
          ("Some debug information here"));
      return GST_FLOW_ERROR;
    }
[..]

      

These types of elements also don't have a relation between the input format and the output format, the input caps simply don't contain the information needed to produce the output caps.

All other elements that need to be configured for the format should implement full caps negotiation, which will be explained in the next few sections.

Transform negotiation

In this negotiation technique, there is a fixed transform between the element input caps and the output caps. This transformation could be parameterized by element properties but not by the content of the stream (see the section called “Fixed negotiation” for that use-case).

The caps that the element can accept depend on the (fixed transformation) downstream caps. The caps that the element can produce depend on the (fixed transformation of) the upstream caps.

This type of element can usually set caps on its source pad from the _event() function on the sink pad when it received the CAPS event. This means that the caps transform function transforms a fixed caps into another fixed caps. Examples of elements include:

  • Videobox. It adds configurable border around a video frame depending on object properties.

  • Identity elements. All elements that don't change the format of the data, only the content. Video and audio effects are an example. Other examples include elements that inspect the stream.

  • Some decoders and encoders, where the output format is defined by input format, like mulawdec and mulawenc. These decoders usually have no headers that define the content of the stream. They are usually more like conversion elements.

Below is an example of a negotiation steps of a typical transform element. In the sink pad CAPS event handler, we compute the caps for the source pad and set those.


  [...]

static gboolean
gst_my_filter_setcaps (GstMyFilter *filter,
		       GstCaps *caps)
{
  GstStructure *structure;
  int rate, channels;
  gboolean ret;
  GstCaps *outcaps;

  structure = gst_caps_get_structure (caps, 0);
  ret = gst_structure_get_int (structure, "rate", &rate);
  ret = ret && gst_structure_get_int (structure, "channels", &channels);
  if (!ret)
    return FALSE;

  outcaps = gst_caps_new_simple ("audio/x-raw",
      "format", G_TYPE_STRING, GST_AUDIO_NE(S16),
      "rate", G_TYPE_INT, rate,
      "channels", G_TYPE_INT, channels, NULL);
  ret = gst_pad_set_caps (filter->srcpad, outcaps);
  gst_caps_unref (outcaps);

  return ret;
}

static gboolean
gst_my_filter_sink_event (GstPad    *pad,
		          GstObject *parent,
		          GstEvent  *event)
{
  gboolean ret;
  GstMyFilter *filter = GST_MY_FILTER (parent);

  switch (GST_EVENT_TYPE (event)) {
    case GST_EVENT_CAPS:
    {
      GstCaps *caps;

      gst_event_parse_caps (event, &caps);
      ret = gst_my_filter_setcaps (filter, caps);
      break;
    }
    default:
      ret = gst_pad_event_default (pad, parent, event);
      break;
  }
  return ret;
}

  [...]

      

Dynamic negotiation

A last negotiation method is the most complex and powerful dynamic negotiation.

Like with the transform negotiation in the section called “Transform negotiation”, dynamic negotiation will perform a transformation on the downstream/upstream caps. Unlike the transform negotiation, this transform will convert fixed caps to unfixed caps. This means that the sink pad input caps can be converted into unfixed (multiple) formats. The source pad will have to choose a format from all the possibilities. It would usually like to choose a format that requires the least amount of effort to produce but it does not have to be. The selection of the format should also depend on the caps that can be accepted downstream (see a QUERY_CAPS function in Implementing a CAPS query function).

A typical flow goes like this:

  • Caps are received on the sink pad of the element.

  • If the element prefers to operate in passthrough mode, check if downstream accepts the caps with the ACCEPT_CAPS query. If it does, we can complete negotiation and we can operate in passthrough mode.

  • Calculate the possible caps for the source pad.

  • Query the downstream peer pad for the list of possible caps.

  • Select from the downstream list the first caps that you can transform to and set this as the output caps. You might have to fixate the caps to some reasonable defaults to construct fixed caps.

Examples of this type of elements include:

  • Converter elements such as videoconvert, audioconvert, audioresample, videoscale, ...

  • Source elements such as audiotestsrc, videotestsrc, v4l2src, pulsesrc, ...

Let's look at the example of an element that can convert between samplerates, so where input and output samplerate don't have to be the same:


static gboolean
gst_my_filter_setcaps (GstMyFilter *filter,
		       GstCaps *caps)
{
  if (gst_pad_set_caps (filter->sinkpad, caps)) {
    filter->passthrough = TRUE;
  } else {
    GstCaps *othercaps, *newcaps;
    GstStructure *s = gst_caps_get_structure (caps, 0), *others;

    /* no passthrough, setup internal conversion */
    gst_structure_get_int (s, "channels", &filter->channels);
    othercaps = gst_pad_get_allowed_caps (filter->srcpad);
    others = gst_caps_get_structure (othercaps, 0);
    gst_structure_set (others,
      "channels", G_TYPE_INT, filter->channels, NULL);

    /* now, the samplerate value can optionally have multiple values, so
     * we "fixate" it, which means that one fixed value is chosen */
    newcaps = gst_caps_copy_nth (othercaps, 0);
    gst_caps_unref (othercaps);
    gst_pad_fixate_caps (filter->srcpad, newcaps);
    if (!gst_pad_set_caps (filter->srcpad, newcaps))
      return FALSE;

    /* we are now set up, configure internally */
    filter->passthrough = FALSE;
    gst_structure_get_int (s, "rate", &filter->from_samplerate);
    others = gst_caps_get_structure (newcaps, 0);
    gst_structure_get_int (others, "rate", &filter->to_samplerate);
  }

  return TRUE;
}

static gboolean
gst_my_filter_sink_event (GstPad    *pad,
		          GstObject *parent,
		          GstEvent  *event)
{
  gboolean ret;
  GstMyFilter *filter = GST_MY_FILTER (parent);

  switch (GST_EVENT_TYPE (event)) {
    case GST_EVENT_CAPS:
    {
      GstCaps *caps;

      gst_event_parse_caps (event, &caps);
      ret = gst_my_filter_setcaps (filter, caps);
      break;
    }
    default:
      ret = gst_pad_event_default (pad, parent, event);
      break;
  }
  return ret;
}

static GstFlowReturn
gst_my_filter_chain (GstPad    *pad,
		     GstObject *parent,
		     GstBuffer *buf)
{
  GstMyFilter *filter = GST_MY_FILTER (parent);
  GstBuffer *out;

  /* push on if in passthrough mode */
  if (filter->passthrough)
    return gst_pad_push (filter->srcpad, buf);

  /* convert, push */
  out = gst_my_filter_convert (filter, buf);
  gst_buffer_unref (buf);

  return gst_pad_push (filter->srcpad, out);
}