Pads and capabilities

As we have seen in Elements, the pads are the element's interface to the outside world. Data streams from one element's source pad to another element's sink pad. The specific type of media that the element can handle will be exposed by the pad's capabilities. We will talk more on capabilities later in this chapter (see Capabilities of a pad).

Pads

A pad type is defined by two properties: its direction and its availability. As we've mentioned before, GStreamer defines two pad directions: source pads and sink pads. This terminology is defined from the view of within the element: elements receive data on their sink pads and generate data on their source pads. Schematically, sink pads are drawn on the left side of an element, whereas source pads are drawn on the right side of an element. In such graphs, data flows from left to right. [1]

Pad directions are very simple compared to pad availability. A pad can have any of three availabilities: always, sometimes and on request. The meaning of those three types is exactly as it says: always pads always exist, sometimes pad exist only in certain cases (and can disappear randomly), and on-request pads appear only if explicitly requested by applications.

Dynamic (or sometimes) pads

Some elements might not have all of their pads when the element is created. This can happen, for example, with an Ogg demuxer element. The element will read the Ogg stream and create dynamic pads for each contained elementary stream (vorbis, theora) when it detects such a stream in the Ogg stream. Likewise, it will delete the pad when the stream ends. This principle is very useful for demuxer elements, for example.

Running gst-inspect oggdemux will show that the element has only one pad: a sink pad called 'sink'. The other pads are “dormant”. You can see this in the pad template because there is an “Exists: Sometimes” property. Depending on the type of Ogg file you play, the pads will be created. We will see that this is very important when you are going to create dynamic pipelines. You can attach a signal handler to an element to inform you when the element has created a new pad from one of its “sometimes” pad templates. The following piece of code is an example of how to do this:

#include <gst/gst.h>

static void
cb_new_pad (GstElement *element,
        GstPad     *pad,
        gpointer    data)
{
  gchar *name;

  name = gst_pad_get_name (pad);
  g_print ("A new pad %s was created\n", name);
  g_free (name);

  /* here, you would setup a new pad link for the newly created pad */
[..]

}

int
main (int   argc,
      char *argv[])
{
  GstElement *pipeline, *source, *demux;
  GMainLoop *loop;

  /* init */
  gst_init (&argc, &argv);

  /* create elements */
  pipeline = gst_pipeline_new ("my_pipeline");
  source = gst_element_factory_make ("filesrc", "source");
  g_object_set (source, "location", argv[1], NULL);
  demux = gst_element_factory_make ("oggdemux", "demuxer");

  /* you would normally check that the elements were created properly */

  /* put together a pipeline */
  gst_bin_add_many (GST_BIN (pipeline), source, demux, NULL);
  gst_element_link_pads (source, "src", demux, "sink");

  /* listen for newly created pads */
  g_signal_connect (demux, "pad-added", G_CALLBACK (cb_new_pad), NULL);

  /* start the pipeline */
  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);
  loop = g_main_loop_new (NULL, FALSE);
  g_main_loop_run (loop);

[..]

}

It is not uncommon to add elements to the pipeline only from within the "pad-added" callback. If you do this, don't forget to set the state of the newly-added elements to the target state of the pipeline using gst_element_set_state () or gst_element_sync_state_with_parent ().

Request pads

An element can also have request pads. These pads are not created automatically but are only created on demand. This is very useful for multiplexers, aggregators and tee elements. Aggregators are elements that merge the content of several input streams together into one output stream. Tee elements are the reverse: they are elements that have one input stream and copy this stream to each of their output pads, which are created on request. Whenever an application needs another copy of the stream, it can simply request a new output pad from the tee element.

The following piece of code shows how you can request a new output pad from a “tee” element:

#include <gst/gst.h>
  
static void
link_to_multiplexer (GstPad     *tolink_pad,
                     GstElement *mux)
{
  GstPad *pad;
  gchar *srcname, *sinkname;

  srcname = gst_pad_get_name (tolink_pad);
  pad = gst_element_get_compatible_pad (mux, tolink_pad, NULL);
  gst_pad_link (tolink_pad, pad);
  sinkname = gst_pad_get_name (pad);
  gst_object_unref (GST_OBJECT (pad));

  g_print ("A new pad %s was created and linked to %s\n", sinkname, srcname);
  g_free (sinkname);
  g_free (srcname);
}

static void
some_function (GstElement *tee)
{
  GstPad * pad;
  gchar *name;

  pad = gst_element_get_request_pad (tee, "src%d");
  name = gst_pad_get_name (pad);
  g_print ("A new pad %s was created\n", name);
  g_free (name);

  /* here, you would link the pad */

  /* [..] */

  /* and, after doing that, free our reference */
  gst_object_unref (GST_OBJECT (pad));
}

The gst_element_get_request_pad () method can be used to get a pad from the element based on the name of the pad template. It is also possible to request a pad that is compatible with another pad template. This is very useful if you want to link an element to a multiplexer element and you need to request a pad that is compatible. The method gst_element_get_compatible_pad () can be used to request a compatible pad, as shown in the next example. It will request a compatible pad from an Ogg multiplexer from any input.

#include <gst/gst.h>
  
static void
link_to_multiplexer (GstPad     *tolink_pad,
                     GstElement *mux)
{
  GstPad *pad;
  gchar *srcname, *sinkname;

  srcname = gst_pad_get_name (tolink_pad);
  pad = gst_element_get_compatible_pad (mux, tolink_pad, NULL);
  gst_pad_link (tolink_pad, pad);
  sinkname = gst_pad_get_name (pad);
  gst_object_unref (GST_OBJECT (pad));

  g_print ("A new pad %s was created and linked to %s\n", sinkname, srcname);
  g_free (sinkname);
  g_free (srcname);
}

static void
some_function (GstElement *tee)
{
  GstPad * pad;
  gchar *name;

  pad = gst_element_get_request_pad (tee, "src%d");
  name = gst_pad_get_name (pad);
  g_print ("A new pad %s was created\n", name);
  g_free (name);

  /* here, you would link the pad */

  /* [..] */

  /* and, after doing that, free our reference */
  gst_object_unref (GST_OBJECT (pad));
}

Capabilities of a pad

Since the pads play a very important role in how the element is viewed by the outside world, a mechanism is implemented to describe the data that can flow or currently flows through the pad by using capabilities. Here, we will briefly describe what capabilities are and how to use them, enough to get an understanding of the concept. For an in-depth look into capabilities and a list of all capabilities defined in GStreamer, see the Plugin Writers Guide

Capabilities are attached to pad templates and to pads. For pad templates, it will describe the types of media that may stream over a pad created from this template. For pads, it can either be a list of possible caps (usually a copy of the pad template's capabilities), in which case the pad is not yet negotiated, or it is the type of media that currently streams over this pad, in which case the pad has been negotiated already.

Dissecting capabilities

A pad's capabilities are described in a GstCaps object. Internally, a GstCaps will contain one or more GstStructure that will describe one media type. A negotiated pad will have capabilities set that contain exactly one structure. Also, this structure will contain only fixed values. These constraints are not true for unnegotiated pads or pad templates.

As an example, below is a dump of the capabilities of the “vorbisdec” element, which you will get by running gst-inspect vorbisdec. You will see two pads: a source and a sink pad. Both of these pads are always available, and both have capabilities attached to them. The sink pad will accept vorbis-encoded audio data, with the media type “audio/x-vorbis”. The source pad will be used to send raw (decoded) audio samples to the next element, with a raw audio media type (in this case, “audio/x-raw”). The source pad will also contain properties for the audio samplerate and the amount of channels, plus some more that you don't need to worry about for now.


Pad Templates:
  SRC template: 'src'
    Availability: Always
    Capabilities:
      audio/x-raw
                 format: F32LE
                   rate: [ 1, 2147483647 ]
               channels: [ 1, 256 ]

  SINK template: 'sink'
    Availability: Always
    Capabilities:
      audio/x-vorbis

Properties and values

Properties are used to describe extra information for capabilities. A property consists of a key (a string) and a value. There are different possible value types that can be used:

  • Basic types, this can be pretty much any GType registered with Glib. Those properties indicate a specific, non-dynamic value for this property. Examples include:

    • An integer value (G_TYPE_INT): the property has this exact value.

    • A boolean value (G_TYPE_BOOLEAN): the property is either TRUE or FALSE.

    • A float value (G_TYPE_FLOAT): the property has this exact floating point value.

    • A string value (G_TYPE_STRING): the property contains a UTF-8 string.

    • A fraction value (GST_TYPE_FRACTION): contains a fraction expressed by an integer numerator and denominator.

  • Range types are GTypes registered by GStreamer to indicate a range of possible values. They are used for indicating allowed audio samplerate values or supported video sizes. The two types defined in GStreamer are:

    • An integer range value (GST_TYPE_INT_RANGE): the property denotes a range of possible integers, with a lower and an upper boundary. The “vorbisdec” element, for example, has a rate property that can be between 8000 and 50000.

    • A float range value (GST_TYPE_FLOAT_RANGE): the property denotes a range of possible floating point values, with a lower and an upper boundary.

    • A fraction range value (GST_TYPE_FRACTION_RANGE): the property denotes a range of possible fraction values, with a lower and an upper boundary.

  • A list value (GST_TYPE_LIST): the property can take any value from a list of basic values given in this list.

    Example: caps that express that either a sample rate of 44100 Hz and a sample rate of 48000 Hz is supported would use a list of integer values, with one value being 44100 and one value being 48000.

  • An array value (GST_TYPE_ARRAY): the property is an array of values. Each value in the array is a full value on its own, too. All values in the array should be of the same elementary type. This means that an array can contain any combination of integers, lists of integers, integer ranges together, and the same for floats or strings, but it can not contain both floats and ints at the same time.

    Example: for audio where there are more than two channels involved the channel layout needs to be specified (for one and two channel audio the channel layout is implicit unless stated otherwise in the caps). So the channel layout would be an array of integer enum values where each enum value represents a loudspeaker position. Unlike a GST_TYPE_LIST, the values in an array will be interpreted as a whole.

What capabilities are used for

Capabilities (short: caps) describe the type of data that is streamed between two pads, or that one pad (template) supports. This makes them very useful for various purposes:

  • Autoplugging: automatically finding elements to link to a pad based on its capabilities. All autopluggers use this method.

  • Compatibility detection: when two pads are linked, GStreamer can verify if the two pads are talking about the same media type. The process of linking two pads and checking if they are compatible is called “caps negotiation”.

  • Metadata: by reading the capabilities from a pad, applications can provide information about the type of media that is being streamed over the pad, which is information about the stream that is currently being played back.

  • Filtering: an application can use capabilities to limit the possible media types that can stream between two pads to a specific subset of their supported stream types. An application can, for example, use “filtered caps” to set a specific (fixed or non-fixed) video size that should stream between two pads. You will see an example of filtered caps later in this manual, in Manually adding or removing data from/to a pipeline. You can do caps filtering by inserting a capsfilter element into your pipeline and setting its “caps” property. Caps filters are often placed after converter elements like audioconvert, audioresample, videoconvert or videoscale to force those converters to convert data to a specific output format at a certain point in a stream.

Using capabilities for metadata

A pad can have a set (i.e. one or more) of capabilities attached to it. Capabilities (GstCaps) are represented as an array of one or more GstStructures, and each GstStructure is an array of fields where each field consists of a field name string (e.g. "width") and a typed value (e.g. G_TYPE_INT or GST_TYPE_INT_RANGE).

Note that there is a distinct difference between the possible capabilities of a pad (ie. usually what you find as caps of pad templates as they are shown in gst-inspect), the allowed caps of a pad (can be the same as the pad's template caps or a subset of them, depending on the possible caps of the peer pad) and lastly negotiated caps (these describe the exact format of a stream or buffer and contain exactly one structure and have no variable bits like ranges or lists, ie. they are fixed caps).

You can get values of properties in a set of capabilities by querying individual properties of one structure. You can get a structure from a caps using gst_caps_get_structure () and the number of structures in a GstCaps using gst_caps_get_size ().

Caps are called simple caps when they contain only one structure, and fixed caps when they contain only one structure and have no variable field types (like ranges or lists of possible values). Two other special types of caps are ANY caps and empty caps.

Here is an example of how to extract the width and height from a set of fixed video caps:

static void
read_video_props (GstCaps *caps)
{
  gint width, height;
  const GstStructure *str;

  g_return_if_fail (gst_caps_is_fixed (caps));

  str = gst_caps_get_structure (caps, 0);
  if (!gst_structure_get_int (str, "width", &width) ||
      !gst_structure_get_int (str, "height", &height)) {
    g_print ("No width/height available\n");
    return;
  }

  g_print ("The video size of this set of capabilities is %dx%d\n",
       width, height);
}

Creating capabilities for filtering

While capabilities are mainly used inside a plugin to describe the media type of the pads, the application programmer often also has to have basic understanding of capabilities in order to interface with the plugins, especially when using filtered caps. When you're using filtered caps or fixation, you're limiting the allowed types of media that can stream between two pads to a subset of their supported media types. You do this using a capsfilter element in your pipeline. In order to do this, you also need to create your own GstCaps. The easiest way to do this is by using the convenience function gst_caps_new_simple ():

static gboolean
link_elements_with_filter (GstElement *element1, GstElement *element2)
{
  gboolean link_ok;
  GstCaps *caps;

  caps = gst_caps_new_simple ("video/x-raw",
          "format", G_TYPE_STRING, "I420",
          "width", G_TYPE_INT, 384,
          "height", G_TYPE_INT, 288,
          "framerate", GST_TYPE_FRACTION, 25, 1,
          NULL);

  link_ok = gst_element_link_filtered (element1, element2, caps);
  gst_caps_unref (caps);

  if (!link_ok) {
    g_warning ("Failed to link element1 and element2!");
  }

  return link_ok;
}

This will force the data flow between those two elements to a certain video format, width, height and framerate (or the linking will fail if that cannot be achieved in the context of the elements involved). Keep in mind that when you use gst_element_link_filtered () it will automatically create a capsfilter element for you and insert it into your bin or pipeline between the two elements you want to connect (this is important if you ever want to disconnect those elements because then you will have to disconnect both elements from the capsfilter instead).

In some cases, you will want to create a more elaborate set of capabilities to filter a link between two pads. Then, this function is too simplistic and you'll want to use the method gst_caps_new_full ():

static gboolean
link_elements_with_filter (GstElement *element1, GstElement *element2)
{
  gboolean link_ok;
  GstCaps *caps;

  caps = gst_caps_new_full (
      gst_structure_new ("video/x-raw",
             "width", G_TYPE_INT, 384,
             "height", G_TYPE_INT, 288,
             "framerate", GST_TYPE_FRACTION, 25, 1,
             NULL),
      gst_structure_new ("video/x-bayer",
             "width", G_TYPE_INT, 384,
             "height", G_TYPE_INT, 288,
             "framerate", GST_TYPE_FRACTION, 25, 1,
             NULL),
      NULL);

  link_ok = gst_element_link_filtered (element1, element2, caps);
  gst_caps_unref (caps);

  if (!link_ok) {
    g_warning ("Failed to link element1 and element2!");
  }

  return link_ok;
}

See the API references for the full API of GstStructure and GstCaps.

Ghost pads

You can see from Visualisation of a GstBin element without ghost pads how a bin has no pads of its own. This is where "ghost pads" come into play.

Visualisation of a GstBin element without ghost pads

A ghost pad is a pad from some element in the bin that can be accessed directly from the bin as well. Compare it to a symbolic link in UNIX filesystems. Using ghost pads on bins, the bin also has a pad and can transparently be used as an element in other parts of your code.

Visualisation of a GstBin element with a ghost pad

Visualisation of a GstBin element with a ghost pad is a representation of a ghost pad. The sink pad of element one is now also a pad of the bin. Because ghost pads look and work like any other pads, they can be added to any type of elements, not just to a GstBin, just like ordinary pads.

A ghostpad is created using the function gst_ghost_pad_new ():

#include <gst/gst.h>

int
main (int   argc,
      char *argv[])
{
  GstElement *bin, *sink;
  GstPad *pad;

  /* init */
  gst_init (&argc, &argv);

  /* create element, add to bin */
  sink = gst_element_factory_make ("fakesink", "sink");
  bin = gst_bin_new ("mybin");
  gst_bin_add (GST_BIN (bin), sink);

  /* add ghostpad */
  pad = gst_element_get_static_pad (sink, "sink");
  gst_element_add_pad (bin, gst_ghost_pad_new ("sink", pad));
  gst_object_unref (GST_OBJECT (pad));

[..]

}

In the above example, the bin now also has a pad: the pad called “sink” of the given element. The bin can, from here on, be used as a substitute for the sink element. You could, for example, link another element to the bin.

  1. In reality, there is no objection to data flowing from a source pad to the sink pad of an element upstream (to the left of this element in drawings). Data will, however, always flow from a source pad of one element to the sink pad of another.

The results of the search are