gstreamer/docs/pwg/other-ntoone.xml

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<chapter id="chapter-other-ntoone" xreflabel="Writing a N-to-1 Element or Demuxer">
  <title>Writing a N-to-1 Element or Demuxer</title>
  <para>
    N-to-1 elements have been previously mentioned and discussed in both
    <xref linkend="chapter-advanced-request"/> and in
    <xref linkend="chapter-loopbased-loopfn"/>. The main noteworthy thing
    about N-to-1 elements is that they should <emphasis>always</emphasis>,
    without any single exception, be <function>_loop ()</function>-based.
    Apart from that, there is not much general that you need to know. We
    will discuss one special type of N-to-1 elements here, these being
    muxers. The first two of these sections apply to N-to-1 elements in
    general, though.
  </para>

  <sect1 id="section-muxer-dataloop" xreflabel="The Data Loop Function">
    <title>The Data Loop Function</title>
    <para>
      As previously mentioned in <xref linkend="chapter-loopbased-loopfn"/>,
      N-to-1 elements generally try to have one buffer from each sink pad
      and then handle the one with the earliest timestamp. There's some
      exceptions to this rule, we will come to those later. This only works
      if all streams actually continuously provide input. There might be
      cases where this is not true, for example subtitles (there might be
      no subtitle for a while), overlay images and so forth. For this
      purpose, there is a <function>_select ()</function> function in
      &GStreamer;. It checks whether input is available on a (list of)
      pad(s). In this way, you can skip over the pads that are 'non-
      continuous'.
    </para>
    <programlisting>
/* Pad selection is currently broken, FIXME some day */
    </programlisting>
  </sect1>

  <sect1 id="section-muxer-events" xreflabel="Events in the Loop Function">
    <title>Events in the Loop Function</title>
    <para>
      N-to-1 elements using a cache will sometimes receive events, and it
      is often unclear how to handle those. For example, how do you seek
      to a frame in an <emphasis>output</emphasis> file (and what's the
      point of it anyway)? So, do discontinuity or seek events make sense,
      and should you use them?
    </para>
    <sect2 id="section-muxevents-discont" xreflabel="Discontinuities and flushes">
      <title>Discontinuities and flushes</title>
      <para>
        Don't do anything. They specify a discontinuity in the output, and
        you should continue to playback as you would otherwise. You
        generally do not need to put a discontinuity in the output stream
        in muxers; you would have to manually start adapting timestamps of
        output frames (if appliccable) to match the previous timescale,
        though. Note that the output data stream should be continuous. For
        other types of N-to-1-elements, it is generally fine to forward
        the discontinuity once it has been received from all pads. This
        depends on the specific element.
      </para>
    </sect2>
    <sect2 id="section-muxevents-seek" xreflabel="Seeks">
      <title>Seeks</title>
      <para>
        Depends on the element. Muxers would generally not implement this,
        because the concept of seeking in an <emphasis>output</emphasis>
        stream at frame level is not very useful. Seeking at byte level
        can be useful, but that is more generally done
        <emphasis>by</emphasis> muxers <emphasis>on</emphasis> sink
        elements.
      </para>
    </sect2>
    <sect2 id="section-muxevents-eos" xreflabel="End-of-Stream">
      <title>End-of-Stream</title>
      <para>
        Speaks for itself.
      </para>
    </sect2>
  </sect1>

  <sect1 id="section-muxer-negotiation" xreflabel="Negotiation">
    <title>Negotiation</title>
    <para>
      Most container formats will have a fair amount of issues with
      changing content on an elementary stream. Therefore, you should
      not allow caps to be changed once you've started using data from
      them. The easiest way to achieve this is by using explicit caps,
      which have been explained before. However, we're going to use them
      in a slightly different way then what you're used to, having the
      core do all the work for us.
    </para>
    <para>
      The idea is that, as long as the stream/file headers have not been
      written yet and no data has been processed yet, a stream is allowed
      to renegotiate. After that point, the caps should be fixed, because
      we can only use a stream once. Caps may then only change within an
      allowed range (think MPEG, where changes in FPS are allowed), or
      sometimes not at all (such as AVI audio). In order to do that, we
      will, after data retrieval and header writing, set an explicit caps
      on each sink pad, that is the minimal caps describing the properties
      of the format that may not change. As an example, for MPEG audio
      inside an MPEG system stream, this would mean a wide caps of
      audio/mpeg with mpegversion=1 and layer=[1,2]. For the same audio type
      in MPEG, though, the samplerate, bitrate, layer and number of channels
      would become static, too. Since the (request) pads will be removed
      when the stream ends, the static caps will cease to exist too, then.
      While the explicit caps  exist, the <function>_link ()</function>-
      function will not be called, since the core will do all necessary
      checks for us. Note that the property of using explicit caps should
      be added along with the actual explicit caps, not any earlier.
    </para>
    <para>
      Below here follows the simple example of an AVI muxer's audio caps
      negotiation. The <function>_link ()</function>-function is fairly
      normal, but the <function>-Loop ()</function>-function does some of
      the tricks mentioned above. There is no <function>_getcaps ()</function>-
      function since the pad template contains all that information already
      (not shown).
    </para>
    <programlisting>
static GstPadLinkReturn
gst_avi_mux_audio_link (GstPad        *pad,
			const GstCaps *caps)
{
  GstAviMux *mux = GST_AVI_MUX (gst_pad_get_parent (pad));
  GstStructure *str = gst_caps_get_structure (caps, 0);
  const gchar *mime = gst_structure_get_name (str);

  if (!strcmp (str, "audio/mpeg")) {
    /* get version, make sure it's 1, get layer, make sure it's 1-3,
     * then create the 2-byte audio tag (0x0055) and fill an audio
     * stream structure (strh/strf). */
    [..]
    return GST_PAD_LINK_OK;
  } else if !strcmp (str, "audio/x-raw-int")) {
    /* See above, but now with the raw audio tag (0x0001). */
    [..]
    return GST_PAD_LINK_OK;
  } else [..]
[..]
}

static void
gst_avi_mux_loop (GstElement *element)
{
  GstAviMux *mux = GST_AVI_MUX (element);
[..]
  /* As we get here, we should have written the header if we hadn't done
   * that before yet, and we're supposed to have an internal buffer from
   * each pad, also from the audio one. So here, we check again whether
   * this is the first run and if so, we set static caps. */
  if (mux->first_cycle) {
    const GList *padlist = gst_element_get_pad_list (element);
    GList *item;

    for (item = padlist; item != NULL; item = item->next) {
      GstPad *pad = item->data;
      GstCaps *caps;

      if (!GST_PAD_IS_SINK (pad))
        continue;

      /* set static caps here */
      if (!strncmp (gst_pad_get_name (pad), "audio_", 6)) {
        /* the strf is the struct you filled in the _link () function. */
        switch (strf->format) {
          case 0x0055: /* mp3 */
            caps = gst_caps_new_simple ("audio/mpeg",
			"mpegversion", G_TYPE_INT, 1,
			"layer",       G_TYPE_INT, 3,
			"bitrate",     G_TYPE_INT, strf->av_bps,
			"rate",        G_TYPE_INT, strf->rate,
			"channels",    G_TYPE_INT, strf->channels,
			NULL);
            break;
          case 0x0001: /* pcm */
            caps = gst_caps_new_simple ("audio/x-raw-int",
					[..]);
            break;
          [..]
        }
      } else if (!strncmp (gst_pad_get_name (pad), "video_", 6)) {
        [..]
      } else {
        g_warning ("oi!");
        continue;
      }

      /* set static caps */
      gst_pad_use_explicit_caps (pad);
      gst_pad_set_explicit_caps (pad, caps);
    }
  }
[..]
  /* Next runs will never be the first again. */
  mux->first_cycle = FALSE;
}
    </programlisting>
    <para>
      Note that there are other ways to achieve that, which might be useful
      for more complex cases. This will do for the simple cases, though.
      This method is provided to simplify negotiation and renegotiation in
      muxers, it is not a complete solution, nor is it a pretty one.
    </para>
  </sect1>

  <sect1 id="section-muxer-markup" xreflabel="Markup vs. data processing">
    <title>Markup vs. data processing</title>
    <para>
      As we noted on demuxers before, we love common programming paradigms
      such as clean, lean and mean code. To achieve that in muxers, it's
      generally a good idea to separate the actual data stream markup from
      the data processing. To illustrate, here's how AVI muxers should
      write out RIFF tag chunks:
    </para>
    <programlisting>
static void
gst_avi_mux_write_chunk (GstAviMux *mux,
			 guint32    id,
			 GstBuffer *data)
{
  GstBuffer *hdr;

  hdr = gst_buffer_new_and_alloc (8);
  ((guint32 *) GST_BUFFER_DATA (buf))[0] = GUINT32_TO_LE (id);
  ((guint32 *) GST_BUFFER_DATA (buf))[1] = GUINT32_TO_LE (GST_BUFFER_SIZE (data));

  gst_pad_push (mux->srcpad, hdr);
  gst_pad_push (mux->srcpad, data);
}

static void
gst_avi_mux_loop (GstElement *element)
{
  GstAviMux *mux = GST_AVI_MUX (element);
  GstBuffer *buf;
[..]
  buf = gst_pad_pull (mux->sinkpad[0]);
[..]
  gst_avi_mux_write_chunk (GST_MAKE_FOURCC ('0','0','d','b'), buf);
}
    </programlisting>
    <para>
      In general, try to program clean code, that should cover pretty
      much everything.
    </para>
  </sect1>
</chapter>