docs/pwg/other-oneton.xml

<chapter id="chapter-other-oneton" xreflabel="Writing a Demuxer or Parser">
  <title>Writing a Demuxer or Parser</title>
  <para>
    Demuxers are the 1-to-N elements that need very special care.
    They are responsible for timestamping raw, unparsed data into
    elementary video or audio streams, and there are many things that you
    can optimize or do wrong. Here, several culprits will be mentioned
    and common solutions will be offered. Parsers are demuxers with only
    one source pad. Also, they only cut the stream into buffers, they
    don't touch the data otherwise.
  </para>
  <para>
    As mentioned previously in <xref linkend="chapter-negotiation"/>,
    demuxers should use fixed caps, since their data type will not change.
  </para>
  <para>
    As discussed in <xref linkend="chapter-scheduling"/>, demuxer elements
    can be written in multiple ways:
  </para>
  <itemizedlist>
    <listitem>
      <para>
        They can be the driving force of the pipeline, by running their own
        task. This works particularly well for elements that need random
        access, for example an AVI demuxer.
      </para>
    </listitem>
    <listitem>
      <para>
        They can also run in push-based mode, which means that an upstream
        element drives the pipeline. This works particularly well for streams
        that may come from network, such as Ogg.
      </para>
    </listitem>
  </itemizedlist>
  <para>
    In addition, audio parsers with one output can, in theory, also be written
    in random access mode. Although simple playback will mostly work if your
    element only accepts one mode, it may be required to implement multiple
    modes to work in combination with all sorts of applications, such as
    editing. Also, performance may become better if you implement multiple
    modes. See <xref linkend="chapter-scheduling"/> to see how an element
    can accept multiple scheduling modes.
  </para>
</chapter>