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6. planar-apis.xml

<section id="planar-apis">
  <title>Single- and multi-planar APIs</title>

  <para>Some devices require data for each input or output video frame
  to be placed in discontiguous memory buffers. In such cases, one
  video frame has to be addressed using more than one memory address, i.e. one
  pointer per "plane". A plane is a sub-buffer of the current frame. For
  examples of such formats see <xref linkend="pixfmt" />.</para>

  <para>Initially, V4L2 API did not support multi-planar buffers and a set of
  extensions has been introduced to handle them. Those extensions constitute
  what is being referred to as the "multi-planar API".</para>

  <para>Some of the V4L2 API calls and structures are interpreted differently,
  depending on whether single- or multi-planar API is being used. An application
  can choose whether to use one or the other by passing a corresponding buffer
  type to its ioctl calls. Multi-planar versions of buffer types are suffixed
  with an `_MPLANE' string. For a list of available multi-planar buffer types
  see &v4l2-buf-type;.
  </para>

  <section>
    <title>Multi-planar formats</title>
    <para>Multi-planar API introduces new multi-planar formats. Those formats
    use a separate set of FourCC codes. It is important to distinguish between
    the multi-planar API and a multi-planar format. Multi-planar API calls can
    handle all single-planar formats as well (as long as they are passed in
    multi-planar API structures), while the single-planar API cannot
    handle multi-planar formats.</para>
  </section>

  <section>
    <title>Calls that distinguish between single and multi-planar APIs</title>
    <variablelist>
      <varlistentry>
        <term>&VIDIOC-QUERYCAP;</term>
        <listitem><para>Two additional multi-planar capabilities are added. They can
        be set together with non-multi-planar ones for devices that handle
        both single- and multi-planar formats.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term>&VIDIOC-G-FMT;, &VIDIOC-S-FMT;, &VIDIOC-TRY-FMT;</term>
        <listitem><para>New structures for describing multi-planar formats are added:
        &v4l2-pix-format-mplane; and &v4l2-plane-pix-format;. Drivers may
        define new multi-planar formats, which have distinct FourCC codes from
        the existing single-planar ones.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>&VIDIOC-QBUF;, &VIDIOC-DQBUF;, &VIDIOC-QUERYBUF;</term>
        <listitem><para>A new &v4l2-plane; structure for describing planes is added.
        Arrays of this structure are passed in the new
        <structfield>m.planes</structfield> field of &v4l2-buffer;.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>&VIDIOC-REQBUFS;</term>
        <listitem><para>Will allocate multi-planar buffers as requested.</para></listitem>
      </varlistentry>
    </variablelist>
  </section>
</section>