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6. vidioc-g-parm.xml

<refentry id="vidioc-g-parm">
  <refmeta>
    <refentrytitle>ioctl VIDIOC_G_PARM, VIDIOC_S_PARM</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>VIDIOC_G_PARM</refname>
    <refname>VIDIOC_S_PARM</refname>
    <refpurpose>Get or set streaming parameters</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcprototype>
	<funcdef>int <function>ioctl</function></funcdef>
	<paramdef>int <parameter>fd</parameter></paramdef>
	<paramdef>int <parameter>request</parameter></paramdef>
	<paramdef>v4l2_streamparm *<parameter>argp</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Arguments</title>

    <variablelist>
      <varlistentry>
	<term><parameter>fd</parameter></term>
	<listitem>
	  <para>&fd;</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>request</parameter></term>
	<listitem>
	  <para>VIDIOC_G_PARM, VIDIOC_S_PARM</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	  <para></para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <para>The current video standard determines a nominal number of
frames per second. If less than this number of frames is to be
captured or output, applications can request frame skipping or
duplicating on the driver side. This is especially useful when using
the <function>read()</function> or <function>write()</function>, which
are not augmented by timestamps or sequence counters, and to avoid
unnecessary data copying.</para>

    <para>Further these ioctls can be used to determine the number of
buffers used internally by a driver in read/write mode. For
implications see the section discussing the &func-read;
function.</para>

    <para>To get and set the streaming parameters applications call
the <constant>VIDIOC_G_PARM</constant> and
<constant>VIDIOC_S_PARM</constant> ioctl, respectively. They take a
pointer to a struct <structname>v4l2_streamparm</structname> which
contains a union holding separate parameters for input and output
devices.</para>

    <table pgwide="1" frame="none" id="v4l2-streamparm">
      <title>struct <structname>v4l2_streamparm</structname></title>
      <tgroup cols="4">
	&cs-ustr;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>type</structfield></entry>
	    <entry></entry>
	    <entry>The buffer (stream) type, same as &v4l2-format;
<structfield>type</structfield>, set by the application. See <xref
	    linkend="v4l2-buf-type" /></entry>
	  </row>
	  <row>
	    <entry>union</entry>
	    <entry><structfield>parm</structfield></entry>
	    <entry></entry>
	    <entry></entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>&v4l2-captureparm;</entry>
	    <entry><structfield>capture</structfield></entry>
	    <entry>Parameters for capture devices, used when
<structfield>type</structfield> is
<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>&v4l2-outputparm;</entry>
	    <entry><structfield>output</structfield></entry>
	    <entry>Parameters for output devices, used when
<structfield>type</structfield> is
<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant>.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>__u8</entry>
	    <entry><structfield>raw_data</structfield>[200]</entry>
	    <entry>A place holder for future extensions.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="v4l2-captureparm">
      <title>struct <structname>v4l2_captureparm</structname></title>
      <tgroup cols="3">
	&cs-str;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>capability</structfield></entry>
	    <entry>See <xref linkend="parm-caps" />.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>capturemode</structfield></entry>
	    <entry>Set by drivers and applications, see <xref linkend="parm-flags" />.</entry>
	  </row>
	  <row>
	    <entry>&v4l2-fract;</entry>
	    <entry><structfield>timeperframe</structfield></entry>
	    <entry><para>This is is the desired period between
successive frames captured by the driver, in seconds. The
field is intended to skip frames on the driver side, saving I/O
bandwidth.</para><para>Applications store here the desired frame
period, drivers return the actual frame period, which must be greater
or equal to the nominal frame period determined by the current video
standard (&v4l2-standard; <structfield>frameperiod</structfield>
field). Changing the video standard (also implicitly by switching the
video input) may reset this parameter to the nominal frame period. To
reset manually applications can just set this field to
zero.</para><para>Drivers support this function only when they set the
<constant>V4L2_CAP_TIMEPERFRAME</constant> flag in the
<structfield>capability</structfield> field.</para></entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>extendedmode</structfield></entry>
	    <entry>Custom (driver specific) streaming parameters. When
unused, applications and drivers must set this field to zero.
Applications using this field should check the driver name and
version, see <xref linkend="querycap" />.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>readbuffers</structfield></entry>
	    <entry>Applications set this field to the desired number
of buffers used internally by the driver in &func-read; mode. Drivers
return the actual number of buffers. When an application requests zero
buffers, drivers should just return the current setting rather than
the minimum or an error code. For details see <xref
		linkend="rw" />.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>reserved</structfield>[4]</entry>
	    <entry>Reserved for future extensions. Drivers and
applications must set the array to zero.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="v4l2-outputparm">
      <title>struct <structname>v4l2_outputparm</structname></title>
      <tgroup cols="3">
	&cs-str;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>capability</structfield></entry>
	    <entry>See <xref linkend="parm-caps" />.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>outputmode</structfield></entry>
	    <entry>Set by drivers and applications, see <xref
	    linkend="parm-flags" />.</entry>
	  </row>
	  <row>
	    <entry>&v4l2-fract;</entry>
	    <entry><structfield>timeperframe</structfield></entry>
	    <entry>This is is the desired period between
successive frames output by the driver, in seconds.</entry>
	  </row>
	  <row>
	    <entry spanname="hspan"><para>The field is intended to
repeat frames on the driver side in &func-write; mode (in streaming
mode timestamps can be used to throttle the output), saving I/O
bandwidth.</para><para>Applications store here the desired frame
period, drivers return the actual frame period, which must be greater
or equal to the nominal frame period determined by the current video
standard (&v4l2-standard; <structfield>frameperiod</structfield>
field). Changing the video standard (also implicitly by switching the
video output) may reset this parameter to the nominal frame period. To
reset manually applications can just set this field to
zero.</para><para>Drivers support this function only when they set the
<constant>V4L2_CAP_TIMEPERFRAME</constant> flag in the
<structfield>capability</structfield> field.</para></entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>extendedmode</structfield></entry>
	    <entry>Custom (driver specific) streaming parameters. When
unused, applications and drivers must set this field to zero.
Applications using this field should check the driver name and
version, see <xref linkend="querycap" />.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>writebuffers</structfield></entry>
	    <entry>Applications set this field to the desired number
of buffers used internally by the driver in
<function>write()</function> mode. Drivers return the actual number of
buffers. When an application requests zero buffers, drivers should
just return the current setting rather than the minimum or an error
code. For details see <xref linkend="rw" />.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>reserved</structfield>[4]</entry>
	    <entry>Reserved for future extensions. Drivers and
applications must set the array to zero.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="parm-caps">
      <title>Streaming Parameters Capabilites</title>
      <tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	  <row>
	    <entry><constant>V4L2_CAP_TIMEPERFRAME</constant></entry>
	    <entry>0x1000</entry>
	    <entry>The frame skipping/repeating controlled by the
<structfield>timeperframe</structfield> field is supported.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="parm-flags">
      <title>Capture Parameters Flags</title>
      <tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	  <row>
	    <entry><constant>V4L2_MODE_HIGHQUALITY</constant></entry>
	    <entry>0x0001</entry>
	    <entry><para>High quality imaging mode. High quality mode
is intended for still imaging applications. The idea is to get the
best possible image quality that the hardware can deliver. It is not
defined how the driver writer may achieve that; it will depend on the
hardware and the ingenuity of the driver writer. High quality mode is
a different mode from the the regular motion video capture modes. In
high quality mode:<itemizedlist>
		  <listitem>
		    <para>The driver may be able to capture higher
resolutions than for motion capture.</para>
		  </listitem>
		  <listitem>
		    <para>The driver may support fewer pixel formats
than motion capture (eg; true color).</para>
		  </listitem>
		  <listitem>
		    <para>The driver may capture and arithmetically
combine multiple successive fields or frames to remove color edge
artifacts and reduce the noise in the video data.
</para>
		  </listitem>
		  <listitem>
		    <para>The driver may capture images in slices like
a scanner in order to handle larger format images than would otherwise
be possible. </para>
		  </listitem>
		  <listitem>
		    <para>An image capture operation may be
significantly slower than motion capture. </para>
		  </listitem>
		  <listitem>
		    <para>Moving objects in the image might have
excessive motion blur. </para>
		  </listitem>
		  <listitem>
		    <para>Capture might only work through the
<function>read()</function> call.</para>
		  </listitem>
		</itemizedlist></para></entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

  </refsect1>

  <refsect1>
    &return-value;
  </refsect1>
</refentry>