333 lines
11 KiB
XML
333 lines
11 KiB
XML
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<refentry id="vidioc-g-parm">
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<refmeta>
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<refentrytitle>ioctl VIDIOC_G_PARM, VIDIOC_S_PARM</refentrytitle>
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&manvol;
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</refmeta>
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<refnamediv>
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<refname>VIDIOC_G_PARM</refname>
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<refname>VIDIOC_S_PARM</refname>
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<refpurpose>Get or set streaming parameters</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>ioctl</function></funcdef>
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<paramdef>int <parameter>fd</parameter></paramdef>
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<paramdef>int <parameter>request</parameter></paramdef>
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<paramdef>v4l2_streamparm *<parameter>argp</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Arguments</title>
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<variablelist>
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<varlistentry>
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<term><parameter>fd</parameter></term>
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<listitem>
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<para>&fd;</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>request</parameter></term>
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<listitem>
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<para>VIDIOC_G_PARM, VIDIOC_S_PARM</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>argp</parameter></term>
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<listitem>
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<para></para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Description</title>
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<para>The current video standard determines a nominal number of
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frames per second. If less than this number of frames is to be
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captured or output, applications can request frame skipping or
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duplicating on the driver side. This is especially useful when using
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the <function>read()</function> or <function>write()</function>, which
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are not augmented by timestamps or sequence counters, and to avoid
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unneccessary data copying.</para>
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<para>Further these ioctls can be used to determine the number of
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buffers used internally by a driver in read/write mode. For
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implications see the section discussing the &func-read;
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function.</para>
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<para>To get and set the streaming parameters applications call
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the <constant>VIDIOC_G_PARM</constant> and
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<constant>VIDIOC_S_PARM</constant> ioctl, respectively. They take a
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pointer to a struct <structname>v4l2_streamparm</structname> which
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contains a union holding separate parameters for input and output
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devices.</para>
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<table pgwide="1" frame="none" id="v4l2-streamparm">
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<title>struct <structname>v4l2_streamparm</structname></title>
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<tgroup cols="4">
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&cs-ustr;
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<tbody valign="top">
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<row>
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<entry>&v4l2-buf-type;</entry>
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<entry><structfield>type</structfield></entry>
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<entry></entry>
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<entry>The buffer (stream) type, same as &v4l2-format;
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<structfield>type</structfield>, set by the application.</entry>
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</row>
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<row>
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<entry>union</entry>
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<entry><structfield>parm</structfield></entry>
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<entry></entry>
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<entry></entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-captureparm;</entry>
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<entry><structfield>capture</structfield></entry>
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<entry>Parameters for capture devices, used when
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<structfield>type</structfield> is
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<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-outputparm;</entry>
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<entry><structfield>output</structfield></entry>
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<entry>Parameters for output devices, used when
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<structfield>type</structfield> is
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<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant>.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>__u8</entry>
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<entry><structfield>raw_data</structfield>[200]</entry>
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<entry>A place holder for future extensions and custom
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(driver defined) buffer types <constant>V4L2_BUF_TYPE_PRIVATE</constant> and
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higher.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table pgwide="1" frame="none" id="v4l2-captureparm">
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<title>struct <structname>v4l2_captureparm</structname></title>
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<tgroup cols="3">
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&cs-str;
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<tbody valign="top">
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<row>
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<entry>__u32</entry>
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<entry><structfield>capability</structfield></entry>
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<entry>See <xref linkend="parm-caps" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>capturemode</structfield></entry>
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<entry>Set by drivers and applications, see <xref linkend="parm-flags" />.</entry>
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</row>
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<row>
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<entry>&v4l2-fract;</entry>
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<entry><structfield>timeperframe</structfield></entry>
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<entry><para>This is is the desired period between
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successive frames captured by the driver, in seconds. The
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field is intended to skip frames on the driver side, saving I/O
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bandwidth.</para><para>Applications store here the desired frame
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period, drivers return the actual frame period, which must be greater
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or equal to the nominal frame period determined by the current video
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standard (&v4l2-standard; <structfield>frameperiod</structfield>
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field). Changing the video standard (also implicitly by switching the
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video input) may reset this parameter to the nominal frame period. To
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reset manually applications can just set this field to
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zero.</para><para>Drivers support this function only when they set the
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<constant>V4L2_CAP_TIMEPERFRAME</constant> flag in the
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<structfield>capability</structfield> field.</para></entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>extendedmode</structfield></entry>
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<entry>Custom (driver specific) streaming parameters. When
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unused, applications and drivers must set this field to zero.
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Applications using this field should check the driver name and
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version, see <xref linkend="querycap" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>readbuffers</structfield></entry>
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<entry>Applications set this field to the desired number
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of buffers used internally by the driver in &func-read; mode. Drivers
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return the actual number of buffers. When an application requests zero
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buffers, drivers should just return the current setting rather than
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the minimum or an error code. For details see <xref
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linkend="rw" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>reserved</structfield>[4]</entry>
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<entry>Reserved for future extensions. Drivers and
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applications must set the array to zero.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table pgwide="1" frame="none" id="v4l2-outputparm">
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<title>struct <structname>v4l2_outputparm</structname></title>
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<tgroup cols="3">
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&cs-str;
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<tbody valign="top">
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<row>
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<entry>__u32</entry>
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<entry><structfield>capability</structfield></entry>
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<entry>See <xref linkend="parm-caps" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>outputmode</structfield></entry>
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<entry>Set by drivers and applications, see <xref
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linkend="parm-flags" />.</entry>
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</row>
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<row>
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<entry>&v4l2-fract;</entry>
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<entry><structfield>timeperframe</structfield></entry>
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<entry>This is is the desired period between
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successive frames output by the driver, in seconds.</entry>
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</row>
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<row>
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<entry spanname="hspan"><para>The field is intended to
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repeat frames on the driver side in &func-write; mode (in streaming
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mode timestamps can be used to throttle the output), saving I/O
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bandwidth.</para><para>Applications store here the desired frame
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period, drivers return the actual frame period, which must be greater
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or equal to the nominal frame period determined by the current video
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standard (&v4l2-standard; <structfield>frameperiod</structfield>
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field). Changing the video standard (also implicitly by switching the
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video output) may reset this parameter to the nominal frame period. To
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reset manually applications can just set this field to
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zero.</para><para>Drivers support this function only when they set the
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<constant>V4L2_CAP_TIMEPERFRAME</constant> flag in the
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<structfield>capability</structfield> field.</para></entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>extendedmode</structfield></entry>
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<entry>Custom (driver specific) streaming parameters. When
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unused, applications and drivers must set this field to zero.
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Applications using this field should check the driver name and
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version, see <xref linkend="querycap" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>writebuffers</structfield></entry>
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<entry>Applications set this field to the desired number
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of buffers used internally by the driver in
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<function>write()</function> mode. Drivers return the actual number of
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buffers. When an application requests zero buffers, drivers should
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just return the current setting rather than the minimum or an error
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code. For details see <xref linkend="rw" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>reserved</structfield>[4]</entry>
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<entry>Reserved for future extensions. Drivers and
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applications must set the array to zero.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table pgwide="1" frame="none" id="parm-caps">
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<title>Streaming Parameters Capabilites</title>
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<tgroup cols="3">
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&cs-def;
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<tbody valign="top">
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<row>
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<entry><constant>V4L2_CAP_TIMEPERFRAME</constant></entry>
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<entry>0x1000</entry>
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<entry>The frame skipping/repeating controlled by the
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<structfield>timeperframe</structfield> field is supported.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table pgwide="1" frame="none" id="parm-flags">
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<title>Capture Parameters Flags</title>
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<tgroup cols="3">
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&cs-def;
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<tbody valign="top">
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<row>
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<entry><constant>V4L2_MODE_HIGHQUALITY</constant></entry>
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<entry>0x0001</entry>
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<entry><para>High quality imaging mode. High quality mode
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is intended for still imaging applications. The idea is to get the
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best possible image quality that the hardware can deliver. It is not
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defined how the driver writer may achieve that; it will depend on the
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hardware and the ingenuity of the driver writer. High quality mode is
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a different mode from the the regular motion video capture modes. In
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high quality mode:<itemizedlist>
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<listitem>
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<para>The driver may be able to capture higher
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resolutions than for motion capture.</para>
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</listitem>
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<listitem>
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<para>The driver may support fewer pixel formats
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than motion capture (eg; true color).</para>
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</listitem>
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<listitem>
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<para>The driver may capture and arithmetically
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combine multiple successive fields or frames to remove color edge
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artifacts and reduce the noise in the video data.
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</para>
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</listitem>
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<listitem>
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<para>The driver may capture images in slices like
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a scanner in order to handle larger format images than would otherwise
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be possible. </para>
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</listitem>
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<listitem>
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<para>An image capture operation may be
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significantly slower than motion capture. </para>
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</listitem>
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<listitem>
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<para>Moving objects in the image might have
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excessive motion blur. </para>
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</listitem>
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<listitem>
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<para>Capture might only work through the
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<function>read()</function> call.</para>
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</listitem>
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</itemizedlist></para></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</refsect1>
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<refsect1>
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&return-value;
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<variablelist>
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<varlistentry>
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<term><errorcode>EINVAL</errorcode></term>
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<listitem>
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<para>This ioctl is not supported.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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</refentry>
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<!--
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Local Variables:
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mode: sgml
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sgml-parent-document: "v4l2.sgml"
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indent-tabs-mode: nil
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End:
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-->
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