CooCenter-System-kernel/Documentation/DocBook/media/v4l/vidioc-enum-frameintervals.xml

<refentry id="vidioc-enum-frameintervals">

  <refmeta>
    <refentrytitle>ioctl VIDIOC_ENUM_FRAMEINTERVALS</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>VIDIOC_ENUM_FRAMEINTERVALS</refname>
    <refpurpose>Enumerate frame intervals</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcprototype>
	<funcdef>int <function>ioctl</function></funcdef>
	<paramdef>int <parameter>fd</parameter></paramdef>
	<paramdef>int <parameter>request</parameter></paramdef>
	<paramdef>struct v4l2_frmivalenum *<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_ENUM_FRAMEINTERVALS</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	  <para>Pointer to a &v4l2-frmivalenum; structure that
contains a pixel format and size and receives a frame interval.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <para>This ioctl allows applications to enumerate all frame
intervals that the device supports for the given pixel format and
frame size.</para>
    <para>The supported pixel formats and frame sizes can be obtained
by using the &VIDIOC-ENUM-FMT; and &VIDIOC-ENUM-FRAMESIZES;
functions.</para>
    <para>The return value and the content of the
<structfield>v4l2_frmivalenum.type</structfield> field depend on the
type of frame intervals the device supports. Here are the semantics of
the function for the different cases:</para>
    <itemizedlist>
      <listitem>
	<para><emphasis role="bold">Discrete:</emphasis> The function
returns success if the given index value (zero-based) is valid. The
application should increase the index by one for each call until
<constant>EINVAL</constant> is returned. The `v4l2_frmivalenum.type`
field is set to `V4L2_FRMIVAL_TYPE_DISCRETE` by the driver. Of the
union only the `discrete` member is valid.</para>
      </listitem>
      <listitem>
	<para><emphasis role="bold">Step-wise:</emphasis> The function
returns success if the given index value is zero and
<constant>EINVAL</constant> for any other index value. The
<structfield>v4l2_frmivalenum.type</structfield> field is set to
<constant>V4L2_FRMIVAL_TYPE_STEPWISE</constant> by the driver. Of the
union only the <structfield>stepwise</structfield> member is
valid.</para>
      </listitem>
      <listitem>
	<para><emphasis role="bold">Continuous:</emphasis> This is a
special case of the step-wise type above. The function returns success
if the given index value is zero and <constant>EINVAL</constant> for
any other index value. The
<structfield>v4l2_frmivalenum.type</structfield> field is set to
<constant>V4L2_FRMIVAL_TYPE_CONTINUOUS</constant> by the driver. Of
the union only the <structfield>stepwise</structfield> member is valid
and the <structfield>step</structfield> value is set to 1.</para>
      </listitem>
    </itemizedlist>

    <para>When the application calls the function with index zero, it
must check the <structfield>type</structfield> field to determine the
type of frame interval enumeration the device supports. Only for the
<constant>V4L2_FRMIVAL_TYPE_DISCRETE</constant> type does it make
sense to increase the index value to receive more frame
intervals.</para>
    <para>Note that the order in which the frame intervals are
returned has no special meaning. In particular does it not say
anything about potential default frame intervals.</para>
    <para>Applications can assume that the enumeration data does not
change without any interaction from the application itself. This means
that the enumeration data is consistent if the application does not
perform any other ioctl calls while it runs the frame interval
enumeration.</para>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <itemizedlist>
      <listitem>
	<para><emphasis role="bold">Frame intervals and frame
rates:</emphasis> The V4L2 API uses frame intervals instead of frame
rates. Given the frame interval the frame rate can be computed as
follows:<screen>frame_rate = 1 / frame_interval</screen></para>
      </listitem>
    </itemizedlist>

  </refsect1>

  <refsect1>
    <title>Structs</title>

    <para>In the structs below, <emphasis>IN</emphasis> denotes a
value that has to be filled in by the application,
<emphasis>OUT</emphasis> denotes values that the driver fills in. The
application should zero out all members except for the
<emphasis>IN</emphasis> fields.</para>

    <table pgwide="1" frame="none" id="v4l2-frmival-stepwise">
      <title>struct <structname>v4l2_frmival_stepwise</structname></title>
      <tgroup cols="3">
	&cs-str;
	<tbody valign="top">
	  <row>
	    <entry>&v4l2-fract;</entry>
	    <entry><structfield>min</structfield></entry>
	    <entry>Minimum frame interval [s].</entry>
	  </row>
	  <row>
	    <entry>&v4l2-fract;</entry>
	    <entry><structfield>max</structfield></entry>
	    <entry>Maximum frame interval [s].</entry>
	  </row>
	  <row>
	    <entry>&v4l2-fract;</entry>
	    <entry><structfield>step</structfield></entry>
	    <entry>Frame interval step size [s].</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="v4l2-frmivalenum">
      <title>struct <structname>v4l2_frmivalenum</structname></title>
      <tgroup cols="4">
	<colspec colname="c1" />
	<colspec colname="c2" />
	<colspec colname="c3" />
	<colspec colname="c4" />
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>index</structfield></entry>
	    <entry></entry>
	    <entry>IN: Index of the given frame interval in the
enumeration.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>pixel_format</structfield></entry>
	    <entry></entry>
	    <entry>IN: Pixel format for which the frame intervals are
enumerated.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>width</structfield></entry>
	    <entry></entry>
	    <entry>IN: Frame width for which the frame intervals are
enumerated.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>height</structfield></entry>
	    <entry></entry>
	    <entry>IN: Frame height for which the frame intervals are
enumerated.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>type</structfield></entry>
	    <entry></entry>
	    <entry>OUT: Frame interval type the device supports.</entry>
	  </row>
	  <row>
	    <entry>union</entry>
	    <entry></entry>
	    <entry></entry>
	    <entry>OUT: Frame interval with the given index.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>&v4l2-fract;</entry>
	    <entry><structfield>discrete</structfield></entry>
	    <entry>Frame interval [s].</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>&v4l2-frmival-stepwise;</entry>
	    <entry><structfield>stepwise</structfield></entry>
	    <entry></entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>reserved[2]</structfield></entry>
	    <entry></entry>
	    <entry>Reserved space for future use. Must be zeroed by drivers and
	    applications.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    <title>Enums</title>

    <table pgwide="1" frame="none" id="v4l2-frmivaltypes">
      <title>enum <structname>v4l2_frmivaltypes</structname></title>
      <tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	  <row>
	    <entry><constant>V4L2_FRMIVAL_TYPE_DISCRETE</constant></entry>
	    <entry>1</entry>
	    <entry>Discrete frame interval.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FRMIVAL_TYPE_CONTINUOUS</constant></entry>
	    <entry>2</entry>
	    <entry>Continuous frame interval.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FRMIVAL_TYPE_STEPWISE</constant></entry>
	    <entry>3</entry>
	    <entry>Step-wise defined frame interval.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>
  </refsect1>

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

</refentry>