mirror of https://gitee.com/openkylin/linux.git
260 lines
8.1 KiB
XML
260 lines
8.1 KiB
XML
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<refentry id="vidioc-enum-frameintervals">
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<refmeta>
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<refentrytitle>ioctl VIDIOC_ENUM_FRAMEINTERVALS</refentrytitle>
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&manvol;
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</refmeta>
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<refnamediv>
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<refname>VIDIOC_ENUM_FRAMEINTERVALS</refname>
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<refpurpose>Enumerate frame intervals</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>struct v4l2_frmivalenum *<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_ENUM_FRAMEINTERVALS</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>Pointer to a &v4l2-frmivalenum; structure that
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contains a pixel format and size and receives a frame interval.</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>This ioctl allows applications to enumerate all frame
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intervals that the device supports for the given pixel format and
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frame size.</para>
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<para>The supported pixel formats and frame sizes can be obtained
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by using the &VIDIOC-ENUM-FMT; and &VIDIOC-ENUM-FRAMESIZES;
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functions.</para>
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<para>The return value and the content of the
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<structfield>v4l2_frmivalenum.type</structfield> field depend on the
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type of frame intervals the device supports. Here are the semantics of
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the function for the different cases:</para>
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<itemizedlist>
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<listitem>
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<para><emphasis role="bold">Discrete:</emphasis> The function
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returns success if the given index value (zero-based) is valid. The
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application should increase the index by one for each call until
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<constant>EINVAL</constant> is returned. The `v4l2_frmivalenum.type`
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field is set to `V4L2_FRMIVAL_TYPE_DISCRETE` by the driver. Of the
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union only the `discrete` member is valid.</para>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Step-wise:</emphasis> The function
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returns success if the given index value is zero and
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<constant>EINVAL</constant> for any other index value. The
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<structfield>v4l2_frmivalenum.type</structfield> field is set to
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<constant>V4L2_FRMIVAL_TYPE_STEPWISE</constant> by the driver. Of the
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union only the <structfield>stepwise</structfield> member is
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valid.</para>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Continuous:</emphasis> This is a
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special case of the step-wise type above. The function returns success
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if the given index value is zero and <constant>EINVAL</constant> for
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any other index value. The
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<structfield>v4l2_frmivalenum.type</structfield> field is set to
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<constant>V4L2_FRMIVAL_TYPE_CONTINUOUS</constant> by the driver. Of
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the union only the <structfield>stepwise</structfield> member is valid
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and the <structfield>step</structfield> value is set to 1.</para>
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</listitem>
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</itemizedlist>
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<para>When the application calls the function with index zero, it
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must check the <structfield>type</structfield> field to determine the
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type of frame interval enumeration the device supports. Only for the
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<constant>V4L2_FRMIVAL_TYPE_DISCRETE</constant> type does it make
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sense to increase the index value to receive more frame
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intervals.</para>
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<para>Note that the order in which the frame intervals are
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returned has no special meaning. In particular does it not say
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anything about potential default frame intervals.</para>
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<para>Applications can assume that the enumeration data does not
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change without any interaction from the application itself. This means
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that the enumeration data is consistent if the application does not
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perform any other ioctl calls while it runs the frame interval
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enumeration.</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<itemizedlist>
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<listitem>
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<para><emphasis role="bold">Frame intervals and frame
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rates:</emphasis> The V4L2 API uses frame intervals instead of frame
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rates. Given the frame interval the frame rate can be computed as
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follows:<screen>frame_rate = 1 / frame_interval</screen></para>
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</listitem>
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</itemizedlist>
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</refsect1>
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<refsect1>
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<title>Structs</title>
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<para>In the structs below, <emphasis>IN</emphasis> denotes a
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value that has to be filled in by the application,
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<emphasis>OUT</emphasis> denotes values that the driver fills in. The
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application should zero out all members except for the
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<emphasis>IN</emphasis> fields.</para>
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<table pgwide="1" frame="none" id="v4l2-frmival-stepwise">
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<title>struct <structname>v4l2_frmival_stepwise</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>&v4l2-fract;</entry>
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<entry><structfield>min</structfield></entry>
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<entry>Minimum frame interval [s].</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>max</structfield></entry>
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<entry>Maximum frame interval [s].</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>step</structfield></entry>
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<entry>Frame interval step size [s].</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-frmivalenum">
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<title>struct <structname>v4l2_frmivalenum</structname></title>
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<tgroup cols="4">
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<colspec colname="c1" />
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<colspec colname="c2" />
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<colspec colname="c3" />
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<colspec colname="c4" />
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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>index</structfield></entry>
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<entry></entry>
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<entry>IN: Index of the given frame interval in the
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enumeration.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>pixel_format</structfield></entry>
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<entry></entry>
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<entry>IN: Pixel format for which the frame intervals are
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enumerated.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>width</structfield></entry>
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<entry></entry>
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<entry>IN: Frame width for which the frame intervals are
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enumerated.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>height</structfield></entry>
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<entry></entry>
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<entry>IN: Frame height for which the frame intervals are
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enumerated.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>type</structfield></entry>
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<entry></entry>
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<entry>OUT: Frame interval type the device supports.</entry>
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</row>
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<row>
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<entry>union</entry>
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<entry></entry>
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<entry></entry>
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<entry>OUT: Frame interval with the given index.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-fract;</entry>
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<entry><structfield>discrete</structfield></entry>
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<entry>Frame interval [s].</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-frmival-stepwise;</entry>
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<entry><structfield>stepwise</structfield></entry>
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<entry></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[2]</structfield></entry>
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<entry></entry>
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<entry>Reserved space for future use.</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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<title>Enums</title>
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<table pgwide="1" frame="none" id="v4l2-frmivaltypes">
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<title>enum <structname>v4l2_frmivaltypes</structname></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_FRMIVAL_TYPE_DISCRETE</constant></entry>
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<entry>1</entry>
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<entry>Discrete frame interval.</entry>
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</row>
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<row>
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<entry><constant>V4L2_FRMIVAL_TYPE_CONTINUOUS</constant></entry>
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<entry>2</entry>
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<entry>Continuous frame interval.</entry>
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</row>
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<row>
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<entry><constant>V4L2_FRMIVAL_TYPE_STEPWISE</constant></entry>
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<entry>3</entry>
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<entry>Step-wise defined frame interval.</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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</refsect1>
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</refentry>
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