/Documentation/DocBook/v4l/vidioc-enum-frameintervals.xml

  1<refentry id="vidioc-enum-frameintervals">
  2
  3  <refmeta>
  4    <refentrytitle>ioctl VIDIOC_ENUM_FRAMEINTERVALS</refentrytitle>
  5    &manvol;
  6  </refmeta>
  7
  8  <refnamediv>
  9    <refname>VIDIOC_ENUM_FRAMEINTERVALS</refname>
 10    <refpurpose>Enumerate frame intervals</refpurpose>
 11  </refnamediv>
 12
 13  <refsynopsisdiv>
 14    <funcsynopsis>
 15      <funcprototype>
 16	<funcdef>int <function>ioctl</function></funcdef>
 17	<paramdef>int <parameter>fd</parameter></paramdef>
 18	<paramdef>int <parameter>request</parameter></paramdef>
 19	<paramdef>struct v4l2_frmivalenum *<parameter>argp</parameter></paramdef>
 20      </funcprototype>
 21    </funcsynopsis>
 22  </refsynopsisdiv>
 23
 24  <refsect1>
 25    <title>Arguments</title>
 26
 27    <variablelist>
 28      <varlistentry>
 29	<term><parameter>fd</parameter></term>
 30	<listitem>
 31	  <para>&fd;</para>
 32	</listitem>
 33      </varlistentry>
 34      <varlistentry>
 35	<term><parameter>request</parameter></term>
 36	<listitem>
 37	  <para>VIDIOC_ENUM_FRAMEINTERVALS</para>
 38	</listitem>
 39      </varlistentry>
 40      <varlistentry>
 41	<term><parameter>argp</parameter></term>
 42	<listitem>
 43	  <para>Pointer to a &v4l2-frmivalenum; structure that
 44contains a pixel format and size and receives a frame interval.</para>
 45	</listitem>
 46      </varlistentry>
 47    </variablelist>
 48  </refsect1>
 49
 50  <refsect1>
 51    <title>Description</title>
 52
 53    <para>This ioctl allows applications to enumerate all frame
 54intervals that the device supports for the given pixel format and
 55frame size.</para>
 56    <para>The supported pixel formats and frame sizes can be obtained
 57by using the &VIDIOC-ENUM-FMT; and &VIDIOC-ENUM-FRAMESIZES;
 58functions.</para>
 59    <para>The return value and the content of the
 60<structfield>v4l2_frmivalenum.type</structfield> field depend on the
 61type of frame intervals the device supports. Here are the semantics of
 62the function for the different cases:</para>
 63    <itemizedlist>
 64      <listitem>
 65	<para><emphasis role="bold">Discrete:</emphasis> The function
 66returns success if the given index value (zero-based) is valid. The
 67application should increase the index by one for each call until
 68<constant>EINVAL</constant> is returned. The `v4l2_frmivalenum.type`
 69field is set to `V4L2_FRMIVAL_TYPE_DISCRETE` by the driver. Of the
 70union only the `discrete` member is valid.</para>
 71      </listitem>
 72      <listitem>
 73	<para><emphasis role="bold">Step-wise:</emphasis> The function
 74returns success if the given index value is zero and
 75<constant>EINVAL</constant> for any other index value. The
 76<structfield>v4l2_frmivalenum.type</structfield> field is set to
 77<constant>V4L2_FRMIVAL_TYPE_STEPWISE</constant> by the driver. Of the
 78union only the <structfield>stepwise</structfield> member is
 79valid.</para>
 80      </listitem>
 81      <listitem>
 82	<para><emphasis role="bold">Continuous:</emphasis> This is a
 83special case of the step-wise type above. The function returns success
 84if the given index value is zero and <constant>EINVAL</constant> for
 85any other index value. The
 86<structfield>v4l2_frmivalenum.type</structfield> field is set to
 87<constant>V4L2_FRMIVAL_TYPE_CONTINUOUS</constant> by the driver. Of
 88the union only the <structfield>stepwise</structfield> member is valid
 89and the <structfield>step</structfield> value is set to 1.</para>
 90      </listitem>
 91    </itemizedlist>
 92
 93    <para>When the application calls the function with index zero, it
 94must check the <structfield>type</structfield> field to determine the
 95type of frame interval enumeration the device supports. Only for the
 96<constant>V4L2_FRMIVAL_TYPE_DISCRETE</constant> type does it make
 97sense to increase the index value to receive more frame
 98intervals.</para>
 99    <para>Note that the order in which the frame intervals are
100returned has no special meaning. In particular does it not say
101anything about potential default frame intervals.</para>
102    <para>Applications can assume that the enumeration data does not
103change without any interaction from the application itself. This means
104that the enumeration data is consistent if the application does not
105perform any other ioctl calls while it runs the frame interval
106enumeration.</para>
107  </refsect1>
108
109  <refsect1>
110    <title>Notes</title>
111
112    <itemizedlist>
113      <listitem>
114	<para><emphasis role="bold">Frame intervals and frame
115rates:</emphasis> The V4L2 API uses frame intervals instead of frame
116rates. Given the frame interval the frame rate can be computed as
117follows:<screen>frame_rate = 1 / frame_interval</screen></para>
118      </listitem>
119    </itemizedlist>
120
121  </refsect1>
122
123  <refsect1>
124    <title>Structs</title>
125
126    <para>In the structs below, <emphasis>IN</emphasis> denotes a
127value that has to be filled in by the application,
128<emphasis>OUT</emphasis> denotes values that the driver fills in. The
129application should zero out all members except for the
130<emphasis>IN</emphasis> fields.</para>
131
132    <table pgwide="1" frame="none" id="v4l2-frmival-stepwise">
133      <title>struct <structname>v4l2_frmival_stepwise</structname></title>
134      <tgroup cols="3">
135	&cs-str;
136	<tbody valign="top">
137	  <row>
138	    <entry>&v4l2-fract;</entry>
139	    <entry><structfield>min</structfield></entry>
140	    <entry>Minimum frame interval [s].</entry>
141	  </row>
142	  <row>
143	    <entry>&v4l2-fract;</entry>
144	    <entry><structfield>max</structfield></entry>
145	    <entry>Maximum frame interval [s].</entry>
146	  </row>
147	  <row>
148	    <entry>&v4l2-fract;</entry>
149	    <entry><structfield>step</structfield></entry>
150	    <entry>Frame interval step size [s].</entry>
151	  </row>
152	</tbody>
153      </tgroup>
154    </table>
155
156    <table pgwide="1" frame="none" id="v4l2-frmivalenum">
157      <title>struct <structname>v4l2_frmivalenum</structname></title>
158      <tgroup cols="4">
159	<colspec colname="c1" />
160	<colspec colname="c2" />
161	<colspec colname="c3" />
162	<colspec colname="c4" />
163	<tbody valign="top">
164	  <row>
165	    <entry>__u32</entry>
166	    <entry><structfield>index</structfield></entry>
167	    <entry></entry>
168	    <entry>IN: Index of the given frame interval in the
169enumeration.</entry>
170	  </row>
171	  <row>
172	    <entry>__u32</entry>
173	    <entry><structfield>pixel_format</structfield></entry>
174	    <entry></entry>
175	    <entry>IN: Pixel format for which the frame intervals are
176enumerated.</entry>
177	  </row>
178	  <row>
179	    <entry>__u32</entry>
180	    <entry><structfield>width</structfield></entry>
181	    <entry></entry>
182	    <entry>IN: Frame width for which the frame intervals are
183enumerated.</entry>
184	  </row>
185	  <row>
186	    <entry>__u32</entry>
187	    <entry><structfield>height</structfield></entry>
188	    <entry></entry>
189	    <entry>IN: Frame height for which the frame intervals are
190enumerated.</entry>
191	  </row>
192	  <row>
193	    <entry>__u32</entry>
194	    <entry><structfield>type</structfield></entry>
195	    <entry></entry>
196	    <entry>OUT: Frame interval type the device supports.</entry>
197	  </row>
198	  <row>
199	    <entry>union</entry>
200	    <entry></entry>
201	    <entry></entry>
202	    <entry>OUT: Frame interval with the given index.</entry>
203	  </row>
204	  <row>
205	    <entry></entry>
206	    <entry>&v4l2-fract;</entry>
207	    <entry><structfield>discrete</structfield></entry>
208	    <entry>Frame interval [s].</entry>
209	  </row>
210	  <row>
211	    <entry></entry>
212	    <entry>&v4l2-frmival-stepwise;</entry>
213	    <entry><structfield>stepwise</structfield></entry>
214	    <entry></entry>
215	  </row>
216	  <row>
217	    <entry>__u32</entry>
218	    <entry><structfield>reserved[2]</structfield></entry>
219	    <entry></entry>
220	    <entry>Reserved space for future use.</entry>
221	  </row>
222	</tbody>
223      </tgroup>
224    </table>
225  </refsect1>
226
227  <refsect1>
228    <title>Enums</title>
229
230    <table pgwide="1" frame="none" id="v4l2-frmivaltypes">
231      <title>enum <structname>v4l2_frmivaltypes</structname></title>
232      <tgroup cols="3">
233	&cs-def;
234	<tbody valign="top">
235	  <row>
236	    <entry><constant>V4L2_FRMIVAL_TYPE_DISCRETE</constant></entry>
237	    <entry>1</entry>
238	    <entry>Discrete frame interval.</entry>
239	  </row>
240	  <row>
241	    <entry><constant>V4L2_FRMIVAL_TYPE_CONTINUOUS</constant></entry>
242	    <entry>2</entry>
243	    <entry>Continuous frame interval.</entry>
244	  </row>
245	  <row>
246	    <entry><constant>V4L2_FRMIVAL_TYPE_STEPWISE</constant></entry>
247	    <entry>3</entry>
248	    <entry>Step-wise defined frame interval.</entry>
249	  </row>
250	</tbody>
251      </tgroup>
252    </table>
253  </refsect1>
254
255  <refsect1>
256    &return-value;
257
258    <para>See the description section above for a list of return
259values that <varname>errno</varname> can have.</para>
260  </refsect1>
261
262</refentry>
263
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