1<refentry id="vidioc-g-parm">
2  <refmeta>
3    <refentrytitle>ioctl VIDIOC_G_PARM, VIDIOC_S_PARM</refentrytitle>
4    &manvol;
5  </refmeta>
6
7  <refnamediv>
8    <refname>VIDIOC_G_PARM</refname>
9    <refname>VIDIOC_S_PARM</refname>
10    <refpurpose>Get or set streaming parameters</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>v4l2_streamparm *<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_G_PARM, VIDIOC_S_PARM</para>
38	</listitem>
39      </varlistentry>
40      <varlistentry>
41	<term><parameter>argp</parameter></term>
42	<listitem>
43	  <para></para>
44	</listitem>
45      </varlistentry>
46    </variablelist>
47  </refsect1>
48
49  <refsect1>
50    <title>Description</title>
51
52    <para>The current video standard determines a nominal number of
53frames per second. If less than this number of frames is to be
54captured or output, applications can request frame skipping or
55duplicating on the driver side. This is especially useful when using
56the <function>read()</function> or <function>write()</function>, which
57are not augmented by timestamps or sequence counters, and to avoid
58unnecessary data copying.</para>
59
60    <para>Further these ioctls can be used to determine the number of
61buffers used internally by a driver in read/write mode. For
62implications see the section discussing the &func-read;
63function.</para>
64
65    <para>To get and set the streaming parameters applications call
66the <constant>VIDIOC_G_PARM</constant> and
67<constant>VIDIOC_S_PARM</constant> ioctl, respectively. They take a
68pointer to a struct <structname>v4l2_streamparm</structname> which
69contains a union holding separate parameters for input and output
70devices.</para>
71
72    <table pgwide="1" frame="none" id="v4l2-streamparm">
73      <title>struct <structname>v4l2_streamparm</structname></title>
74      <tgroup cols="4">
75	&cs-ustr;
76	<tbody valign="top">
77	  <row>
78	    <entry>&v4l2-buf-type;</entry>
79	    <entry><structfield>type</structfield></entry>
80	    <entry></entry>
81	    <entry>The buffer (stream) type, same as &v4l2-format;
82<structfield>type</structfield>, set by the application.</entry>
83	  </row>
84	  <row>
85	    <entry>union</entry>
86	    <entry><structfield>parm</structfield></entry>
87	    <entry></entry>
88	    <entry></entry>
89	  </row>
90	  <row>
91	    <entry></entry>
92	    <entry>&v4l2-captureparm;</entry>
93	    <entry><structfield>capture</structfield></entry>
94	    <entry>Parameters for capture devices, used when
95<structfield>type</structfield> is
96<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>.</entry>
97	  </row>
98	  <row>
99	    <entry></entry>
100	    <entry>&v4l2-outputparm;</entry>
101	    <entry><structfield>output</structfield></entry>
102	    <entry>Parameters for output devices, used when
103<structfield>type</structfield> is
104<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant>.</entry>
105	  </row>
106	  <row>
107	    <entry></entry>
108	    <entry>__u8</entry>
109	    <entry><structfield>raw_data</structfield>[200]</entry>
110	    <entry>A place holder for future extensions and custom
111(driver defined) buffer types <constant>V4L2_BUF_TYPE_PRIVATE</constant> and
112higher.</entry>
113	  </row>
114	</tbody>
115      </tgroup>
116    </table>
117
118    <table pgwide="1" frame="none" id="v4l2-captureparm">
119      <title>struct <structname>v4l2_captureparm</structname></title>
120      <tgroup cols="3">
121	&cs-str;
122	<tbody valign="top">
123	  <row>
124	    <entry>__u32</entry>
125	    <entry><structfield>capability</structfield></entry>
126	    <entry>See <xref linkend="parm-caps" />.</entry>
127	  </row>
128	  <row>
129	    <entry>__u32</entry>
130	    <entry><structfield>capturemode</structfield></entry>
131	    <entry>Set by drivers and applications, see <xref linkend="parm-flags" />.</entry>
132	  </row>
133	  <row>
134	    <entry>&v4l2-fract;</entry>
135	    <entry><structfield>timeperframe</structfield></entry>
136	    <entry><para>This is is the desired period between
137successive frames captured by the driver, in seconds. The
138field is intended to skip frames on the driver side, saving I/O
139bandwidth.</para><para>Applications store here the desired frame
140period, drivers return the actual frame period, which must be greater
141or equal to the nominal frame period determined by the current video
142standard (&v4l2-standard; <structfield>frameperiod</structfield>
143field). Changing the video standard (also implicitly by switching the
144video input) may reset this parameter to the nominal frame period. To
145reset manually applications can just set this field to
146zero.</para><para>Drivers support this function only when they set the
147<constant>V4L2_CAP_TIMEPERFRAME</constant> flag in the
148<structfield>capability</structfield> field.</para></entry>
149	  </row>
150	  <row>
151	    <entry>__u32</entry>
152	    <entry><structfield>extendedmode</structfield></entry>
153	    <entry>Custom (driver specific) streaming parameters. When
154unused, applications and drivers must set this field to zero.
155Applications using this field should check the driver name and
156version, see <xref linkend="querycap" />.</entry>
157	  </row>
158	  <row>
159	    <entry>__u32</entry>
160	    <entry><structfield>readbuffers</structfield></entry>
161	    <entry>Applications set this field to the desired number
162of buffers used internally by the driver in &func-read; mode. Drivers
163return the actual number of buffers. When an application requests zero
164buffers, drivers should just return the current setting rather than
165the minimum or an error code. For details see <xref
166		linkend="rw" />.</entry>
167	  </row>
168	  <row>
169	    <entry>__u32</entry>
170	    <entry><structfield>reserved</structfield>[4]</entry>
171	    <entry>Reserved for future extensions. Drivers and
172applications must set the array to zero.</entry>
173	  </row>
174	</tbody>
175      </tgroup>
176    </table>
177
178    <table pgwide="1" frame="none" id="v4l2-outputparm">
179      <title>struct <structname>v4l2_outputparm</structname></title>
180      <tgroup cols="3">
181	&cs-str;
182	<tbody valign="top">
183	  <row>
184	    <entry>__u32</entry>
185	    <entry><structfield>capability</structfield></entry>
186	    <entry>See <xref linkend="parm-caps" />.</entry>
187	  </row>
188	  <row>
189	    <entry>__u32</entry>
190	    <entry><structfield>outputmode</structfield></entry>
191	    <entry>Set by drivers and applications, see <xref
192	    linkend="parm-flags" />.</entry>
193	  </row>
194	  <row>
195	    <entry>&v4l2-fract;</entry>
196	    <entry><structfield>timeperframe</structfield></entry>
197	    <entry>This is is the desired period between
198successive frames output by the driver, in seconds.</entry>
199	  </row>
200	  <row>
201	    <entry spanname="hspan"><para>The field is intended to
202repeat frames on the driver side in &func-write; mode (in streaming
203mode timestamps can be used to throttle the output), saving I/O
204bandwidth.</para><para>Applications store here the desired frame
205period, drivers return the actual frame period, which must be greater
206or equal to the nominal frame period determined by the current video
207standard (&v4l2-standard; <structfield>frameperiod</structfield>
208field). Changing the video standard (also implicitly by switching the
209video output) may reset this parameter to the nominal frame period. To
210reset manually applications can just set this field to
211zero.</para><para>Drivers support this function only when they set the
212<constant>V4L2_CAP_TIMEPERFRAME</constant> flag in the
213<structfield>capability</structfield> field.</para></entry>
214	  </row>
215	  <row>
216	    <entry>__u32</entry>
217	    <entry><structfield>extendedmode</structfield></entry>
218	    <entry>Custom (driver specific) streaming parameters. When
219unused, applications and drivers must set this field to zero.
220Applications using this field should check the driver name and
221version, see <xref linkend="querycap" />.</entry>
222	  </row>
223	  <row>
224	    <entry>__u32</entry>
225	    <entry><structfield>writebuffers</structfield></entry>
226	    <entry>Applications set this field to the desired number
227of buffers used internally by the driver in
228<function>write()</function> mode. Drivers return the actual number of
229buffers. When an application requests zero buffers, drivers should
230just return the current setting rather than the minimum or an error
231code. For details see <xref linkend="rw" />.</entry>
232	  </row>
233	  <row>
234	    <entry>__u32</entry>
235	    <entry><structfield>reserved</structfield>[4]</entry>
236	    <entry>Reserved for future extensions. Drivers and
237applications must set the array to zero.</entry>
238	  </row>
239	</tbody>
240      </tgroup>
241    </table>
242
243    <table pgwide="1" frame="none" id="parm-caps">
244      <title>Streaming Parameters Capabilites</title>
245      <tgroup cols="3">
246	&cs-def;
247	<tbody valign="top">
248	  <row>
249	    <entry><constant>V4L2_CAP_TIMEPERFRAME</constant></entry>
250	    <entry>0x1000</entry>
251	    <entry>The frame skipping/repeating controlled by the
252<structfield>timeperframe</structfield> field is supported.</entry>
253	  </row>
254	</tbody>
255      </tgroup>
256    </table>
257
258    <table pgwide="1" frame="none" id="parm-flags">
259      <title>Capture Parameters Flags</title>
260      <tgroup cols="3">
261	&cs-def;
262	<tbody valign="top">
263	  <row>
264	    <entry><constant>V4L2_MODE_HIGHQUALITY</constant></entry>
265	    <entry>0x0001</entry>
266	    <entry><para>High quality imaging mode. High quality mode
267is intended for still imaging applications. The idea is to get the
268best possible image quality that the hardware can deliver. It is not
269defined how the driver writer may achieve that; it will depend on the
270hardware and the ingenuity of the driver writer. High quality mode is
271a different mode from the the regular motion video capture modes. In
272high quality mode:<itemizedlist>
273		  <listitem>
274		    <para>The driver may be able to capture higher
275resolutions than for motion capture.</para>
276		  </listitem>
277		  <listitem>
278		    <para>The driver may support fewer pixel formats
279than motion capture (eg; true color).</para>
280		  </listitem>
281		  <listitem>
282		    <para>The driver may capture and arithmetically
283combine multiple successive fields or frames to remove color edge
284artifacts and reduce the noise in the video data.
285</para>
286		  </listitem>
287		  <listitem>
288		    <para>The driver may capture images in slices like
289a scanner in order to handle larger format images than would otherwise
290be possible. </para>
291		  </listitem>
292		  <listitem>
293		    <para>An image capture operation may be
294significantly slower than motion capture. </para>
295		  </listitem>
296		  <listitem>
297		    <para>Moving objects in the image might have
298excessive motion blur. </para>
299		  </listitem>
300		  <listitem>
301		    <para>Capture might only work through the
302<function>read()</function> call.</para>
303		  </listitem>
304		</itemizedlist></para></entry>
305	  </row>
306	</tbody>
307      </tgroup>
308    </table>
309
310  </refsect1>
311
312  <refsect1>
313    &return-value;
314  </refsect1>
315</refentry>
316