summaryrefslogtreecommitdiff
path: root/Documentation/media/uapi/v4l/vidioc-g-parm.rst
blob: 7116e0decddc817021fb8216b79e48f608be4a43 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
.. -*- coding: utf-8; mode: rst -*-

.. _VIDIOC_G_PARM:

**********************************
ioctl VIDIOC_G_PARM, VIDIOC_S_PARM
**********************************

Name
====

VIDIOC_G_PARM - VIDIOC_S_PARM - Get or set streaming parameters


Synopsis
========

.. cpp:function:: int ioctl( int fd, int request, v4l2_streamparm *argp )


Arguments
=========

``fd``
    File descriptor returned by :ref:`open() <func-open>`.

``request``
    VIDIOC_G_PARM, VIDIOC_S_PARM

``argp``


Description
===========

The current video standard determines a nominal number of frames per
second. If less than this number of frames is to be captured or output,
applications can request frame skipping or duplicating on the driver
side. This is especially useful when using the :ref:`read() <func-read>` or
:ref:`write() <func-write>`, which are not augmented by timestamps or sequence
counters, and to avoid unnecessary data copying.

Further these ioctls can be used to determine the number of buffers used
internally by a driver in read/write mode. For implications see the
section discussing the :ref:`read() <func-read>` function.

To get and set the streaming parameters applications call the
:ref:`VIDIOC_G_PARM <VIDIOC_G_PARM>` and :ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl, respectively. They take a
pointer to a struct :ref:`struct v4l2_streamparm <v4l2-streamparm>` which contains a
union holding separate parameters for input and output devices.


.. _v4l2-streamparm:

.. flat-table:: struct v4l2_streamparm
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 1 2


    -  .. row 1

       -  __u32

       -  ``type``

       -
       -  The buffer (stream) type, same as struct
	  :ref:`v4l2_format <v4l2-format>` ``type``, set by the
	  application. See :ref:`v4l2-buf-type`

    -  .. row 2

       -  union

       -  ``parm``

       -
       -

    -  .. row 3

       -
       -  struct :ref:`v4l2_captureparm <v4l2-captureparm>`

       -  ``capture``

       -  Parameters for capture devices, used when ``type`` is
	  ``V4L2_BUF_TYPE_VIDEO_CAPTURE``.

    -  .. row 4

       -
       -  struct :ref:`v4l2_outputparm <v4l2-outputparm>`

       -  ``output``

       -  Parameters for output devices, used when ``type`` is
	  ``V4L2_BUF_TYPE_VIDEO_OUTPUT``.

    -  .. row 5

       -
       -  __u8

       -  ``raw_data``\ [200]

       -  A place holder for future extensions.



.. _v4l2-captureparm:

.. flat-table:: struct v4l2_captureparm
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2


    -  .. row 1

       -  __u32

       -  ``capability``

       -  See :ref:`parm-caps`.

    -  .. row 2

       -  __u32

       -  ``capturemode``

       -  Set by drivers and applications, see :ref:`parm-flags`.

    -  .. row 3

       -  struct :ref:`v4l2_fract <v4l2-fract>`

       -  ``timeperframe``

       -  This is the desired period between successive frames captured by
	  the driver, in seconds. The field is intended to skip frames on
	  the driver side, saving I/O bandwidth.

	  Applications store here the desired frame period, drivers return
	  the actual frame period, which must be greater or equal to the
	  nominal frame period determined by the current video standard
	  (struct :ref:`v4l2_standard <v4l2-standard>` ``frameperiod``
	  field). Changing the video standard (also implicitly by switching
	  the video input) may reset this parameter to the nominal frame
	  period. To reset manually applications can just set this field to
	  zero.

	  Drivers support this function only when they set the
	  ``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field.

    -  .. row 4

       -  __u32

       -  ``extendedmode``

       -  Custom (driver specific) streaming parameters. When unused,
	  applications and drivers must set this field to zero. Applications
	  using this field should check the driver name and version, see
	  :ref:`querycap`.

    -  .. row 5

       -  __u32

       -  ``readbuffers``

       -  Applications set this field to the desired number of buffers used
	  internally by the driver in :ref:`read() <func-read>` mode.
	  Drivers return the actual number of buffers. When an application
	  requests zero buffers, drivers should just return the current
	  setting rather than the minimum or an error code. For details see
	  :ref:`rw`.

    -  .. row 6

       -  __u32

       -  ``reserved``\ [4]

       -  Reserved for future extensions. Drivers and applications must set
	  the array to zero.



.. _v4l2-outputparm:

.. flat-table:: struct v4l2_outputparm
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2


    -  .. row 1

       -  __u32

       -  ``capability``

       -  See :ref:`parm-caps`.

    -  .. row 2

       -  __u32

       -  ``outputmode``

       -  Set by drivers and applications, see :ref:`parm-flags`.

    -  .. row 3

       -  struct :ref:`v4l2_fract <v4l2-fract>`

       -  ``timeperframe``

       -  This is the desired period between successive frames output by the
	  driver, in seconds.

    -  .. row 4

       -  :cspan:`2`

	  The field is intended to repeat frames on the driver side in
	  :ref:`write() <func-write>` mode (in streaming mode timestamps
	  can be used to throttle the output), saving I/O bandwidth.

	  Applications store here the desired frame period, drivers return
	  the actual frame period, which must be greater or equal to the
	  nominal frame period determined by the current video standard
	  (struct :ref:`v4l2_standard <v4l2-standard>` ``frameperiod``
	  field). Changing the video standard (also implicitly by switching
	  the video output) may reset this parameter to the nominal frame
	  period. To reset manually applications can just set this field to
	  zero.

	  Drivers support this function only when they set the
	  ``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field.

    -  .. row 5

       -  __u32

       -  ``extendedmode``

       -  Custom (driver specific) streaming parameters. When unused,
	  applications and drivers must set this field to zero. Applications
	  using this field should check the driver name and version, see
	  :ref:`querycap`.

    -  .. row 6

       -  __u32

       -  ``writebuffers``

       -  Applications set this field to the desired number of buffers used
	  internally by the driver in :ref:`write() <func-write>` mode. Drivers
	  return the actual number of buffers. When an application requests
	  zero buffers, drivers should just return the current setting
	  rather than the minimum or an error code. For details see
	  :ref:`rw`.

    -  .. row 7

       -  __u32

       -  ``reserved``\ [4]

       -  Reserved for future extensions. Drivers and applications must set
	  the array to zero.



.. _parm-caps:

.. flat-table:: Streaming Parameters Capabilites
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 1 4


    -  .. row 1

       -  ``V4L2_CAP_TIMEPERFRAME``

       -  0x1000

       -  The frame skipping/repeating controlled by the ``timeperframe``
	  field is supported.



.. _parm-flags:

.. flat-table:: Capture Parameters Flags
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 1 4


    -  .. row 1

       -  ``V4L2_MODE_HIGHQUALITY``

       -  0x0001

       -  High quality imaging mode. High quality mode is intended for still
	  imaging applications. The idea is to get the best possible image
	  quality that the hardware can deliver. It is not defined how the
	  driver writer may achieve that; it will depend on the hardware and
	  the ingenuity of the driver writer. High quality mode is a
	  different mode from the regular motion video capture modes. In
	  high quality mode:

	  -  The driver may be able to capture higher resolutions than for
	     motion capture.

	  -  The driver may support fewer pixel formats than motion capture
	     (eg; true color).

	  -  The driver may capture and arithmetically combine multiple
	     successive fields or frames to remove color edge artifacts and
	     reduce the noise in the video data.

	  -  The driver may capture images in slices like a scanner in order
	     to handle larger format images than would otherwise be
	     possible.

	  -  An image capture operation may be significantly slower than
	     motion capture.

	  -  Moving objects in the image might have excessive motion blur.

	  -  Capture might only work through the :ref:`read() <func-read>` call.


Return Value
============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.