1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
9 Devices typically have a number of user-settable controls such as
10 brightness, saturation and so on, which would be presented to the user
11 on a graphical user interface. But, different devices will have
12 different controls available, and furthermore, the range of possible
13 values, and the default value will vary from device to device. The
14 control ioctls provide the information and a mechanism to create a nice
15 user interface for these controls that will work correctly with any
18 All controls are accessed using an ID value. V4L2 defines several IDs
19 for specific purposes. Drivers can also implement their own custom
20 controls using ``V4L2_CID_PRIVATE_BASE`` [#f1]_ and higher values. The
21 pre-defined control IDs have the prefix ``V4L2_CID_``, and are listed in
22 :ref:`control-id`. The ID is used when querying the attributes of a
23 control, and when getting or setting the current value.
25 Generally applications should present controls to the user without
26 assumptions about their purpose. Each control comes with a name string
27 the user is supposed to understand. When the purpose is non-intuitive
28 the driver writer should provide a user manual, a user interface plug-in
29 or a driver specific panel application. Predefined IDs were introduced
30 to change a few controls programmatically, for example to mute a device
31 during a channel switch.
33 Drivers may enumerate different controls after switching the current
34 video input or output, tuner or modulator, or audio input or output.
35 Different in the sense of other bounds, another default and current
36 value, step size or other menu items. A control with a certain *custom*
37 ID can also change name and type.
39 If a control is not applicable to the current configuration of the
40 device (for example, it doesn't apply to the current video input)
41 drivers set the ``V4L2_CTRL_FLAG_INACTIVE`` flag.
43 Control values are stored globally, they do not change when switching
44 except to stay within the reported bounds. They also do not change e. g.
45 when the device is opened or closed, when the tuner radio frequency is
46 changed or generally never without application request.
48 V4L2 specifies an event mechanism to notify applications when controls
50 :ref:`VIDIOC_SUBSCRIBE_EVENT`, event
51 ``V4L2_EVENT_CTRL``), panel applications might want to make use of that
52 in order to always reflect the correct control value.
54 All controls use machine endianness.
63 First predefined ID, equal to ``V4L2_CID_BRIGHTNESS``.
65 ``V4L2_CID_USER_BASE``
66 Synonym of ``V4L2_CID_BASE``.
68 ``V4L2_CID_BRIGHTNESS`` ``(integer)``
69 Picture brightness, or more precisely, the black level.
71 ``V4L2_CID_CONTRAST`` ``(integer)``
72 Picture contrast or luma gain.
74 ``V4L2_CID_SATURATION`` ``(integer)``
75 Picture color saturation or chroma gain.
77 ``V4L2_CID_HUE`` ``(integer)``
80 ``V4L2_CID_AUDIO_VOLUME`` ``(integer)``
81 Overall audio volume. Note some drivers also provide an OSS or ALSA
84 ``V4L2_CID_AUDIO_BALANCE`` ``(integer)``
85 Audio stereo balance. Minimum corresponds to all the way left,
88 ``V4L2_CID_AUDIO_BASS`` ``(integer)``
89 Audio bass adjustment.
91 ``V4L2_CID_AUDIO_TREBLE`` ``(integer)``
92 Audio treble adjustment.
94 ``V4L2_CID_AUDIO_MUTE`` ``(boolean)``
95 Mute audio, i. e. set the volume to zero, however without affecting
96 ``V4L2_CID_AUDIO_VOLUME``. Like ALSA drivers, V4L2 drivers must mute
97 at load time to avoid excessive noise. Actually the entire device
98 should be reset to a low power consumption state.
100 ``V4L2_CID_AUDIO_LOUDNESS`` ``(boolean)``
101 Loudness mode (bass boost).
103 ``V4L2_CID_BLACK_LEVEL`` ``(integer)``
104 Another name for brightness (not a synonym of
105 ``V4L2_CID_BRIGHTNESS``). This control is deprecated and should not
106 be used in new drivers and applications.
108 ``V4L2_CID_AUTO_WHITE_BALANCE`` ``(boolean)``
109 Automatic white balance (cameras).
111 ``V4L2_CID_DO_WHITE_BALANCE`` ``(button)``
112 This is an action control. When set (the value is ignored), the
113 device will do a white balance and then hold the current setting.
114 Contrast this with the boolean ``V4L2_CID_AUTO_WHITE_BALANCE``,
115 which, when activated, keeps adjusting the white balance.
117 ``V4L2_CID_RED_BALANCE`` ``(integer)``
120 ``V4L2_CID_BLUE_BALANCE`` ``(integer)``
123 ``V4L2_CID_GAMMA`` ``(integer)``
126 ``V4L2_CID_WHITENESS`` ``(integer)``
127 Whiteness for grey-scale devices. This is a synonym for
128 ``V4L2_CID_GAMMA``. This control is deprecated and should not be
129 used in new drivers and applications.
131 ``V4L2_CID_EXPOSURE`` ``(integer)``
132 Exposure (cameras). [Unit?]
134 ``V4L2_CID_AUTOGAIN`` ``(boolean)``
135 Automatic gain/exposure control.
137 ``V4L2_CID_GAIN`` ``(integer)``
140 Primarily used to control gain on e.g. TV tuners but also on
141 webcams. Most devices control only digital gain with this control
142 but on some this could include analogue gain as well. Devices that
143 recognise the difference between digital and analogue gain use
144 controls ``V4L2_CID_DIGITAL_GAIN`` and ``V4L2_CID_ANALOGUE_GAIN``.
148 ``V4L2_CID_HFLIP`` ``(boolean)``
149 Mirror the picture horizontally.
153 ``V4L2_CID_VFLIP`` ``(boolean)``
154 Mirror the picture vertically.
156 .. _v4l2-power-line-frequency:
158 ``V4L2_CID_POWER_LINE_FREQUENCY`` ``(enum)``
159 Enables a power line frequency filter to avoid flicker. Possible
160 values for ``enum v4l2_power_line_frequency`` are:
162 ========================================== ==
163 ``V4L2_CID_POWER_LINE_FREQUENCY_DISABLED`` 0
164 ``V4L2_CID_POWER_LINE_FREQUENCY_50HZ`` 1
165 ``V4L2_CID_POWER_LINE_FREQUENCY_60HZ`` 2
166 ``V4L2_CID_POWER_LINE_FREQUENCY_AUTO`` 3
167 ========================================== ==
169 ``V4L2_CID_HUE_AUTO`` ``(boolean)``
170 Enables automatic hue control by the device. The effect of setting
171 ``V4L2_CID_HUE`` while automatic hue control is enabled is
172 undefined, drivers should ignore such request.
174 ``V4L2_CID_WHITE_BALANCE_TEMPERATURE`` ``(integer)``
175 This control specifies the white balance settings as a color
176 temperature in Kelvin. A driver should have a minimum of 2800
177 (incandescent) to 6500 (daylight). For more information about color
179 `Wikipedia <http://en.wikipedia.org/wiki/Color_temperature>`__.
181 ``V4L2_CID_SHARPNESS`` ``(integer)``
182 Adjusts the sharpness filters in a camera. The minimum value
183 disables the filters, higher values give a sharper picture.
185 ``V4L2_CID_BACKLIGHT_COMPENSATION`` ``(integer)``
186 Adjusts the backlight compensation in a camera. The minimum value
187 disables backlight compensation.
189 ``V4L2_CID_CHROMA_AGC`` ``(boolean)``
190 Chroma automatic gain control.
192 ``V4L2_CID_CHROMA_GAIN`` ``(integer)``
193 Adjusts the Chroma gain control (for use when chroma AGC is
196 ``V4L2_CID_COLOR_KILLER`` ``(boolean)``
197 Enable the color killer (i. e. force a black & white image in case
198 of a weak video signal).
202 ``V4L2_CID_COLORFX`` ``(enum)``
203 Selects a color effect. The following values are defined:
207 .. tabularcolumns:: |p{5.7cm}|p{11.8cm}|
214 * - ``V4L2_COLORFX_NONE``
215 - Color effect is disabled.
216 * - ``V4L2_COLORFX_ANTIQUE``
217 - An aging (old photo) effect.
218 * - ``V4L2_COLORFX_ART_FREEZE``
219 - Frost color effect.
220 * - ``V4L2_COLORFX_AQUA``
221 - Water color, cool tone.
222 * - ``V4L2_COLORFX_BW``
224 * - ``V4L2_COLORFX_EMBOSS``
225 - Emboss, the highlights and shadows replace light/dark boundaries
226 and low contrast areas are set to a gray background.
227 * - ``V4L2_COLORFX_GRASS_GREEN``
229 * - ``V4L2_COLORFX_NEGATIVE``
231 * - ``V4L2_COLORFX_SEPIA``
233 * - ``V4L2_COLORFX_SKETCH``
235 * - ``V4L2_COLORFX_SKIN_WHITEN``
237 * - ``V4L2_COLORFX_SKY_BLUE``
239 * - ``V4L2_COLORFX_SOLARIZATION``
240 - Solarization, the image is partially reversed in tone, only color
241 values above or below a certain threshold are inverted.
242 * - ``V4L2_COLORFX_SILHOUETTE``
243 - Silhouette (outline).
244 * - ``V4L2_COLORFX_VIVID``
246 * - ``V4L2_COLORFX_SET_CBCR``
247 - The Cb and Cr chroma components are replaced by fixed coefficients
248 determined by ``V4L2_CID_COLORFX_CBCR`` control.
249 * - ``V4L2_COLORFX_SET_RGB``
250 - The RGB components are replaced by the fixed RGB components determined
251 by ``V4L2_CID_COLORFX_RGB`` control.
254 ``V4L2_CID_COLORFX_RGB`` ``(integer)``
255 Determines the Red, Green, and Blue coefficients for
256 ``V4L2_COLORFX_SET_RGB`` color effect.
257 Bits [7:0] of the supplied 32 bit value are interpreted as Blue component,
258 bits [15:8] as Green component, bits [23:16] as Red component, and
259 bits [31:24] must be zero.
261 ``V4L2_CID_COLORFX_CBCR`` ``(integer)``
262 Determines the Cb and Cr coefficients for ``V4L2_COLORFX_SET_CBCR``
263 color effect. Bits [7:0] of the supplied 32 bit value are
264 interpreted as Cr component, bits [15:8] as Cb component and bits
265 [31:16] must be zero.
267 ``V4L2_CID_AUTOBRIGHTNESS`` ``(boolean)``
268 Enable Automatic Brightness.
270 ``V4L2_CID_ROTATE`` ``(integer)``
271 Rotates the image by specified angle. Common angles are 90, 270 and
272 180. Rotating the image to 90 and 270 will reverse the height and
273 width of the display window. It is necessary to set the new height
274 and width of the picture using the
275 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl according to the
276 rotation angle selected.
278 ``V4L2_CID_BG_COLOR`` ``(integer)``
279 Sets the background color on the current output device. Background
280 color needs to be specified in the RGB24 format. The supplied 32 bit
281 value is interpreted as bits 0-7 Red color information, bits 8-15
282 Green color information, bits 16-23 Blue color information and bits
285 ``V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2`` ``(boolean)``
286 Switch on or off the illuminator 1 or 2 of the device (usually a
289 ``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE`` ``(integer)``
290 This is a read-only control that can be read by the application and
291 used as a hint to determine the number of CAPTURE buffers to pass to
292 REQBUFS. The value is the minimum number of CAPTURE buffers that is
293 necessary for hardware to work.
295 ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT`` ``(integer)``
296 This is a read-only control that can be read by the application and
297 used as a hint to determine the number of OUTPUT buffers to pass to
298 REQBUFS. The value is the minimum number of OUTPUT buffers that is
299 necessary for hardware to work.
301 .. _v4l2-alpha-component:
303 ``V4L2_CID_ALPHA_COMPONENT`` ``(integer)``
304 Sets the alpha color component. When a capture device (or capture
305 queue of a mem-to-mem device) produces a frame format that includes
306 an alpha component (e.g.
307 :ref:`packed RGB image formats <pixfmt-rgb>`) and the alpha value
308 is not defined by the device or the mem-to-mem input data this
309 control lets you select the alpha component value of all pixels.
310 When an output device (or output queue of a mem-to-mem device)
311 consumes a frame format that doesn't include an alpha component and
312 the device supports alpha channel processing this control lets you
313 set the alpha component value of all pixels for further processing
317 End of the predefined control IDs (currently
318 ``V4L2_CID_ALPHA_COMPONENT`` + 1).
320 ``V4L2_CID_PRIVATE_BASE``
321 ID of the first custom (driver specific) control. Applications
322 depending on particular custom controls should check the driver name
323 and version, see :ref:`querycap`.
325 Applications can enumerate the available controls with the
326 :ref:`VIDIOC_QUERYCTRL` and
327 :ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a
328 control value with the :ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and
329 :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls. Drivers must implement
330 ``VIDIOC_QUERYCTRL``, ``VIDIOC_G_CTRL`` and ``VIDIOC_S_CTRL`` when the
331 device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or
332 more menu type controls.
335 .. _enum_all_controls:
337 Example: Enumerating all controls
338 =================================
342 struct v4l2_queryctrl queryctrl;
343 struct v4l2_querymenu querymenu;
345 static void enumerate_menu(__u32 id)
347 printf(" Menu items:\\n");
349 memset(&querymenu, 0, sizeof(querymenu));
352 for (querymenu.index = queryctrl.minimum;
353 querymenu.index <= queryctrl.maximum;
355 if (0 == ioctl(fd, VIDIOC_QUERYMENU, &querymenu)) {
356 printf(" %s\\n", querymenu.name);
361 memset(&queryctrl, 0, sizeof(queryctrl));
363 queryctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL;
364 while (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
365 if (!(queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)) {
366 printf("Control %s\\n", queryctrl.name);
368 if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
369 enumerate_menu(queryctrl.id);
372 queryctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
374 if (errno != EINVAL) {
375 perror("VIDIOC_QUERYCTRL");
379 Example: Enumerating all controls including compound controls
380 =============================================================
384 struct v4l2_query_ext_ctrl query_ext_ctrl;
386 memset(&query_ext_ctrl, 0, sizeof(query_ext_ctrl));
388 query_ext_ctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND;
389 while (0 == ioctl(fd, VIDIOC_QUERY_EXT_CTRL, &query_ext_ctrl)) {
390 if (!(query_ext_ctrl.flags & V4L2_CTRL_FLAG_DISABLED)) {
391 printf("Control %s\\n", query_ext_ctrl.name);
393 if (query_ext_ctrl.type == V4L2_CTRL_TYPE_MENU)
394 enumerate_menu(query_ext_ctrl.id);
397 query_ext_ctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND;
399 if (errno != EINVAL) {
400 perror("VIDIOC_QUERY_EXT_CTRL");
404 Example: Enumerating all user controls (old style)
405 ==================================================
410 memset(&queryctrl, 0, sizeof(queryctrl));
412 for (queryctrl.id = V4L2_CID_BASE;
413 queryctrl.id < V4L2_CID_LASTP1;
415 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
416 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
419 printf("Control %s\\n", queryctrl.name);
421 if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
422 enumerate_menu(queryctrl.id);
427 perror("VIDIOC_QUERYCTRL");
432 for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
434 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
435 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
438 printf("Control %s\\n", queryctrl.name);
440 if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
441 enumerate_menu(queryctrl.id);
446 perror("VIDIOC_QUERYCTRL");
452 Example: Changing controls
453 ==========================
457 struct v4l2_queryctrl queryctrl;
458 struct v4l2_control control;
460 memset(&queryctrl, 0, sizeof(queryctrl));
461 queryctrl.id = V4L2_CID_BRIGHTNESS;
463 if (-1 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
464 if (errno != EINVAL) {
465 perror("VIDIOC_QUERYCTRL");
468 printf("V4L2_CID_BRIGHTNESS is not supported\n");
470 } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
471 printf("V4L2_CID_BRIGHTNESS is not supported\n");
473 memset(&control, 0, sizeof (control));
474 control.id = V4L2_CID_BRIGHTNESS;
475 control.value = queryctrl.default_value;
477 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)) {
478 perror("VIDIOC_S_CTRL");
483 memset(&control, 0, sizeof(control));
484 control.id = V4L2_CID_CONTRAST;
486 if (0 == ioctl(fd, VIDIOC_G_CTRL, &control)) {
489 /* The driver may clamp the value or return ERANGE, ignored here */
491 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)
492 && errno != ERANGE) {
493 perror("VIDIOC_S_CTRL");
496 /* Ignore if V4L2_CID_CONTRAST is unsupported */
497 } else if (errno != EINVAL) {
498 perror("VIDIOC_G_CTRL");
502 control.id = V4L2_CID_AUDIO_MUTE;
503 control.value = 1; /* silence */
506 ioctl(fd, VIDIOC_S_CTRL, &control);
509 The use of ``V4L2_CID_PRIVATE_BASE`` is problematic because different
510 drivers may use the same ``V4L2_CID_PRIVATE_BASE`` ID for different
511 controls. This makes it hard to programmatically set such controls
512 since the meaning of the control with that ID is driver dependent. In
513 order to resolve this drivers use unique IDs and the
514 ``V4L2_CID_PRIVATE_BASE`` IDs are mapped to those unique IDs by the
515 kernel. Consider these ``V4L2_CID_PRIVATE_BASE`` IDs as aliases to
518 Many applications today still use the ``V4L2_CID_PRIVATE_BASE`` IDs
519 instead of using :ref:`VIDIOC_QUERYCTRL` with
520 the ``V4L2_CTRL_FLAG_NEXT_CTRL`` flag to enumerate all IDs, so
521 support for ``V4L2_CID_PRIVATE_BASE`` is still around.