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``.
146 ``V4L2_CID_HFLIP`` ``(boolean)``
147 Mirror the picture horizontally.
149 ``V4L2_CID_VFLIP`` ``(boolean)``
150 Mirror the picture vertically.
152 .. _v4l2-power-line-frequency:
154 ``V4L2_CID_POWER_LINE_FREQUENCY`` ``(enum)``
155 Enables a power line frequency filter to avoid flicker. Possible
156 values for ``enum v4l2_power_line_frequency`` are:
158 ========================================== ==
159 ``V4L2_CID_POWER_LINE_FREQUENCY_DISABLED`` 0
160 ``V4L2_CID_POWER_LINE_FREQUENCY_50HZ`` 1
161 ``V4L2_CID_POWER_LINE_FREQUENCY_60HZ`` 2
162 ``V4L2_CID_POWER_LINE_FREQUENCY_AUTO`` 3
163 ========================================== ==
165 ``V4L2_CID_HUE_AUTO`` ``(boolean)``
166 Enables automatic hue control by the device. The effect of setting
167 ``V4L2_CID_HUE`` while automatic hue control is enabled is
168 undefined, drivers should ignore such request.
170 ``V4L2_CID_WHITE_BALANCE_TEMPERATURE`` ``(integer)``
171 This control specifies the white balance settings as a color
172 temperature in Kelvin. A driver should have a minimum of 2800
173 (incandescent) to 6500 (daylight). For more information about color
175 `Wikipedia <http://en.wikipedia.org/wiki/Color_temperature>`__.
177 ``V4L2_CID_SHARPNESS`` ``(integer)``
178 Adjusts the sharpness filters in a camera. The minimum value
179 disables the filters, higher values give a sharper picture.
181 ``V4L2_CID_BACKLIGHT_COMPENSATION`` ``(integer)``
182 Adjusts the backlight compensation in a camera. The minimum value
183 disables backlight compensation.
185 ``V4L2_CID_CHROMA_AGC`` ``(boolean)``
186 Chroma automatic gain control.
188 ``V4L2_CID_CHROMA_GAIN`` ``(integer)``
189 Adjusts the Chroma gain control (for use when chroma AGC is
192 ``V4L2_CID_COLOR_KILLER`` ``(boolean)``
193 Enable the color killer (i. e. force a black & white image in case
194 of a weak video signal).
198 ``V4L2_CID_COLORFX`` ``(enum)``
199 Selects a color effect. The following values are defined:
203 .. tabularcolumns:: |p{5.7cm}|p{11.8cm}|
210 * - ``V4L2_COLORFX_NONE``
211 - Color effect is disabled.
212 * - ``V4L2_COLORFX_ANTIQUE``
213 - An aging (old photo) effect.
214 * - ``V4L2_COLORFX_ART_FREEZE``
215 - Frost color effect.
216 * - ``V4L2_COLORFX_AQUA``
217 - Water color, cool tone.
218 * - ``V4L2_COLORFX_BW``
220 * - ``V4L2_COLORFX_EMBOSS``
221 - Emboss, the highlights and shadows replace light/dark boundaries
222 and low contrast areas are set to a gray background.
223 * - ``V4L2_COLORFX_GRASS_GREEN``
225 * - ``V4L2_COLORFX_NEGATIVE``
227 * - ``V4L2_COLORFX_SEPIA``
229 * - ``V4L2_COLORFX_SKETCH``
231 * - ``V4L2_COLORFX_SKIN_WHITEN``
233 * - ``V4L2_COLORFX_SKY_BLUE``
235 * - ``V4L2_COLORFX_SOLARIZATION``
236 - Solarization, the image is partially reversed in tone, only color
237 values above or below a certain threshold are inverted.
238 * - ``V4L2_COLORFX_SILHOUETTE``
239 - Silhouette (outline).
240 * - ``V4L2_COLORFX_VIVID``
242 * - ``V4L2_COLORFX_SET_CBCR``
243 - The Cb and Cr chroma components are replaced by fixed coefficients
244 determined by ``V4L2_CID_COLORFX_CBCR`` control.
248 ``V4L2_CID_COLORFX_CBCR`` ``(integer)``
249 Determines the Cb and Cr coefficients for ``V4L2_COLORFX_SET_CBCR``
250 color effect. Bits [7:0] of the supplied 32 bit value are
251 interpreted as Cr component, bits [15:8] as Cb component and bits
252 [31:16] must be zero.
254 ``V4L2_CID_AUTOBRIGHTNESS`` ``(boolean)``
255 Enable Automatic Brightness.
257 ``V4L2_CID_ROTATE`` ``(integer)``
258 Rotates the image by specified angle. Common angles are 90, 270 and
259 180. Rotating the image to 90 and 270 will reverse the height and
260 width of the display window. It is necessary to set the new height
261 and width of the picture using the
262 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl according to the
263 rotation angle selected.
265 ``V4L2_CID_BG_COLOR`` ``(integer)``
266 Sets the background color on the current output device. Background
267 color needs to be specified in the RGB24 format. The supplied 32 bit
268 value is interpreted as bits 0-7 Red color information, bits 8-15
269 Green color information, bits 16-23 Blue color information and bits
272 ``V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2`` ``(boolean)``
273 Switch on or off the illuminator 1 or 2 of the device (usually a
276 ``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE`` ``(integer)``
277 This is a read-only control that can be read by the application and
278 used as a hint to determine the number of CAPTURE buffers to pass to
279 REQBUFS. The value is the minimum number of CAPTURE buffers that is
280 necessary for hardware to work.
282 ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT`` ``(integer)``
283 This is a read-only control that can be read by the application and
284 used as a hint to determine the number of OUTPUT buffers to pass to
285 REQBUFS. The value is the minimum number of OUTPUT buffers that is
286 necessary for hardware to work.
288 .. _v4l2-alpha-component:
290 ``V4L2_CID_ALPHA_COMPONENT`` ``(integer)``
291 Sets the alpha color component. When a capture device (or capture
292 queue of a mem-to-mem device) produces a frame format that includes
293 an alpha component (e.g.
294 :ref:`packed RGB image formats <pixfmt-rgb>`) and the alpha value
295 is not defined by the device or the mem-to-mem input data this
296 control lets you select the alpha component value of all pixels.
297 When an output device (or output queue of a mem-to-mem device)
298 consumes a frame format that doesn't include an alpha component and
299 the device supports alpha channel processing this control lets you
300 set the alpha component value of all pixels for further processing
304 End of the predefined control IDs (currently
305 ``V4L2_CID_ALPHA_COMPONENT`` + 1).
307 ``V4L2_CID_PRIVATE_BASE``
308 ID of the first custom (driver specific) control. Applications
309 depending on particular custom controls should check the driver name
310 and version, see :ref:`querycap`.
312 Applications can enumerate the available controls with the
313 :ref:`VIDIOC_QUERYCTRL` and
314 :ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a
315 control value with the :ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and
316 :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls. Drivers must implement
317 ``VIDIOC_QUERYCTRL``, ``VIDIOC_G_CTRL`` and ``VIDIOC_S_CTRL`` when the
318 device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or
319 more menu type controls.
322 .. _enum_all_controls:
324 Example: Enumerating all controls
325 =================================
329 struct v4l2_queryctrl queryctrl;
330 struct v4l2_querymenu querymenu;
332 static void enumerate_menu(__u32 id)
334 printf(" Menu items:\\n");
336 memset(&querymenu, 0, sizeof(querymenu));
339 for (querymenu.index = queryctrl.minimum;
340 querymenu.index <= queryctrl.maximum;
342 if (0 == ioctl(fd, VIDIOC_QUERYMENU, &querymenu)) {
343 printf(" %s\\n", querymenu.name);
348 memset(&queryctrl, 0, sizeof(queryctrl));
350 queryctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL;
351 while (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
352 if (!(queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)) {
353 printf("Control %s\\n", queryctrl.name);
355 if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
356 enumerate_menu(queryctrl.id);
359 queryctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
361 if (errno != EINVAL) {
362 perror("VIDIOC_QUERYCTRL");
366 Example: Enumerating all controls including compound controls
367 =============================================================
371 struct v4l2_query_ext_ctrl query_ext_ctrl;
373 memset(&query_ext_ctrl, 0, sizeof(query_ext_ctrl));
375 query_ext_ctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND;
376 while (0 == ioctl(fd, VIDIOC_QUERY_EXT_CTRL, &query_ext_ctrl)) {
377 if (!(query_ext_ctrl.flags & V4L2_CTRL_FLAG_DISABLED)) {
378 printf("Control %s\\n", query_ext_ctrl.name);
380 if (query_ext_ctrl.type == V4L2_CTRL_TYPE_MENU)
381 enumerate_menu(query_ext_ctrl.id);
384 query_ext_ctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND;
386 if (errno != EINVAL) {
387 perror("VIDIOC_QUERY_EXT_CTRL");
391 Example: Enumerating all user controls (old style)
392 ==================================================
397 memset(&queryctrl, 0, sizeof(queryctrl));
399 for (queryctrl.id = V4L2_CID_BASE;
400 queryctrl.id < V4L2_CID_LASTP1;
402 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
403 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
406 printf("Control %s\\n", queryctrl.name);
408 if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
409 enumerate_menu(queryctrl.id);
414 perror("VIDIOC_QUERYCTRL");
419 for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
421 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
422 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
425 printf("Control %s\\n", queryctrl.name);
427 if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
428 enumerate_menu(queryctrl.id);
433 perror("VIDIOC_QUERYCTRL");
439 Example: Changing controls
440 ==========================
444 struct v4l2_queryctrl queryctrl;
445 struct v4l2_control control;
447 memset(&queryctrl, 0, sizeof(queryctrl));
448 queryctrl.id = V4L2_CID_BRIGHTNESS;
450 if (-1 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
451 if (errno != EINVAL) {
452 perror("VIDIOC_QUERYCTRL");
455 printf("V4L2_CID_BRIGHTNESS is not supportedn");
457 } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
458 printf("V4L2_CID_BRIGHTNESS is not supportedn");
460 memset(&control, 0, sizeof (control));
461 control.id = V4L2_CID_BRIGHTNESS;
462 control.value = queryctrl.default_value;
464 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)) {
465 perror("VIDIOC_S_CTRL");
470 memset(&control, 0, sizeof(control));
471 control.id = V4L2_CID_CONTRAST;
473 if (0 == ioctl(fd, VIDIOC_G_CTRL, &control)) {
476 /* The driver may clamp the value or return ERANGE, ignored here */
478 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)
479 && errno != ERANGE) {
480 perror("VIDIOC_S_CTRL");
483 /* Ignore if V4L2_CID_CONTRAST is unsupported */
484 } else if (errno != EINVAL) {
485 perror("VIDIOC_G_CTRL");
489 control.id = V4L2_CID_AUDIO_MUTE;
490 control.value = 1; /* silence */
493 ioctl(fd, VIDIOC_S_CTRL, &control);
496 The use of ``V4L2_CID_PRIVATE_BASE`` is problematic because different
497 drivers may use the same ``V4L2_CID_PRIVATE_BASE`` ID for different
498 controls. This makes it hard to programmatically set such controls
499 since the meaning of the control with that ID is driver dependent. In
500 order to resolve this drivers use unique IDs and the
501 ``V4L2_CID_PRIVATE_BASE`` IDs are mapped to those unique IDs by the
502 kernel. Consider these ``V4L2_CID_PRIVATE_BASE`` IDs as aliases to
505 Many applications today still use the ``V4L2_CID_PRIVATE_BASE`` IDs
506 instead of using :ref:`VIDIOC_QUERYCTRL` with
507 the ``V4L2_CTRL_FLAG_NEXT_CTRL`` flag to enumerate all IDs, so
508 support for ``V4L2_CID_PRIVATE_BASE`` is still around.