1 .. -*- coding: utf-8; mode: rst -*-
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 ``V4L2_CID_HFLIP`` ``(boolean)``
141 Mirror the picture horizontally.
143 ``V4L2_CID_VFLIP`` ``(boolean)``
144 Mirror the picture vertically.
146 .. _v4l2-power-line-frequency:
148 ``V4L2_CID_POWER_LINE_FREQUENCY`` ``(enum)``
149 Enables a power line frequency filter to avoid flicker. Possible
150 values for ``enum v4l2_power_line_frequency`` are:
151 ``V4L2_CID_POWER_LINE_FREQUENCY_DISABLED`` (0),
152 ``V4L2_CID_POWER_LINE_FREQUENCY_50HZ`` (1),
153 ``V4L2_CID_POWER_LINE_FREQUENCY_60HZ`` (2) and
154 ``V4L2_CID_POWER_LINE_FREQUENCY_AUTO`` (3).
156 ``V4L2_CID_HUE_AUTO`` ``(boolean)``
157 Enables automatic hue control by the device. The effect of setting
158 ``V4L2_CID_HUE`` while automatic hue control is enabled is
159 undefined, drivers should ignore such request.
161 ``V4L2_CID_WHITE_BALANCE_TEMPERATURE`` ``(integer)``
162 This control specifies the white balance settings as a color
163 temperature in Kelvin. A driver should have a minimum of 2800
164 (incandescent) to 6500 (daylight). For more information about color
166 `Wikipedia <http://en.wikipedia.org/wiki/Color_temperature>`__.
168 ``V4L2_CID_SHARPNESS`` ``(integer)``
169 Adjusts the sharpness filters in a camera. The minimum value
170 disables the filters, higher values give a sharper picture.
172 ``V4L2_CID_BACKLIGHT_COMPENSATION`` ``(integer)``
173 Adjusts the backlight compensation in a camera. The minimum value
174 disables backlight compensation.
176 ``V4L2_CID_CHROMA_AGC`` ``(boolean)``
177 Chroma automatic gain control.
179 ``V4L2_CID_CHROMA_GAIN`` ``(integer)``
180 Adjusts the Chroma gain control (for use when chroma AGC is
183 ``V4L2_CID_COLOR_KILLER`` ``(boolean)``
184 Enable the color killer (i. e. force a black & white image in case
185 of a weak video signal).
189 ``V4L2_CID_COLORFX`` ``(enum)``
190 Selects a color effect. The following values are defined:
194 .. tabularcolumns:: |p{5.5cm}|p{12cm}|
203 - ``V4L2_COLORFX_NONE``
205 - Color effect is disabled.
209 - ``V4L2_COLORFX_ANTIQUE``
211 - An aging (old photo) effect.
215 - ``V4L2_COLORFX_ART_FREEZE``
217 - Frost color effect.
221 - ``V4L2_COLORFX_AQUA``
223 - Water color, cool tone.
227 - ``V4L2_COLORFX_BW``
233 - ``V4L2_COLORFX_EMBOSS``
235 - Emboss, the highlights and shadows replace light/dark boundaries
236 and low contrast areas are set to a gray background.
240 - ``V4L2_COLORFX_GRASS_GREEN``
246 - ``V4L2_COLORFX_NEGATIVE``
252 - ``V4L2_COLORFX_SEPIA``
258 - ``V4L2_COLORFX_SKETCH``
264 - ``V4L2_COLORFX_SKIN_WHITEN``
270 - ``V4L2_COLORFX_SKY_BLUE``
276 - ``V4L2_COLORFX_SOLARIZATION``
278 - Solarization, the image is partially reversed in tone, only color
279 values above or below a certain threshold are inverted.
283 - ``V4L2_COLORFX_SILHOUETTE``
285 - Silhouette (outline).
289 - ``V4L2_COLORFX_VIVID``
295 - ``V4L2_COLORFX_SET_CBCR``
297 - The Cb and Cr chroma components are replaced by fixed coefficients
298 determined by ``V4L2_CID_COLORFX_CBCR`` control.
302 ``V4L2_CID_COLORFX_CBCR`` ``(integer)``
303 Determines the Cb and Cr coefficients for ``V4L2_COLORFX_SET_CBCR``
304 color effect. Bits [7:0] of the supplied 32 bit value are
305 interpreted as Cr component, bits [15:8] as Cb component and bits
306 [31:16] must be zero.
308 ``V4L2_CID_AUTOBRIGHTNESS`` ``(boolean)``
309 Enable Automatic Brightness.
311 ``V4L2_CID_ROTATE`` ``(integer)``
312 Rotates the image by specified angle. Common angles are 90, 270 and
313 180. Rotating the image to 90 and 270 will reverse the height and
314 width of the display window. It is necessary to set the new height
315 and width of the picture using the
316 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl according to the
317 rotation angle selected.
319 ``V4L2_CID_BG_COLOR`` ``(integer)``
320 Sets the background color on the current output device. Background
321 color needs to be specified in the RGB24 format. The supplied 32 bit
322 value is interpreted as bits 0-7 Red color information, bits 8-15
323 Green color information, bits 16-23 Blue color information and bits
326 ``V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2`` ``(boolean)``
327 Switch on or off the illuminator 1 or 2 of the device (usually a
330 ``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE`` ``(integer)``
331 This is a read-only control that can be read by the application and
332 used as a hint to determine the number of CAPTURE buffers to pass to
333 REQBUFS. The value is the minimum number of CAPTURE buffers that is
334 necessary for hardware to work.
336 ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT`` ``(integer)``
337 This is a read-only control that can be read by the application and
338 used as a hint to determine the number of OUTPUT buffers to pass to
339 REQBUFS. The value is the minimum number of OUTPUT buffers that is
340 necessary for hardware to work.
342 .. _v4l2-alpha-component:
344 ``V4L2_CID_ALPHA_COMPONENT`` ``(integer)``
345 Sets the alpha color component. When a capture device (or capture
346 queue of a mem-to-mem device) produces a frame format that includes
347 an alpha component (e.g.
348 :ref:`packed RGB image formats <rgb-formats>`) and the alpha value
349 is not defined by the device or the mem-to-mem input data this
350 control lets you select the alpha component value of all pixels.
351 When an output device (or output queue of a mem-to-mem device)
352 consumes a frame format that doesn't include an alpha component and
353 the device supports alpha channel processing this control lets you
354 set the alpha component value of all pixels for further processing
358 End of the predefined control IDs (currently
359 ``V4L2_CID_ALPHA_COMPONENT`` + 1).
361 ``V4L2_CID_PRIVATE_BASE``
362 ID of the first custom (driver specific) control. Applications
363 depending on particular custom controls should check the driver name
364 and version, see :ref:`querycap`.
366 Applications can enumerate the available controls with the
367 :ref:`VIDIOC_QUERYCTRL` and
368 :ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a
369 control value with the :ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and
370 :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls. Drivers must implement
371 ``VIDIOC_QUERYCTRL``, ``VIDIOC_G_CTRL`` and ``VIDIOC_S_CTRL`` when the
372 device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or
373 more menu type controls.
376 .. _enum_all_controls:
378 Example: Enumerating all user controls
379 ======================================
384 struct v4l2_queryctrl queryctrl;
385 struct v4l2_querymenu querymenu;
387 static void enumerate_menu(void)
389 printf(" Menu items:\\n");
391 memset(&querymenu, 0, sizeof(querymenu));
392 querymenu.id = queryctrl.id;
394 for (querymenu.index = queryctrl.minimum;
395 querymenu.index <= queryctrl.maximum;
397 if (0 == ioctl(fd, VIDIOC_QUERYMENU, &querymenu)) {
398 printf(" %s\\n", querymenu.name);
403 memset(&queryctrl, 0, sizeof(queryctrl));
405 for (queryctrl.id = V4L2_CID_BASE;
406 queryctrl.id < V4L2_CID_LASTP1;
408 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
409 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
412 printf("Control %s\\n", queryctrl.name);
414 if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
420 perror("VIDIOC_QUERYCTRL");
425 for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
427 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
428 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
431 printf("Control %s\\n", queryctrl.name);
433 if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
439 perror("VIDIOC_QUERYCTRL");
445 Example: Enumerating all user controls (alternative)
446 ====================================================
450 memset(&queryctrl, 0, sizeof(queryctrl));
452 queryctrl.id = V4L2_CTRL_CLASS_USER | V4L2_CTRL_FLAG_NEXT_CTRL;
453 while (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
454 if (V4L2_CTRL_ID2CLASS(queryctrl.id) != V4L2_CTRL_CLASS_USER)
456 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
459 printf("Control %s\\n", queryctrl.name);
461 if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
464 queryctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
466 if (errno != EINVAL) {
467 perror("VIDIOC_QUERYCTRL");
471 Example: Changing controls
472 ==========================
476 struct v4l2_queryctrl queryctrl;
477 struct v4l2_control control;
479 memset(&queryctrl, 0, sizeof(queryctrl));
480 queryctrl.id = V4L2_CID_BRIGHTNESS;
482 if (-1 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
483 if (errno != EINVAL) {
484 perror("VIDIOC_QUERYCTRL");
487 printf("V4L2_CID_BRIGHTNESS is not supportedn");
489 } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
490 printf("V4L2_CID_BRIGHTNESS is not supportedn");
492 memset(&control, 0, sizeof (control));
493 control.id = V4L2_CID_BRIGHTNESS;
494 control.value = queryctrl.default_value;
496 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)) {
497 perror("VIDIOC_S_CTRL");
502 memset(&control, 0, sizeof(control));
503 control.id = V4L2_CID_CONTRAST;
505 if (0 == ioctl(fd, VIDIOC_G_CTRL, &control)) {
508 /* The driver may clamp the value or return ERANGE, ignored here */
510 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)
511 && errno != ERANGE) {
512 perror("VIDIOC_S_CTRL");
515 /* Ignore if V4L2_CID_CONTRAST is unsupported */
516 } else if (errno != EINVAL) {
517 perror("VIDIOC_G_CTRL");
521 control.id = V4L2_CID_AUDIO_MUTE;
522 control.value = 1; /* silence */
525 ioctl(fd, VIDIOC_S_CTRL, &control);
528 The use of ``V4L2_CID_PRIVATE_BASE`` is problematic because different
529 drivers may use the same ``V4L2_CID_PRIVATE_BASE`` ID for different
530 controls. This makes it hard to programatically set such controls
531 since the meaning of the control with that ID is driver dependent. In
532 order to resolve this drivers use unique IDs and the
533 ``V4L2_CID_PRIVATE_BASE`` IDs are mapped to those unique IDs by the
534 kernel. Consider these ``V4L2_CID_PRIVATE_BASE`` IDs as aliases to
537 Many applications today still use the ``V4L2_CID_PRIVATE_BASE`` IDs
538 instead of using :ref:`VIDIOC_QUERYCTRL` with
539 the ``V4L2_CTRL_FLAG_NEXT_CTRL`` flag to enumerate all IDs, so
540 support for ``V4L2_CID_PRIVATE_BASE`` is still around.