1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 3.. _control: 4 5************* 6User Controls 7************* 8 9Devices typically have a number of user-settable controls such as 10brightness, saturation and so on, which would be presented to the user 11on a graphical user interface. But, different devices will have 12different controls available, and furthermore, the range of possible 13values, and the default value will vary from device to device. The 14control ioctls provide the information and a mechanism to create a nice 15user interface for these controls that will work correctly with any 16device. 17 18All controls are accessed using an ID value. V4L2 defines several IDs 19for specific purposes. Drivers can also implement their own custom 20controls using ``V4L2_CID_PRIVATE_BASE`` [#f1]_ and higher values. The 21pre-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 23control, and when getting or setting the current value. 24 25Generally applications should present controls to the user without 26assumptions about their purpose. Each control comes with a name string 27the user is supposed to understand. When the purpose is non-intuitive 28the driver writer should provide a user manual, a user interface plug-in 29or a driver specific panel application. Predefined IDs were introduced 30to change a few controls programmatically, for example to mute a device 31during a channel switch. 32 33Drivers may enumerate different controls after switching the current 34video input or output, tuner or modulator, or audio input or output. 35Different in the sense of other bounds, another default and current 36value, step size or other menu items. A control with a certain *custom* 37ID can also change name and type. 38 39If a control is not applicable to the current configuration of the 40device (for example, it doesn't apply to the current video input) 41drivers set the ``V4L2_CTRL_FLAG_INACTIVE`` flag. 42 43Control values are stored globally, they do not change when switching 44except to stay within the reported bounds. They also do not change e. g. 45when the device is opened or closed, when the tuner radio frequency is 46changed or generally never without application request. 47 48V4L2 specifies an event mechanism to notify applications when controls 49change value (see 50:ref:`VIDIOC_SUBSCRIBE_EVENT`, event 51``V4L2_EVENT_CTRL``), panel applications might want to make use of that 52in order to always reflect the correct control value. 53 54All controls use machine endianness. 55 56 57.. _control-id: 58 59Control IDs 60=========== 61 62``V4L2_CID_BASE`` 63 First predefined ID, equal to ``V4L2_CID_BRIGHTNESS``. 64 65``V4L2_CID_USER_BASE`` 66 Synonym of ``V4L2_CID_BASE``. 67 68``V4L2_CID_BRIGHTNESS`` ``(integer)`` 69 Picture brightness, or more precisely, the black level. 70 71``V4L2_CID_CONTRAST`` ``(integer)`` 72 Picture contrast or luma gain. 73 74``V4L2_CID_SATURATION`` ``(integer)`` 75 Picture color saturation or chroma gain. 76 77``V4L2_CID_HUE`` ``(integer)`` 78 Hue or color balance. 79 80``V4L2_CID_AUDIO_VOLUME`` ``(integer)`` 81 Overall audio volume. Note some drivers also provide an OSS or ALSA 82 mixer interface. 83 84``V4L2_CID_AUDIO_BALANCE`` ``(integer)`` 85 Audio stereo balance. Minimum corresponds to all the way left, 86 maximum to right. 87 88``V4L2_CID_AUDIO_BASS`` ``(integer)`` 89 Audio bass adjustment. 90 91``V4L2_CID_AUDIO_TREBLE`` ``(integer)`` 92 Audio treble adjustment. 93 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. 99 100``V4L2_CID_AUDIO_LOUDNESS`` ``(boolean)`` 101 Loudness mode (bass boost). 102 103``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. 107 108``V4L2_CID_AUTO_WHITE_BALANCE`` ``(boolean)`` 109 Automatic white balance (cameras). 110 111``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. 116 117``V4L2_CID_RED_BALANCE`` ``(integer)`` 118 Red chroma balance. 119 120``V4L2_CID_BLUE_BALANCE`` ``(integer)`` 121 Blue chroma balance. 122 123``V4L2_CID_GAMMA`` ``(integer)`` 124 Gamma adjust. 125 126``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. 130 131``V4L2_CID_EXPOSURE`` ``(integer)`` 132 Exposure (cameras). [Unit?] 133 134``V4L2_CID_AUTOGAIN`` ``(boolean)`` 135 Automatic gain/exposure control. 136 137``V4L2_CID_GAIN`` ``(integer)`` 138 Gain control. 139 140 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``. 145 146``V4L2_CID_HFLIP`` ``(boolean)`` 147 Mirror the picture horizontally. 148 149``V4L2_CID_VFLIP`` ``(boolean)`` 150 Mirror the picture vertically. 151 152.. _v4l2-power-line-frequency: 153 154``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: 157 158 ========================================== == 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 ========================================== == 164 165``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. 169 170``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 174 temperature see 175 `Wikipedia <http://en.wikipedia.org/wiki/Color_temperature>`__. 176 177``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. 180 181``V4L2_CID_BACKLIGHT_COMPENSATION`` ``(integer)`` 182 Adjusts the backlight compensation in a camera. The minimum value 183 disables backlight compensation. 184 185``V4L2_CID_CHROMA_AGC`` ``(boolean)`` 186 Chroma automatic gain control. 187 188``V4L2_CID_CHROMA_GAIN`` ``(integer)`` 189 Adjusts the Chroma gain control (for use when chroma AGC is 190 disabled). 191 192``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). 195 196.. _v4l2-colorfx: 197 198``V4L2_CID_COLORFX`` ``(enum)`` 199 Selects a color effect. The following values are defined: 200 201 202 203.. tabularcolumns:: |p{5.7cm}|p{11.8cm}| 204 205.. flat-table:: 206 :header-rows: 0 207 :stub-columns: 0 208 :widths: 11 24 209 210 * - ``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`` 219 - Black and white. 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`` 224 - Grass green. 225 * - ``V4L2_COLORFX_NEGATIVE`` 226 - Negative. 227 * - ``V4L2_COLORFX_SEPIA`` 228 - Sepia tone. 229 * - ``V4L2_COLORFX_SKETCH`` 230 - Sketch. 231 * - ``V4L2_COLORFX_SKIN_WHITEN`` 232 - Skin whiten. 233 * - ``V4L2_COLORFX_SKY_BLUE`` 234 - 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`` 241 - Vivid colors. 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. 245 246 247 248``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. 253 254``V4L2_CID_AUTOBRIGHTNESS`` ``(boolean)`` 255 Enable Automatic Brightness. 256 257``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. 264 265``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 270 24-31 must be zero. 271 272``V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2`` ``(boolean)`` 273 Switch on or off the illuminator 1 or 2 of the device (usually a 274 microscope). 275 276``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. 281 282``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. 287 288.. _v4l2-alpha-component: 289 290``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 301 in the device. 302 303``V4L2_CID_LASTP1`` 304 End of the predefined control IDs (currently 305 ``V4L2_CID_ALPHA_COMPONENT`` + 1). 306 307``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`. 311 312Applications can enumerate the available controls with the 313:ref:`VIDIOC_QUERYCTRL` and 314:ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a 315control 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 318device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or 319more menu type controls. 320 321 322.. _enum_all_controls: 323 324Example: Enumerating all controls 325================================= 326 327.. code-block:: c 328 329 struct v4l2_queryctrl queryctrl; 330 struct v4l2_querymenu querymenu; 331 332 static void enumerate_menu(__u32 id) 333 { 334 printf(" Menu items:\\n"); 335 336 memset(&querymenu, 0, sizeof(querymenu)); 337 querymenu.id = id; 338 339 for (querymenu.index = queryctrl.minimum; 340 querymenu.index <= queryctrl.maximum; 341 querymenu.index++) { 342 if (0 == ioctl(fd, VIDIOC_QUERYMENU, &querymenu)) { 343 printf(" %s\\n", querymenu.name); 344 } 345 } 346 } 347 348 memset(&queryctrl, 0, sizeof(queryctrl)); 349 350 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); 354 355 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 356 enumerate_menu(queryctrl.id); 357 } 358 359 queryctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL; 360 } 361 if (errno != EINVAL) { 362 perror("VIDIOC_QUERYCTRL"); 363 exit(EXIT_FAILURE); 364 } 365 366Example: Enumerating all controls including compound controls 367============================================================= 368 369.. code-block:: c 370 371 struct v4l2_query_ext_ctrl query_ext_ctrl; 372 373 memset(&query_ext_ctrl, 0, sizeof(query_ext_ctrl)); 374 375 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); 379 380 if (query_ext_ctrl.type == V4L2_CTRL_TYPE_MENU) 381 enumerate_menu(query_ext_ctrl.id); 382 } 383 384 query_ext_ctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND; 385 } 386 if (errno != EINVAL) { 387 perror("VIDIOC_QUERY_EXT_CTRL"); 388 exit(EXIT_FAILURE); 389 } 390 391Example: Enumerating all user controls (old style) 392================================================== 393 394.. code-block:: c 395 396 397 memset(&queryctrl, 0, sizeof(queryctrl)); 398 399 for (queryctrl.id = V4L2_CID_BASE; 400 queryctrl.id < V4L2_CID_LASTP1; 401 queryctrl.id++) { 402 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 403 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) 404 continue; 405 406 printf("Control %s\\n", queryctrl.name); 407 408 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 409 enumerate_menu(queryctrl.id); 410 } else { 411 if (errno == EINVAL) 412 continue; 413 414 perror("VIDIOC_QUERYCTRL"); 415 exit(EXIT_FAILURE); 416 } 417 } 418 419 for (queryctrl.id = V4L2_CID_PRIVATE_BASE;; 420 queryctrl.id++) { 421 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 422 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) 423 continue; 424 425 printf("Control %s\\n", queryctrl.name); 426 427 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 428 enumerate_menu(queryctrl.id); 429 } else { 430 if (errno == EINVAL) 431 break; 432 433 perror("VIDIOC_QUERYCTRL"); 434 exit(EXIT_FAILURE); 435 } 436 } 437 438 439Example: Changing controls 440========================== 441 442.. code-block:: c 443 444 struct v4l2_queryctrl queryctrl; 445 struct v4l2_control control; 446 447 memset(&queryctrl, 0, sizeof(queryctrl)); 448 queryctrl.id = V4L2_CID_BRIGHTNESS; 449 450 if (-1 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 451 if (errno != EINVAL) { 452 perror("VIDIOC_QUERYCTRL"); 453 exit(EXIT_FAILURE); 454 } else { 455 printf("V4L2_CID_BRIGHTNESS is not supportedn"); 456 } 457 } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) { 458 printf("V4L2_CID_BRIGHTNESS is not supportedn"); 459 } else { 460 memset(&control, 0, sizeof (control)); 461 control.id = V4L2_CID_BRIGHTNESS; 462 control.value = queryctrl.default_value; 463 464 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)) { 465 perror("VIDIOC_S_CTRL"); 466 exit(EXIT_FAILURE); 467 } 468 } 469 470 memset(&control, 0, sizeof(control)); 471 control.id = V4L2_CID_CONTRAST; 472 473 if (0 == ioctl(fd, VIDIOC_G_CTRL, &control)) { 474 control.value += 1; 475 476 /* The driver may clamp the value or return ERANGE, ignored here */ 477 478 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control) 479 && errno != ERANGE) { 480 perror("VIDIOC_S_CTRL"); 481 exit(EXIT_FAILURE); 482 } 483 /* Ignore if V4L2_CID_CONTRAST is unsupported */ 484 } else if (errno != EINVAL) { 485 perror("VIDIOC_G_CTRL"); 486 exit(EXIT_FAILURE); 487 } 488 489 control.id = V4L2_CID_AUDIO_MUTE; 490 control.value = 1; /* silence */ 491 492 /* Errors ignored */ 493 ioctl(fd, VIDIOC_S_CTRL, &control); 494 495.. [#f1] 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 503 the real IDs. 504 505 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. 509