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: 147 148``V4L2_CID_HFLIP`` ``(boolean)`` 149 Mirror the picture horizontally. 150 151.. _v4l2-cid-vflip: 152 153``V4L2_CID_VFLIP`` ``(boolean)`` 154 Mirror the picture vertically. 155 156.. _v4l2-power-line-frequency: 157 158``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: 161 162 ========================================== == 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 ========================================== == 168 169``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. 173 174``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 178 temperature see 179 `Wikipedia <http://en.wikipedia.org/wiki/Color_temperature>`__. 180 181``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. 184 185``V4L2_CID_BACKLIGHT_COMPENSATION`` ``(integer)`` 186 Adjusts the backlight compensation in a camera. The minimum value 187 disables backlight compensation. 188 189``V4L2_CID_CHROMA_AGC`` ``(boolean)`` 190 Chroma automatic gain control. 191 192``V4L2_CID_CHROMA_GAIN`` ``(integer)`` 193 Adjusts the Chroma gain control (for use when chroma AGC is 194 disabled). 195 196``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). 199 200.. _v4l2-colorfx: 201 202``V4L2_CID_COLORFX`` ``(enum)`` 203 Selects a color effect. The following values are defined: 204 205 206 207.. tabularcolumns:: |p{5.7cm}|p{11.8cm}| 208 209.. flat-table:: 210 :header-rows: 0 211 :stub-columns: 0 212 :widths: 11 24 213 214 * - ``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`` 223 - Black and white. 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`` 228 - Grass green. 229 * - ``V4L2_COLORFX_NEGATIVE`` 230 - Negative. 231 * - ``V4L2_COLORFX_SEPIA`` 232 - Sepia tone. 233 * - ``V4L2_COLORFX_SKETCH`` 234 - Sketch. 235 * - ``V4L2_COLORFX_SKIN_WHITEN`` 236 - Skin whiten. 237 * - ``V4L2_COLORFX_SKY_BLUE`` 238 - 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`` 245 - Vivid colors. 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. 252 253 254``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. 260 261``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. 266 267``V4L2_CID_AUTOBRIGHTNESS`` ``(boolean)`` 268 Enable Automatic Brightness. 269 270``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. 277 278``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 283 24-31 must be zero. 284 285``V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2`` ``(boolean)`` 286 Switch on or off the illuminator 1 or 2 of the device (usually a 287 microscope). 288 289``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. 294 295``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. 300 301.. _v4l2-alpha-component: 302 303``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 314 in the device. 315 316``V4L2_CID_LASTP1`` 317 End of the predefined control IDs (currently 318 ``V4L2_CID_ALPHA_COMPONENT`` + 1). 319 320``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`. 324 325Applications can enumerate the available controls with the 326:ref:`VIDIOC_QUERYCTRL` and 327:ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a 328control 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 331device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or 332more menu type controls. 333 334 335.. _enum_all_controls: 336 337Example: Enumerating all controls 338================================= 339 340.. code-block:: c 341 342 struct v4l2_queryctrl queryctrl; 343 struct v4l2_querymenu querymenu; 344 345 static void enumerate_menu(__u32 id) 346 { 347 printf(" Menu items:\\n"); 348 349 memset(&querymenu, 0, sizeof(querymenu)); 350 querymenu.id = id; 351 352 for (querymenu.index = queryctrl.minimum; 353 querymenu.index <= queryctrl.maximum; 354 querymenu.index++) { 355 if (0 == ioctl(fd, VIDIOC_QUERYMENU, &querymenu)) { 356 printf(" %s\\n", querymenu.name); 357 } 358 } 359 } 360 361 memset(&queryctrl, 0, sizeof(queryctrl)); 362 363 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); 367 368 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 369 enumerate_menu(queryctrl.id); 370 } 371 372 queryctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL; 373 } 374 if (errno != EINVAL) { 375 perror("VIDIOC_QUERYCTRL"); 376 exit(EXIT_FAILURE); 377 } 378 379Example: Enumerating all controls including compound controls 380============================================================= 381 382.. code-block:: c 383 384 struct v4l2_query_ext_ctrl query_ext_ctrl; 385 386 memset(&query_ext_ctrl, 0, sizeof(query_ext_ctrl)); 387 388 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); 392 393 if (query_ext_ctrl.type == V4L2_CTRL_TYPE_MENU) 394 enumerate_menu(query_ext_ctrl.id); 395 } 396 397 query_ext_ctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND; 398 } 399 if (errno != EINVAL) { 400 perror("VIDIOC_QUERY_EXT_CTRL"); 401 exit(EXIT_FAILURE); 402 } 403 404Example: Enumerating all user controls (old style) 405================================================== 406 407.. code-block:: c 408 409 410 memset(&queryctrl, 0, sizeof(queryctrl)); 411 412 for (queryctrl.id = V4L2_CID_BASE; 413 queryctrl.id < V4L2_CID_LASTP1; 414 queryctrl.id++) { 415 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 416 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) 417 continue; 418 419 printf("Control %s\\n", queryctrl.name); 420 421 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 422 enumerate_menu(queryctrl.id); 423 } else { 424 if (errno == EINVAL) 425 continue; 426 427 perror("VIDIOC_QUERYCTRL"); 428 exit(EXIT_FAILURE); 429 } 430 } 431 432 for (queryctrl.id = V4L2_CID_PRIVATE_BASE;; 433 queryctrl.id++) { 434 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 435 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) 436 continue; 437 438 printf("Control %s\\n", queryctrl.name); 439 440 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 441 enumerate_menu(queryctrl.id); 442 } else { 443 if (errno == EINVAL) 444 break; 445 446 perror("VIDIOC_QUERYCTRL"); 447 exit(EXIT_FAILURE); 448 } 449 } 450 451 452Example: Changing controls 453========================== 454 455.. code-block:: c 456 457 struct v4l2_queryctrl queryctrl; 458 struct v4l2_control control; 459 460 memset(&queryctrl, 0, sizeof(queryctrl)); 461 queryctrl.id = V4L2_CID_BRIGHTNESS; 462 463 if (-1 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 464 if (errno != EINVAL) { 465 perror("VIDIOC_QUERYCTRL"); 466 exit(EXIT_FAILURE); 467 } else { 468 printf("V4L2_CID_BRIGHTNESS is not supported\n"); 469 } 470 } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) { 471 printf("V4L2_CID_BRIGHTNESS is not supported\n"); 472 } else { 473 memset(&control, 0, sizeof (control)); 474 control.id = V4L2_CID_BRIGHTNESS; 475 control.value = queryctrl.default_value; 476 477 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)) { 478 perror("VIDIOC_S_CTRL"); 479 exit(EXIT_FAILURE); 480 } 481 } 482 483 memset(&control, 0, sizeof(control)); 484 control.id = V4L2_CID_CONTRAST; 485 486 if (0 == ioctl(fd, VIDIOC_G_CTRL, &control)) { 487 control.value += 1; 488 489 /* The driver may clamp the value or return ERANGE, ignored here */ 490 491 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control) 492 && errno != ERANGE) { 493 perror("VIDIOC_S_CTRL"); 494 exit(EXIT_FAILURE); 495 } 496 /* Ignore if V4L2_CID_CONTRAST is unsupported */ 497 } else if (errno != EINVAL) { 498 perror("VIDIOC_G_CTRL"); 499 exit(EXIT_FAILURE); 500 } 501 502 control.id = V4L2_CID_AUDIO_MUTE; 503 control.value = 1; /* silence */ 504 505 /* Errors ignored */ 506 ioctl(fd, VIDIOC_S_CTRL, &control); 507 508.. [#f1] 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 516 the real IDs. 517 518 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. 522