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 ``V4L2_CID_POWER_LINE_FREQUENCY_DISABLED`` (0), 158 ``V4L2_CID_POWER_LINE_FREQUENCY_50HZ`` (1), 159 ``V4L2_CID_POWER_LINE_FREQUENCY_60HZ`` (2) and 160 ``V4L2_CID_POWER_LINE_FREQUENCY_AUTO`` (3). 161 162``V4L2_CID_HUE_AUTO`` ``(boolean)`` 163 Enables automatic hue control by the device. The effect of setting 164 ``V4L2_CID_HUE`` while automatic hue control is enabled is 165 undefined, drivers should ignore such request. 166 167``V4L2_CID_WHITE_BALANCE_TEMPERATURE`` ``(integer)`` 168 This control specifies the white balance settings as a color 169 temperature in Kelvin. A driver should have a minimum of 2800 170 (incandescent) to 6500 (daylight). For more information about color 171 temperature see 172 `Wikipedia <http://en.wikipedia.org/wiki/Color_temperature>`__. 173 174``V4L2_CID_SHARPNESS`` ``(integer)`` 175 Adjusts the sharpness filters in a camera. The minimum value 176 disables the filters, higher values give a sharper picture. 177 178``V4L2_CID_BACKLIGHT_COMPENSATION`` ``(integer)`` 179 Adjusts the backlight compensation in a camera. The minimum value 180 disables backlight compensation. 181 182``V4L2_CID_CHROMA_AGC`` ``(boolean)`` 183 Chroma automatic gain control. 184 185``V4L2_CID_CHROMA_GAIN`` ``(integer)`` 186 Adjusts the Chroma gain control (for use when chroma AGC is 187 disabled). 188 189``V4L2_CID_COLOR_KILLER`` ``(boolean)`` 190 Enable the color killer (i. e. force a black & white image in case 191 of a weak video signal). 192 193.. _v4l2-colorfx: 194 195``V4L2_CID_COLORFX`` ``(enum)`` 196 Selects a color effect. The following values are defined: 197 198 199 200.. tabularcolumns:: |p{5.5cm}|p{12cm}| 201 202.. flat-table:: 203 :header-rows: 0 204 :stub-columns: 0 205 :widths: 11 24 206 207 * - ``V4L2_COLORFX_NONE`` 208 - Color effect is disabled. 209 * - ``V4L2_COLORFX_ANTIQUE`` 210 - An aging (old photo) effect. 211 * - ``V4L2_COLORFX_ART_FREEZE`` 212 - Frost color effect. 213 * - ``V4L2_COLORFX_AQUA`` 214 - Water color, cool tone. 215 * - ``V4L2_COLORFX_BW`` 216 - Black and white. 217 * - ``V4L2_COLORFX_EMBOSS`` 218 - Emboss, the highlights and shadows replace light/dark boundaries 219 and low contrast areas are set to a gray background. 220 * - ``V4L2_COLORFX_GRASS_GREEN`` 221 - Grass green. 222 * - ``V4L2_COLORFX_NEGATIVE`` 223 - Negative. 224 * - ``V4L2_COLORFX_SEPIA`` 225 - Sepia tone. 226 * - ``V4L2_COLORFX_SKETCH`` 227 - Sketch. 228 * - ``V4L2_COLORFX_SKIN_WHITEN`` 229 - Skin whiten. 230 * - ``V4L2_COLORFX_SKY_BLUE`` 231 - Sky blue. 232 * - ``V4L2_COLORFX_SOLARIZATION`` 233 - Solarization, the image is partially reversed in tone, only color 234 values above or below a certain threshold are inverted. 235 * - ``V4L2_COLORFX_SILHOUETTE`` 236 - Silhouette (outline). 237 * - ``V4L2_COLORFX_VIVID`` 238 - Vivid colors. 239 * - ``V4L2_COLORFX_SET_CBCR`` 240 - The Cb and Cr chroma components are replaced by fixed coefficients 241 determined by ``V4L2_CID_COLORFX_CBCR`` control. 242 243 244 245``V4L2_CID_COLORFX_CBCR`` ``(integer)`` 246 Determines the Cb and Cr coefficients for ``V4L2_COLORFX_SET_CBCR`` 247 color effect. Bits [7:0] of the supplied 32 bit value are 248 interpreted as Cr component, bits [15:8] as Cb component and bits 249 [31:16] must be zero. 250 251``V4L2_CID_AUTOBRIGHTNESS`` ``(boolean)`` 252 Enable Automatic Brightness. 253 254``V4L2_CID_ROTATE`` ``(integer)`` 255 Rotates the image by specified angle. Common angles are 90, 270 and 256 180. Rotating the image to 90 and 270 will reverse the height and 257 width of the display window. It is necessary to set the new height 258 and width of the picture using the 259 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl according to the 260 rotation angle selected. 261 262``V4L2_CID_BG_COLOR`` ``(integer)`` 263 Sets the background color on the current output device. Background 264 color needs to be specified in the RGB24 format. The supplied 32 bit 265 value is interpreted as bits 0-7 Red color information, bits 8-15 266 Green color information, bits 16-23 Blue color information and bits 267 24-31 must be zero. 268 269``V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2`` ``(boolean)`` 270 Switch on or off the illuminator 1 or 2 of the device (usually a 271 microscope). 272 273``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE`` ``(integer)`` 274 This is a read-only control that can be read by the application and 275 used as a hint to determine the number of CAPTURE buffers to pass to 276 REQBUFS. The value is the minimum number of CAPTURE buffers that is 277 necessary for hardware to work. 278 279``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT`` ``(integer)`` 280 This is a read-only control that can be read by the application and 281 used as a hint to determine the number of OUTPUT buffers to pass to 282 REQBUFS. The value is the minimum number of OUTPUT buffers that is 283 necessary for hardware to work. 284 285.. _v4l2-alpha-component: 286 287``V4L2_CID_ALPHA_COMPONENT`` ``(integer)`` 288 Sets the alpha color component. When a capture device (or capture 289 queue of a mem-to-mem device) produces a frame format that includes 290 an alpha component (e.g. 291 :ref:`packed RGB image formats <pixfmt-rgb>`) and the alpha value 292 is not defined by the device or the mem-to-mem input data this 293 control lets you select the alpha component value of all pixels. 294 When an output device (or output queue of a mem-to-mem device) 295 consumes a frame format that doesn't include an alpha component and 296 the device supports alpha channel processing this control lets you 297 set the alpha component value of all pixels for further processing 298 in the device. 299 300``V4L2_CID_LASTP1`` 301 End of the predefined control IDs (currently 302 ``V4L2_CID_ALPHA_COMPONENT`` + 1). 303 304``V4L2_CID_PRIVATE_BASE`` 305 ID of the first custom (driver specific) control. Applications 306 depending on particular custom controls should check the driver name 307 and version, see :ref:`querycap`. 308 309Applications can enumerate the available controls with the 310:ref:`VIDIOC_QUERYCTRL` and 311:ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a 312control value with the :ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and 313:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls. Drivers must implement 314``VIDIOC_QUERYCTRL``, ``VIDIOC_G_CTRL`` and ``VIDIOC_S_CTRL`` when the 315device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or 316more menu type controls. 317 318 319.. _enum_all_controls: 320 321Example: Enumerating all controls 322================================= 323 324.. code-block:: c 325 326 struct v4l2_queryctrl queryctrl; 327 struct v4l2_querymenu querymenu; 328 329 static void enumerate_menu(__u32 id) 330 { 331 printf(" Menu items:\\n"); 332 333 memset(&querymenu, 0, sizeof(querymenu)); 334 querymenu.id = id; 335 336 for (querymenu.index = queryctrl.minimum; 337 querymenu.index <= queryctrl.maximum; 338 querymenu.index++) { 339 if (0 == ioctl(fd, VIDIOC_QUERYMENU, &querymenu)) { 340 printf(" %s\\n", querymenu.name); 341 } 342 } 343 } 344 345 memset(&queryctrl, 0, sizeof(queryctrl)); 346 347 queryctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL; 348 while (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 349 if (!(queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)) { 350 printf("Control %s\\n", queryctrl.name); 351 352 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 353 enumerate_menu(queryctrl.id); 354 } 355 356 queryctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL; 357 } 358 if (errno != EINVAL) { 359 perror("VIDIOC_QUERYCTRL"); 360 exit(EXIT_FAILURE); 361 } 362 363Example: Enumerating all controls including compound controls 364============================================================= 365 366.. code-block:: c 367 368 struct v4l2_query_ext_ctrl query_ext_ctrl; 369 370 memset(&query_ext_ctrl, 0, sizeof(query_ext_ctrl)); 371 372 query_ext_ctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND; 373 while (0 == ioctl(fd, VIDIOC_QUERY_EXT_CTRL, &query_ext_ctrl)) { 374 if (!(query_ext_ctrl.flags & V4L2_CTRL_FLAG_DISABLED)) { 375 printf("Control %s\\n", query_ext_ctrl.name); 376 377 if (query_ext_ctrl.type == V4L2_CTRL_TYPE_MENU) 378 enumerate_menu(query_ext_ctrl.id); 379 } 380 381 query_ext_ctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND; 382 } 383 if (errno != EINVAL) { 384 perror("VIDIOC_QUERY_EXT_CTRL"); 385 exit(EXIT_FAILURE); 386 } 387 388Example: Enumerating all user controls (old style) 389================================================== 390 391.. code-block:: c 392 393 394 memset(&queryctrl, 0, sizeof(queryctrl)); 395 396 for (queryctrl.id = V4L2_CID_BASE; 397 queryctrl.id < V4L2_CID_LASTP1; 398 queryctrl.id++) { 399 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 400 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) 401 continue; 402 403 printf("Control %s\\n", queryctrl.name); 404 405 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 406 enumerate_menu(queryctrl.id); 407 } else { 408 if (errno == EINVAL) 409 continue; 410 411 perror("VIDIOC_QUERYCTRL"); 412 exit(EXIT_FAILURE); 413 } 414 } 415 416 for (queryctrl.id = V4L2_CID_PRIVATE_BASE;; 417 queryctrl.id++) { 418 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 419 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) 420 continue; 421 422 printf("Control %s\\n", queryctrl.name); 423 424 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 425 enumerate_menu(queryctrl.id); 426 } else { 427 if (errno == EINVAL) 428 break; 429 430 perror("VIDIOC_QUERYCTRL"); 431 exit(EXIT_FAILURE); 432 } 433 } 434 435 436Example: Changing controls 437========================== 438 439.. code-block:: c 440 441 struct v4l2_queryctrl queryctrl; 442 struct v4l2_control control; 443 444 memset(&queryctrl, 0, sizeof(queryctrl)); 445 queryctrl.id = V4L2_CID_BRIGHTNESS; 446 447 if (-1 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 448 if (errno != EINVAL) { 449 perror("VIDIOC_QUERYCTRL"); 450 exit(EXIT_FAILURE); 451 } else { 452 printf("V4L2_CID_BRIGHTNESS is not supportedn"); 453 } 454 } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) { 455 printf("V4L2_CID_BRIGHTNESS is not supportedn"); 456 } else { 457 memset(&control, 0, sizeof (control)); 458 control.id = V4L2_CID_BRIGHTNESS; 459 control.value = queryctrl.default_value; 460 461 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)) { 462 perror("VIDIOC_S_CTRL"); 463 exit(EXIT_FAILURE); 464 } 465 } 466 467 memset(&control, 0, sizeof(control)); 468 control.id = V4L2_CID_CONTRAST; 469 470 if (0 == ioctl(fd, VIDIOC_G_CTRL, &control)) { 471 control.value += 1; 472 473 /* The driver may clamp the value or return ERANGE, ignored here */ 474 475 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control) 476 && errno != ERANGE) { 477 perror("VIDIOC_S_CTRL"); 478 exit(EXIT_FAILURE); 479 } 480 /* Ignore if V4L2_CID_CONTRAST is unsupported */ 481 } else if (errno != EINVAL) { 482 perror("VIDIOC_G_CTRL"); 483 exit(EXIT_FAILURE); 484 } 485 486 control.id = V4L2_CID_AUDIO_MUTE; 487 control.value = 1; /* silence */ 488 489 /* Errors ignored */ 490 ioctl(fd, VIDIOC_S_CTRL, &control); 491 492.. [#f1] 493 The use of ``V4L2_CID_PRIVATE_BASE`` is problematic because different 494 drivers may use the same ``V4L2_CID_PRIVATE_BASE`` ID for different 495 controls. This makes it hard to programmatically set such controls 496 since the meaning of the control with that ID is driver dependent. In 497 order to resolve this drivers use unique IDs and the 498 ``V4L2_CID_PRIVATE_BASE`` IDs are mapped to those unique IDs by the 499 kernel. Consider these ``V4L2_CID_PRIVATE_BASE`` IDs as aliases to 500 the real IDs. 501 502 Many applications today still use the ``V4L2_CID_PRIVATE_BASE`` IDs 503 instead of using :ref:`VIDIOC_QUERYCTRL` with 504 the ``V4L2_CTRL_FLAG_NEXT_CTRL`` flag to enumerate all IDs, so 505 support for ``V4L2_CID_PRIVATE_BASE`` is still around. 506