control.rst (17556B)
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 * - ``V4L2_COLORFX_SET_RGB`` 246 - The RGB components are replaced by the fixed RGB components determined 247 by ``V4L2_CID_COLORFX_RGB`` control. 248 249 250``V4L2_CID_COLORFX_RGB`` ``(integer)`` 251 Determines the Red, Green, and Blue coefficients for 252 ``V4L2_COLORFX_SET_RGB`` color effect. 253 Bits [7:0] of the supplied 32 bit value are interpreted as Blue component, 254 bits [15:8] as Green component, bits [23:16] as Red component, and 255 bits [31:24] must be zero. 256 257``V4L2_CID_COLORFX_CBCR`` ``(integer)`` 258 Determines the Cb and Cr coefficients for ``V4L2_COLORFX_SET_CBCR`` 259 color effect. Bits [7:0] of the supplied 32 bit value are 260 interpreted as Cr component, bits [15:8] as Cb component and bits 261 [31:16] must be zero. 262 263``V4L2_CID_AUTOBRIGHTNESS`` ``(boolean)`` 264 Enable Automatic Brightness. 265 266``V4L2_CID_ROTATE`` ``(integer)`` 267 Rotates the image by specified angle. Common angles are 90, 270 and 268 180. Rotating the image to 90 and 270 will reverse the height and 269 width of the display window. It is necessary to set the new height 270 and width of the picture using the 271 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl according to the 272 rotation angle selected. 273 274``V4L2_CID_BG_COLOR`` ``(integer)`` 275 Sets the background color on the current output device. Background 276 color needs to be specified in the RGB24 format. The supplied 32 bit 277 value is interpreted as bits 0-7 Red color information, bits 8-15 278 Green color information, bits 16-23 Blue color information and bits 279 24-31 must be zero. 280 281``V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2`` ``(boolean)`` 282 Switch on or off the illuminator 1 or 2 of the device (usually a 283 microscope). 284 285``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE`` ``(integer)`` 286 This is a read-only control that can be read by the application and 287 used as a hint to determine the number of CAPTURE buffers to pass to 288 REQBUFS. The value is the minimum number of CAPTURE buffers that is 289 necessary for hardware to work. 290 291``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT`` ``(integer)`` 292 This is a read-only control that can be read by the application and 293 used as a hint to determine the number of OUTPUT buffers to pass to 294 REQBUFS. The value is the minimum number of OUTPUT buffers that is 295 necessary for hardware to work. 296 297.. _v4l2-alpha-component: 298 299``V4L2_CID_ALPHA_COMPONENT`` ``(integer)`` 300 Sets the alpha color component. When a capture device (or capture 301 queue of a mem-to-mem device) produces a frame format that includes 302 an alpha component (e.g. 303 :ref:`packed RGB image formats <pixfmt-rgb>`) and the alpha value 304 is not defined by the device or the mem-to-mem input data this 305 control lets you select the alpha component value of all pixels. 306 When an output device (or output queue of a mem-to-mem device) 307 consumes a frame format that doesn't include an alpha component and 308 the device supports alpha channel processing this control lets you 309 set the alpha component value of all pixels for further processing 310 in the device. 311 312``V4L2_CID_LASTP1`` 313 End of the predefined control IDs (currently 314 ``V4L2_CID_ALPHA_COMPONENT`` + 1). 315 316``V4L2_CID_PRIVATE_BASE`` 317 ID of the first custom (driver specific) control. Applications 318 depending on particular custom controls should check the driver name 319 and version, see :ref:`querycap`. 320 321Applications can enumerate the available controls with the 322:ref:`VIDIOC_QUERYCTRL` and 323:ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a 324control value with the :ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and 325:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls. Drivers must implement 326``VIDIOC_QUERYCTRL``, ``VIDIOC_G_CTRL`` and ``VIDIOC_S_CTRL`` when the 327device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or 328more menu type controls. 329 330 331.. _enum_all_controls: 332 333Example: Enumerating all controls 334================================= 335 336.. code-block:: c 337 338 struct v4l2_queryctrl queryctrl; 339 struct v4l2_querymenu querymenu; 340 341 static void enumerate_menu(__u32 id) 342 { 343 printf(" Menu items:\\n"); 344 345 memset(&querymenu, 0, sizeof(querymenu)); 346 querymenu.id = id; 347 348 for (querymenu.index = queryctrl.minimum; 349 querymenu.index <= queryctrl.maximum; 350 querymenu.index++) { 351 if (0 == ioctl(fd, VIDIOC_QUERYMENU, &querymenu)) { 352 printf(" %s\\n", querymenu.name); 353 } 354 } 355 } 356 357 memset(&queryctrl, 0, sizeof(queryctrl)); 358 359 queryctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL; 360 while (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 361 if (!(queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)) { 362 printf("Control %s\\n", queryctrl.name); 363 364 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 365 enumerate_menu(queryctrl.id); 366 } 367 368 queryctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL; 369 } 370 if (errno != EINVAL) { 371 perror("VIDIOC_QUERYCTRL"); 372 exit(EXIT_FAILURE); 373 } 374 375Example: Enumerating all controls including compound controls 376============================================================= 377 378.. code-block:: c 379 380 struct v4l2_query_ext_ctrl query_ext_ctrl; 381 382 memset(&query_ext_ctrl, 0, sizeof(query_ext_ctrl)); 383 384 query_ext_ctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND; 385 while (0 == ioctl(fd, VIDIOC_QUERY_EXT_CTRL, &query_ext_ctrl)) { 386 if (!(query_ext_ctrl.flags & V4L2_CTRL_FLAG_DISABLED)) { 387 printf("Control %s\\n", query_ext_ctrl.name); 388 389 if (query_ext_ctrl.type == V4L2_CTRL_TYPE_MENU) 390 enumerate_menu(query_ext_ctrl.id); 391 } 392 393 query_ext_ctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND; 394 } 395 if (errno != EINVAL) { 396 perror("VIDIOC_QUERY_EXT_CTRL"); 397 exit(EXIT_FAILURE); 398 } 399 400Example: Enumerating all user controls (old style) 401================================================== 402 403.. code-block:: c 404 405 406 memset(&queryctrl, 0, sizeof(queryctrl)); 407 408 for (queryctrl.id = V4L2_CID_BASE; 409 queryctrl.id < V4L2_CID_LASTP1; 410 queryctrl.id++) { 411 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 412 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) 413 continue; 414 415 printf("Control %s\\n", queryctrl.name); 416 417 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 418 enumerate_menu(queryctrl.id); 419 } else { 420 if (errno == EINVAL) 421 continue; 422 423 perror("VIDIOC_QUERYCTRL"); 424 exit(EXIT_FAILURE); 425 } 426 } 427 428 for (queryctrl.id = V4L2_CID_PRIVATE_BASE;; 429 queryctrl.id++) { 430 if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 431 if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) 432 continue; 433 434 printf("Control %s\\n", queryctrl.name); 435 436 if (queryctrl.type == V4L2_CTRL_TYPE_MENU) 437 enumerate_menu(queryctrl.id); 438 } else { 439 if (errno == EINVAL) 440 break; 441 442 perror("VIDIOC_QUERYCTRL"); 443 exit(EXIT_FAILURE); 444 } 445 } 446 447 448Example: Changing controls 449========================== 450 451.. code-block:: c 452 453 struct v4l2_queryctrl queryctrl; 454 struct v4l2_control control; 455 456 memset(&queryctrl, 0, sizeof(queryctrl)); 457 queryctrl.id = V4L2_CID_BRIGHTNESS; 458 459 if (-1 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) { 460 if (errno != EINVAL) { 461 perror("VIDIOC_QUERYCTRL"); 462 exit(EXIT_FAILURE); 463 } else { 464 printf("V4L2_CID_BRIGHTNESS is not supportedn"); 465 } 466 } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) { 467 printf("V4L2_CID_BRIGHTNESS is not supportedn"); 468 } else { 469 memset(&control, 0, sizeof (control)); 470 control.id = V4L2_CID_BRIGHTNESS; 471 control.value = queryctrl.default_value; 472 473 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)) { 474 perror("VIDIOC_S_CTRL"); 475 exit(EXIT_FAILURE); 476 } 477 } 478 479 memset(&control, 0, sizeof(control)); 480 control.id = V4L2_CID_CONTRAST; 481 482 if (0 == ioctl(fd, VIDIOC_G_CTRL, &control)) { 483 control.value += 1; 484 485 /* The driver may clamp the value or return ERANGE, ignored here */ 486 487 if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control) 488 && errno != ERANGE) { 489 perror("VIDIOC_S_CTRL"); 490 exit(EXIT_FAILURE); 491 } 492 /* Ignore if V4L2_CID_CONTRAST is unsupported */ 493 } else if (errno != EINVAL) { 494 perror("VIDIOC_G_CTRL"); 495 exit(EXIT_FAILURE); 496 } 497 498 control.id = V4L2_CID_AUDIO_MUTE; 499 control.value = 1; /* silence */ 500 501 /* Errors ignored */ 502 ioctl(fd, VIDIOC_S_CTRL, &control); 503 504.. [#f1] 505 The use of ``V4L2_CID_PRIVATE_BASE`` is problematic because different 506 drivers may use the same ``V4L2_CID_PRIVATE_BASE`` ID for different 507 controls. This makes it hard to programmatically set such controls 508 since the meaning of the control with that ID is driver dependent. In 509 order to resolve this drivers use unique IDs and the 510 ``V4L2_CID_PRIVATE_BASE`` IDs are mapped to those unique IDs by the 511 kernel. Consider these ``V4L2_CID_PRIVATE_BASE`` IDs as aliases to 512 the real IDs. 513 514 Many applications today still use the ``V4L2_CID_PRIVATE_BASE`` IDs 515 instead of using :ref:`VIDIOC_QUERYCTRL` with 516 the ``V4L2_CTRL_FLAG_NEXT_CTRL`` flag to enumerate all IDs, so 517 support for ``V4L2_CID_PRIVATE_BASE`` is still around.