Documentation/media/uapi/v4l/control.rst

   1 .. -*- coding: utf-8; mode: rst -*-
   2
   3 .. _control:
   4
   5 *************
   6 User Controls
   7 *************
   8
   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
  16 device.
  17
  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.
  24
  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.
  32
  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.
  38
  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.
  42
  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.
  47
  48 V4L2 specifies an event mechanism to notify applications when controls
  49 change value (see
  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.
  53
  54 All controls use machine endianness.
  55
  56
  57 .. _control-id:
  58
  59 Control 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 ``V4L2_CID_HFLIP`` ``(boolean)``
 141     Mirror the picture horizontally.
 142
 143 ``V4L2_CID_VFLIP`` ``(boolean)``
 144     Mirror the picture vertically.
 145
 146 .. _v4l2-power-line-frequency:
 147
 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).
 155
 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.
 160
 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
 165     temperature see
 166     `Wikipedia <http://en.wikipedia.org/wiki/Color_temperature>`__.
 167
 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.
 171
 172 ``V4L2_CID_BACKLIGHT_COMPENSATION`` ``(integer)``
 173     Adjusts the backlight compensation in a camera. The minimum value
 174     disables backlight compensation.
 175
 176 ``V4L2_CID_CHROMA_AGC`` ``(boolean)``
 177     Chroma automatic gain control.
 178
 179 ``V4L2_CID_CHROMA_GAIN`` ``(integer)``
 180     Adjusts the Chroma gain control (for use when chroma AGC is
 181     disabled).
 182
 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).
 186
 187 .. _v4l2-colorfx:
 188
 189 ``V4L2_CID_COLORFX`` ``(enum)``
 190     Selects a color effect. The following values are defined:
 191
 192
 193
 194 .. flat-table::
 195     :header-rows:  0
 196     :stub-columns: 0
 197
 198
 199     -  .. row 1
 200
 201        -  ``V4L2_COLORFX_NONE``
 202
 203        -  Color effect is disabled.
 204
 205     -  .. row 2
 206
 207        -  ``V4L2_COLORFX_ANTIQUE``
 208
 209        -  An aging (old photo) effect.
 210
 211     -  .. row 3
 212
 213        -  ``V4L2_COLORFX_ART_FREEZE``
 214
 215        -  Frost color effect.
 216
 217     -  .. row 4
 218
 219        -  ``V4L2_COLORFX_AQUA``
 220
 221        -  Water color, cool tone.
 222
 223     -  .. row 5
 224
 225        -  ``V4L2_COLORFX_BW``
 226
 227        -  Black and white.
 228
 229     -  .. row 6
 230
 231        -  ``V4L2_COLORFX_EMBOSS``
 232
 233        -  Emboss, the highlights and shadows replace light/dark boundaries
 234           and low contrast areas are set to a gray background.
 235
 236     -  .. row 7
 237
 238        -  ``V4L2_COLORFX_GRASS_GREEN``
 239
 240        -  Grass green.
 241
 242     -  .. row 8
 243
 244        -  ``V4L2_COLORFX_NEGATIVE``
 245
 246        -  Negative.
 247
 248     -  .. row 9
 249
 250        -  ``V4L2_COLORFX_SEPIA``
 251
 252        -  Sepia tone.
 253
 254     -  .. row 10
 255
 256        -  ``V4L2_COLORFX_SKETCH``
 257
 258        -  Sketch.
 259
 260     -  .. row 11
 261
 262        -  ``V4L2_COLORFX_SKIN_WHITEN``
 263
 264        -  Skin whiten.
 265
 266     -  .. row 12
 267
 268        -  ``V4L2_COLORFX_SKY_BLUE``
 269
 270        -  Sky blue.
 271
 272     -  .. row 13
 273
 274        -  ``V4L2_COLORFX_SOLARIZATION``
 275
 276        -  Solarization, the image is partially reversed in tone, only color
 277           values above or below a certain threshold are inverted.
 278
 279     -  .. row 14
 280
 281        -  ``V4L2_COLORFX_SILHOUETTE``
 282
 283        -  Silhouette (outline).
 284
 285     -  .. row 15
 286
 287        -  ``V4L2_COLORFX_VIVID``
 288
 289        -  Vivid colors.
 290
 291     -  .. row 16
 292
 293        -  ``V4L2_COLORFX_SET_CBCR``
 294
 295        -  The Cb and Cr chroma components are replaced by fixed coefficients
 296           determined by ``V4L2_CID_COLORFX_CBCR`` control.
 297
 298
 299
 300 ``V4L2_CID_COLORFX_CBCR`` ``(integer)``
 301     Determines the Cb and Cr coefficients for ``V4L2_COLORFX_SET_CBCR``
 302     color effect. Bits [7:0] of the supplied 32 bit value are
 303     interpreted as Cr component, bits [15:8] as Cb component and bits
 304     [31:16] must be zero.
 305
 306 ``V4L2_CID_AUTOBRIGHTNESS`` ``(boolean)``
 307     Enable Automatic Brightness.
 308
 309 ``V4L2_CID_ROTATE`` ``(integer)``
 310     Rotates the image by specified angle. Common angles are 90, 270 and
 311     180. Rotating the image to 90 and 270 will reverse the height and
 312     width of the display window. It is necessary to set the new height
 313     and width of the picture using the
 314     :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl according to the
 315     rotation angle selected.
 316
 317 ``V4L2_CID_BG_COLOR`` ``(integer)``
 318     Sets the background color on the current output device. Background
 319     color needs to be specified in the RGB24 format. The supplied 32 bit
 320     value is interpreted as bits 0-7 Red color information, bits 8-15
 321     Green color information, bits 16-23 Blue color information and bits
 322     24-31 must be zero.
 323
 324 ``V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2`` ``(boolean)``
 325     Switch on or off the illuminator 1 or 2 of the device (usually a
 326     microscope).
 327
 328 ``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE`` ``(integer)``
 329     This is a read-only control that can be read by the application and
 330     used as a hint to determine the number of CAPTURE buffers to pass to
 331     REQBUFS. The value is the minimum number of CAPTURE buffers that is
 332     necessary for hardware to work.
 333
 334 ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT`` ``(integer)``
 335     This is a read-only control that can be read by the application and
 336     used as a hint to determine the number of OUTPUT buffers to pass to
 337     REQBUFS. The value is the minimum number of OUTPUT buffers that is
 338     necessary for hardware to work.
 339
 340 .. _v4l2-alpha-component:
 341
 342 ``V4L2_CID_ALPHA_COMPONENT`` ``(integer)``
 343     Sets the alpha color component. When a capture device (or capture
 344     queue of a mem-to-mem device) produces a frame format that includes
 345     an alpha component (e.g.
 346     :ref:`packed RGB image formats <rgb-formats>`) and the alpha value
 347     is not defined by the device or the mem-to-mem input data this
 348     control lets you select the alpha component value of all pixels.
 349     When an output device (or output queue of a mem-to-mem device)
 350     consumes a frame format that doesn't include an alpha component and
 351     the device supports alpha channel processing this control lets you
 352     set the alpha component value of all pixels for further processing
 353     in the device.
 354
 355 ``V4L2_CID_LASTP1``
 356     End of the predefined control IDs (currently
 357     ``V4L2_CID_ALPHA_COMPONENT`` + 1).
 358
 359 ``V4L2_CID_PRIVATE_BASE``
 360     ID of the first custom (driver specific) control. Applications
 361     depending on particular custom controls should check the driver name
 362     and version, see :ref:`querycap`.
 363
 364 Applications can enumerate the available controls with the
 365 :ref:`VIDIOC_QUERYCTRL` and
 366 :ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a
 367 control value with the :ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and
 368 :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls. Drivers must implement
 369 ``VIDIOC_QUERYCTRL``, ``VIDIOC_G_CTRL`` and ``VIDIOC_S_CTRL`` when the
 370 device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or
 371 more menu type controls.
 372
 373
 374 .. _enum_all_controls:
 375
 376 Example: Enumerating all user controls
 377 ======================================
 378
 379 .. code-block:: c
 380
 381
 382     struct v4l2_queryctrl queryctrl;
 383     struct v4l2_querymenu querymenu;
 384
 385     static void enumerate_menu(void)
 386     {
 387         printf("  Menu items:\\n");
 388
 389         memset(&querymenu, 0, sizeof(querymenu));
 390         querymenu.id = queryctrl.id;
 391
 392         for (querymenu.index = queryctrl.minimum;
 393              querymenu.index <= queryctrl.maximum;
 394              querymenu.index++) {
 395             if (0 == ioctl(fd, VIDIOC_QUERYMENU, &querymenu)) {
 396                 printf("  %s\\n", querymenu.name);
 397             }
 398         }
 399     }
 400
 401     memset(&queryctrl, 0, sizeof(queryctrl));
 402
 403     for (queryctrl.id = V4L2_CID_BASE;
 404          queryctrl.id < V4L2_CID_LASTP1;
 405          queryctrl.id++) {
 406         if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
 407             if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
 408                 continue;
 409
 410             printf("Control %s\\n", queryctrl.name);
 411
 412             if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
 413                 enumerate_menu();
 414         } else {
 415             if (errno == EINVAL)
 416                 continue;
 417
 418             perror("VIDIOC_QUERYCTRL");
 419             exit(EXIT_FAILURE);
 420         }
 421     }
 422
 423     for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
 424          queryctrl.id++) {
 425         if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
 426             if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
 427                 continue;
 428
 429             printf("Control %s\\n", queryctrl.name);
 430
 431             if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
 432                 enumerate_menu();
 433         } else {
 434             if (errno == EINVAL)
 435                 break;
 436
 437             perror("VIDIOC_QUERYCTRL");
 438             exit(EXIT_FAILURE);
 439         }
 440     }
 441
 442
 443 Example: Enumerating all user controls (alternative)
 444 ====================================================
 445
 446 .. code-block:: c
 447
 448     memset(&queryctrl, 0, sizeof(queryctrl));
 449
 450     queryctrl.id = V4L2_CTRL_CLASS_USER | V4L2_CTRL_FLAG_NEXT_CTRL;
 451     while (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
 452         if (V4L2_CTRL_ID2CLASS(queryctrl.id) != V4L2_CTRL_CLASS_USER)
 453             break;
 454         if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
 455             continue;
 456
 457         printf("Control %s\\n", queryctrl.name);
 458
 459         if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
 460             enumerate_menu();
 461
 462         queryctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
 463     }
 464     if (errno != EINVAL) {
 465         perror("VIDIOC_QUERYCTRL");
 466         exit(EXIT_FAILURE);
 467     }
 468
 469 Example: Changing controls
 470 ==========================
 471
 472 .. code-block:: c
 473
 474     struct v4l2_queryctrl queryctrl;
 475     struct v4l2_control control;
 476
 477     memset(&queryctrl, 0, sizeof(queryctrl));
 478     queryctrl.id = V4L2_CID_BRIGHTNESS;
 479
 480     if (-1 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
 481         if (errno != EINVAL) {
 482             perror("VIDIOC_QUERYCTRL");
 483             exit(EXIT_FAILURE);
 484         } else {
 485             printf("V4L2_CID_BRIGHTNESS is not supportedn");
 486         }
 487     } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
 488         printf("V4L2_CID_BRIGHTNESS is not supportedn");
 489     } else {
 490         memset(&control, 0, sizeof (control));
 491         control.id = V4L2_CID_BRIGHTNESS;
 492         control.value = queryctrl.default_value;
 493
 494         if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)) {
 495             perror("VIDIOC_S_CTRL");
 496             exit(EXIT_FAILURE);
 497         }
 498     }
 499
 500     memset(&control, 0, sizeof(control));
 501     control.id = V4L2_CID_CONTRAST;
 502
 503     if (0 == ioctl(fd, VIDIOC_G_CTRL, &control)) {
 504         control.value += 1;
 505
 506         /* The driver may clamp the value or return ERANGE, ignored here */
 507
 508         if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)
 509             && errno != ERANGE) {
 510             perror("VIDIOC_S_CTRL");
 511             exit(EXIT_FAILURE);
 512         }
 513     /* Ignore if V4L2_CID_CONTRAST is unsupported */
 514     } else if (errno != EINVAL) {
 515         perror("VIDIOC_G_CTRL");
 516         exit(EXIT_FAILURE);
 517     }
 518
 519     control.id = V4L2_CID_AUDIO_MUTE;
 520     control.value = 1; /* silence */
 521
 522     /* Errors ignored */
 523     ioctl(fd, VIDIOC_S_CTRL, &control);
 524
 525 .. [#f1]
 526    The use of ``V4L2_CID_PRIVATE_BASE`` is problematic because different
 527    drivers may use the same ``V4L2_CID_PRIVATE_BASE`` ID for different
 528    controls. This makes it hard to programatically set such controls
 529    since the meaning of the control with that ID is driver dependent. In
 530    order to resolve this drivers use unique IDs and the
 531    ``V4L2_CID_PRIVATE_BASE`` IDs are mapped to those unique IDs by the
 532    kernel. Consider these ``V4L2_CID_PRIVATE_BASE`` IDs as aliases to
 533    the real IDs.
 534
 535    Many applications today still use the ``V4L2_CID_PRIVATE_BASE`` IDs
 536    instead of using :ref:`VIDIOC_QUERYCTRL` with
 537    the ``V4L2_CTRL_FLAG_NEXT_CTRL`` flag to enumerate all IDs, so
 538    support for ``V4L2_CID_PRIVATE_BASE`` is still around.