media/v4l/control.rst

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _control:

*************
User Controls
*************

Devices typically have a number of user-settable controls such as
brightness, saturation and so on, which would be presented to the user
on a graphical user interface. But, different devices will have
different controls available, and furthermore, the range of possible
values, and the default value will vary from device to device. The
control ioctls provide the information and a mechanism to create a nice
user interface for these controls that will work correctly with any
device.

All controls are accessed using an ID value. V4L2 defines several IDs
for specific purposes. Drivers can also implement their own custom
controls using ``V4L2_CID_PRIVATE_BASE``  [#f1]_ and higher values. The
pre-defined control IDs have the prefix ``V4L2_CID_``, and are listed in
:ref:`control-id`. The ID is used when querying the attributes of a
control, and when getting or setting the current value.

Generally applications should present controls to the user without
assumptions about their purpose. Each control comes with a name string
the user is supposed to understand. When the purpose is non-intuitive
the driver writer should provide a user manual, a user interface plug-in
or a driver specific panel application. Predefined IDs were introduced
to change a few controls programmatically, for example to mute a device
during a channel switch.

Drivers may enumerate different controls after switching the current
video input or output, tuner or modulator, or audio input or output.
Different in the sense of other bounds, another default and current
value, step size or other menu items. A control with a certain *custom*
ID can also change name and type.

If a control is not applicable to the current configuration of the
device (for example, it doesn't apply to the current video input)
drivers set the ``V4L2_CTRL_FLAG_INACTIVE`` flag.

Control values are stored globally, they do not change when switching
except to stay within the reported bounds. They also do not change e. g.
when the device is opened or closed, when the tuner radio frequency is
changed or generally never without application request.

V4L2 specifies an event mechanism to notify applications when controls
change value (see
:ref:`VIDIOC_SUBSCRIBE_EVENT`, event
``V4L2_EVENT_CTRL``), panel applications might want to make use of that
in order to always reflect the correct control value.

All controls use machine endianness.


.. _control-id:

Control IDs
===========

``V4L2_CID_BASE``
    First predefined ID, equal to ``V4L2_CID_BRIGHTNESS``.

``V4L2_CID_USER_BASE``
    Synonym of ``V4L2_CID_BASE``.

``V4L2_CID_BRIGHTNESS`` ``(integer)``
    Picture brightness, or more precisely, the black level.

``V4L2_CID_CONTRAST`` ``(integer)``
    Picture contrast or luma gain.

``V4L2_CID_SATURATION`` ``(integer)``
    Picture color saturation or chroma gain.

``V4L2_CID_HUE`` ``(integer)``
    Hue or color balance.

``V4L2_CID_AUDIO_VOLUME`` ``(integer)``
    Overall audio volume. Note some drivers also provide an OSS or ALSA
    mixer interface.

``V4L2_CID_AUDIO_BALANCE`` ``(integer)``
    Audio stereo balance. Minimum corresponds to all the way left,
    maximum to right.

``V4L2_CID_AUDIO_BASS`` ``(integer)``
    Audio bass adjustment.

``V4L2_CID_AUDIO_TREBLE`` ``(integer)``
    Audio treble adjustment.

``V4L2_CID_AUDIO_MUTE`` ``(boolean)``
    Mute audio, i. e. set the volume to zero, however without affecting
    ``V4L2_CID_AUDIO_VOLUME``. Like ALSA drivers, V4L2 drivers must mute
    at load time to avoid excessive noise. Actually the entire device
    should be reset to a low power consumption state.

``V4L2_CID_AUDIO_LOUDNESS`` ``(boolean)``
    Loudness mode (bass boost).

``V4L2_CID_BLACK_LEVEL`` ``(integer)``
    Another name for brightness (not a synonym of
    ``V4L2_CID_BRIGHTNESS``). This control is deprecated and should not
    be used in new drivers and applications.

``V4L2_CID_AUTO_WHITE_BALANCE`` ``(boolean)``
    Automatic white balance (cameras).

``V4L2_CID_DO_WHITE_BALANCE`` ``(button)``
    This is an action control. When set (the value is ignored), the
    device will do a white balance and then hold the current setting.
    Contrast this with the boolean ``V4L2_CID_AUTO_WHITE_BALANCE``,
    which, when activated, keeps adjusting the white balance.

``V4L2_CID_RED_BALANCE`` ``(integer)``
    Red chroma balance.

``V4L2_CID_BLUE_BALANCE`` ``(integer)``
    Blue chroma balance.

``V4L2_CID_GAMMA`` ``(integer)``
    Gamma adjust.

``V4L2_CID_WHITENESS`` ``(integer)``
    Whiteness for grey-scale devices. This is a synonym for
    ``V4L2_CID_GAMMA``. This control is deprecated and should not be
    used in new drivers and applications.

``V4L2_CID_EXPOSURE`` ``(integer)``
    Exposure (cameras). [Unit?]

``V4L2_CID_AUTOGAIN`` ``(boolean)``
    Automatic gain/exposure control.

``V4L2_CID_GAIN`` ``(integer)``
    Gain control.

    Primarily used to control gain on e.g. TV tuners but also on
    webcams. Most devices control only digital gain with this control
    but on some this could include analogue gain as well. Devices that
    recognise the difference between digital and analogue gain use
    controls ``V4L2_CID_DIGITAL_GAIN`` and ``V4L2_CID_ANALOGUE_GAIN``.

.. _v4l2-cid-hflip:

``V4L2_CID_HFLIP`` ``(boolean)``
    Mirror the picture horizontally.

.. _v4l2-cid-vflip:

``V4L2_CID_VFLIP`` ``(boolean)``
    Mirror the picture vertically.

.. _v4l2-power-line-frequency:

``V4L2_CID_POWER_LINE_FREQUENCY`` ``(enum)``
    Enables a power line frequency filter to avoid flicker. Possible
    values for ``enum v4l2_power_line_frequency`` are:

    ==========================================  ==
    ``V4L2_CID_POWER_LINE_FREQUENCY_DISABLED``	 0
    ``V4L2_CID_POWER_LINE_FREQUENCY_50HZ``	 1
    ``V4L2_CID_POWER_LINE_FREQUENCY_60HZ``	 2
    ``V4L2_CID_POWER_LINE_FREQUENCY_AUTO``	 3
    ==========================================  ==

``V4L2_CID_HUE_AUTO`` ``(boolean)``
    Enables automatic hue control by the device. The effect of setting
    ``V4L2_CID_HUE`` while automatic hue control is enabled is
    undefined, drivers should ignore such request.

``V4L2_CID_WHITE_BALANCE_TEMPERATURE`` ``(integer)``
    This control specifies the white balance settings as a color
    temperature in Kelvin. A driver should have a minimum of 2800
    (incandescent) to 6500 (daylight). For more information about color
    temperature see
    `Wikipedia <http://en.wikipedia.org/wiki/Color_temperature>`__.

``V4L2_CID_SHARPNESS`` ``(integer)``
    Adjusts the sharpness filters in a camera. The minimum value
    disables the filters, higher values give a sharper picture.

``V4L2_CID_BACKLIGHT_COMPENSATION`` ``(integer)``
    Adjusts the backlight compensation in a camera. The minimum value
    disables backlight compensation.

``V4L2_CID_CHROMA_AGC`` ``(boolean)``
    Chroma automatic gain control.

``V4L2_CID_CHROMA_GAIN`` ``(integer)``
    Adjusts the Chroma gain control (for use when chroma AGC is
    disabled).

``V4L2_CID_COLOR_KILLER`` ``(boolean)``
    Enable the color killer (i. e. force a black & white image in case
    of a weak video signal).

.. _v4l2-colorfx:

``V4L2_CID_COLORFX`` ``(enum)``
    Selects a color effect. The following values are defined:


.. tabularcolumns:: |p{5.7cm}|p{11.8cm}|

.. flat-table::
    :header-rows:  0
    :stub-columns: 0
    :widths: 11 24

    * - ``V4L2_COLORFX_NONE``
      - Color effect is disabled.
    * - ``V4L2_COLORFX_ANTIQUE``
      - An aging (old photo) effect.
    * - ``V4L2_COLORFX_ART_FREEZE``
      - Frost color effect.
    * - ``V4L2_COLORFX_AQUA``
      - Water color, cool tone.
    * - ``V4L2_COLORFX_BW``
      - Black and white.
    * - ``V4L2_COLORFX_EMBOSS``
      - Emboss, the highlights and shadows replace light/dark boundaries
	and low contrast areas are set to a gray background.
    * - ``V4L2_COLORFX_GRASS_GREEN``
      - Grass green.
    * - ``V4L2_COLORFX_NEGATIVE``
      - Negative.
    * - ``V4L2_COLORFX_SEPIA``
      - Sepia tone.
    * - ``V4L2_COLORFX_SKETCH``
      - Sketch.
    * - ``V4L2_COLORFX_SKIN_WHITEN``
      - Skin whiten.
    * - ``V4L2_COLORFX_SKY_BLUE``
      - Sky blue.
    * - ``V4L2_COLORFX_SOLARIZATION``
      - Solarization, the image is partially reversed in tone, only color
	values above or below a certain threshold are inverted.
    * - ``V4L2_COLORFX_SILHOUETTE``
      - Silhouette (outline).
    * - ``V4L2_COLORFX_VIVID``
      - Vivid colors.
    * - ``V4L2_COLORFX_SET_CBCR``
      - The Cb and Cr chroma components are replaced by fixed coefficients
	determined by ``V4L2_CID_COLORFX_CBCR`` control.
    * - ``V4L2_COLORFX_SET_RGB``
      - The RGB components are replaced by the fixed RGB components determined
        by ``V4L2_CID_COLORFX_RGB`` control.


``V4L2_CID_COLORFX_RGB`` ``(integer)``
    Determines the Red, Green, and Blue coefficients for
    ``V4L2_COLORFX_SET_RGB`` color effect.
    Bits [7:0] of the supplied 32 bit value are interpreted as Blue component,
    bits [15:8] as Green component, bits [23:16] as Red component, and
    bits [31:24] must be zero.

``V4L2_CID_COLORFX_CBCR`` ``(integer)``
    Determines the Cb and Cr coefficients for ``V4L2_COLORFX_SET_CBCR``
    color effect. Bits [7:0] of the supplied 32 bit value are
    interpreted as Cr component, bits [15:8] as Cb component and bits
    [31:16] must be zero.

``V4L2_CID_AUTOBRIGHTNESS`` ``(boolean)``
    Enable Automatic Brightness.

``V4L2_CID_ROTATE`` ``(integer)``
    Rotates the image by specified angle. Common angles are 90, 270 and
    180. Rotating the image to 90 and 270 will reverse the height and
    width of the display window. It is necessary to set the new height
    and width of the picture using the
    :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl according to the
    rotation angle selected.

``V4L2_CID_BG_COLOR`` ``(integer)``
    Sets the background color on the current output device. Background
    color needs to be specified in the RGB24 format. The supplied 32 bit
    value is interpreted as bits 0-7 Red color information, bits 8-15
    Green color information, bits 16-23 Blue color information and bits
    24-31 must be zero.

``V4L2_CID_ILLUMINATORS_1 V4L2_CID_ILLUMINATORS_2`` ``(boolean)``
    Switch on or off the illuminator 1 or 2 of the device (usually a
    microscope).

``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE`` ``(integer)``
    This is a read-only control that can be read by the application and
    used as a hint to determine the number of CAPTURE buffers to pass to
    REQBUFS. The value is the minimum number of CAPTURE buffers that is
    necessary for hardware to work. This control is required for stateful
    decoders.

``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT`` ``(integer)``
    This is a read-only control that can be read by the application and
    used as a hint to determine the number of OUTPUT buffers to pass to
    REQBUFS. The value is the minimum number of OUTPUT buffers that is
    necessary for hardware to work. This control is required for stateful
    encoders.

.. _v4l2-alpha-component:

``V4L2_CID_ALPHA_COMPONENT`` ``(integer)``
    Sets the alpha color component. When a capture device (or capture
    queue of a mem-to-mem device) produces a frame format that includes
    an alpha component (e.g.
    :ref:`packed RGB image formats <pixfmt-rgb>`) and the alpha value
    is not defined by the device or the mem-to-mem input data this
    control lets you select the alpha component value of all pixels.
    When an output device (or output queue of a mem-to-mem device)
    consumes a frame format that doesn't include an alpha component and
    the device supports alpha channel processing this control lets you
    set the alpha component value of all pixels for further processing
    in the device.

``V4L2_CID_LASTP1``
    End of the predefined control IDs (currently
    ``V4L2_CID_ALPHA_COMPONENT`` + 1).

``V4L2_CID_PRIVATE_BASE``
    ID of the first custom (driver specific) control. Applications
    depending on particular custom controls should check the driver name
    and version, see :ref:`querycap`.

Applications can enumerate the available controls with the
:ref:`VIDIOC_QUERYCTRL` and
:ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a
control value with the :ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and
:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls. Drivers must implement
``VIDIOC_QUERYCTRL``, ``VIDIOC_G_CTRL`` and ``VIDIOC_S_CTRL`` when the
device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or
more menu type controls.


.. _enum_all_controls:

Example: Enumerating all controls
=================================

.. code-block:: c

    struct v4l2_queryctrl queryctrl;
    struct v4l2_querymenu querymenu;

    static void enumerate_menu(__u32 id)
    {
	printf("  Menu items:\\n");

	memset(&querymenu, 0, sizeof(querymenu));
	querymenu.id = id;

	for (querymenu.index = queryctrl.minimum;
	     querymenu.index <= queryctrl.maximum;
	     querymenu.index++) {
	    if (0 == ioctl(fd, VIDIOC_QUERYMENU, &querymenu)) {
		printf("  %s\\n", querymenu.name);
	    }
	}
    }

    memset(&queryctrl, 0, sizeof(queryctrl));

    queryctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL;
    while (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
	if (!(queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)) {
	    printf("Control %s\\n", queryctrl.name);

	    if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
	        enumerate_menu(queryctrl.id);
        }

	queryctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
    }
    if (errno != EINVAL) {
	perror("VIDIOC_QUERYCTRL");
	exit(EXIT_FAILURE);
    }

Example: Enumerating all controls including compound controls
=============================================================

.. code-block:: c

    struct v4l2_query_ext_ctrl query_ext_ctrl;

    memset(&query_ext_ctrl, 0, sizeof(query_ext_ctrl));

    query_ext_ctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND;
    while (0 == ioctl(fd, VIDIOC_QUERY_EXT_CTRL, &query_ext_ctrl)) {
	if (!(query_ext_ctrl.flags & V4L2_CTRL_FLAG_DISABLED)) {
	    printf("Control %s\\n", query_ext_ctrl.name);

	    if (query_ext_ctrl.type == V4L2_CTRL_TYPE_MENU)
	        enumerate_menu(query_ext_ctrl.id);
        }

	query_ext_ctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL | V4L2_CTRL_FLAG_NEXT_COMPOUND;
    }
    if (errno != EINVAL) {
	perror("VIDIOC_QUERY_EXT_CTRL");
	exit(EXIT_FAILURE);
    }

Example: Enumerating all user controls (old style)
==================================================

.. code-block:: c


    memset(&queryctrl, 0, sizeof(queryctrl));

    for (queryctrl.id = V4L2_CID_BASE;
	 queryctrl.id < V4L2_CID_LASTP1;
	 queryctrl.id++) {
	if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
	    if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
		continue;

	    printf("Control %s\\n", queryctrl.name);

	    if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
		enumerate_menu(queryctrl.id);
	} else {
	    if (errno == EINVAL)
		continue;

	    perror("VIDIOC_QUERYCTRL");
	    exit(EXIT_FAILURE);
	}
    }

    for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
	 queryctrl.id++) {
	if (0 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
	    if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
		continue;

	    printf("Control %s\\n", queryctrl.name);

	    if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
		enumerate_menu(queryctrl.id);
	} else {
	    if (errno == EINVAL)
		break;

	    perror("VIDIOC_QUERYCTRL");
	    exit(EXIT_FAILURE);
	}
    }


Example: Changing controls
==========================

.. code-block:: c

    struct v4l2_queryctrl queryctrl;
    struct v4l2_control control;

    memset(&queryctrl, 0, sizeof(queryctrl));
    queryctrl.id = V4L2_CID_BRIGHTNESS;

    if (-1 == ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)) {
	if (errno != EINVAL) {
	    perror("VIDIOC_QUERYCTRL");
	    exit(EXIT_FAILURE);
	} else {
	    printf("V4L2_CID_BRIGHTNESS is not supported\n");
	}
    } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
	printf("V4L2_CID_BRIGHTNESS is not supported\n");
    } else {
	memset(&control, 0, sizeof (control));
	control.id = V4L2_CID_BRIGHTNESS;
	control.value = queryctrl.default_value;

	if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)) {
	    perror("VIDIOC_S_CTRL");
	    exit(EXIT_FAILURE);
	}
    }

    memset(&control, 0, sizeof(control));
    control.id = V4L2_CID_CONTRAST;

    if (0 == ioctl(fd, VIDIOC_G_CTRL, &control)) {
	control.value += 1;

	/* The driver may clamp the value or return ERANGE, ignored here */

	if (-1 == ioctl(fd, VIDIOC_S_CTRL, &control)
	    && errno != ERANGE) {
	    perror("VIDIOC_S_CTRL");
	    exit(EXIT_FAILURE);
	}
    /* Ignore if V4L2_CID_CONTRAST is unsupported */
    } else if (errno != EINVAL) {
	perror("VIDIOC_G_CTRL");
	exit(EXIT_FAILURE);
    }

    control.id = V4L2_CID_AUDIO_MUTE;
    control.value = 1; /* silence */

    /* Errors ignored */
    ioctl(fd, VIDIOC_S_CTRL, &control);

.. [#f1]
   The use of ``V4L2_CID_PRIVATE_BASE`` is problematic because different
   drivers may use the same ``V4L2_CID_PRIVATE_BASE`` ID for different
   controls. This makes it hard to programmatically set such controls
   since the meaning of the control with that ID is driver dependent. In
   order to resolve this drivers use unique IDs and the
   ``V4L2_CID_PRIVATE_BASE`` IDs are mapped to those unique IDs by the
   kernel. Consider these ``V4L2_CID_PRIVATE_BASE`` IDs as aliases to
   the real IDs.

   Many applications today still use the ``V4L2_CID_PRIVATE_BASE`` IDs
   instead of using :ref:`VIDIOC_QUERYCTRL` with
   the ``V4L2_CTRL_FLAG_NEXT_CTRL`` flag to enumerate all IDs, so
   support for ``V4L2_CID_PRIVATE_BASE`` is still around.