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 and higher values. The pre-defined control IDs have the prefix V4L2_CID_, and are listed in Table 1.1, “Control IDs”. 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.[9] 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. Since V4L2 specifies no event mechanism, panel applications intended to cooperate with other panel applications (be they built into a larger application, as a TV viewer) may need to regularly poll control values to update their user interface.[10]

All controls use machine endianness.

Table 1.1. Control IDs

IDTypeDescription
V4L2_CID_BASE First predefined ID, equal to V4L2_CID_BRIGHTNESS.
V4L2_CID_USER_BASE Synonym of V4L2_CID_BASE.
V4L2_CID_BRIGHTNESSintegerPicture brightness, or more precisely, the black level.
V4L2_CID_CONTRASTintegerPicture contrast or luma gain.
V4L2_CID_SATURATIONintegerPicture color saturation or chroma gain.
V4L2_CID_HUEintegerHue or color balance.
V4L2_CID_AUDIO_VOLUMEintegerOverall audio volume. Note some drivers also provide an OSS or ALSA mixer interface.
V4L2_CID_AUDIO_BALANCEintegerAudio stereo balance. Minimum corresponds to all the way left, maximum to right.
V4L2_CID_AUDIO_BASSintegerAudio bass adjustment.
V4L2_CID_AUDIO_TREBLEintegerAudio treble adjustment.
V4L2_CID_AUDIO_MUTEbooleanMute 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_LOUDNESSbooleanLoudness mode (bass boost).
V4L2_CID_BLACK_LEVELintegerAnother 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_BALANCEbooleanAutomatic white balance (cameras).
V4L2_CID_DO_WHITE_BALANCEbuttonThis 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_BALANCEintegerRed chroma balance.
V4L2_CID_BLUE_BALANCEintegerBlue chroma balance.
V4L2_CID_GAMMAintegerGamma adjust.
V4L2_CID_WHITENESSintegerWhiteness 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_EXPOSUREintegerExposure (cameras). [Unit?]
V4L2_CID_AUTOGAINbooleanAutomatic gain/exposure control.
V4L2_CID_GAINintegerGain control.
V4L2_CID_HFLIPbooleanMirror the picture horizontally.
V4L2_CID_VFLIPbooleanMirror the picture vertically.
V4L2_CID_HCENTER_DEPRECATED (formerly V4L2_CID_HCENTER)integerHorizontal image centering. This control is deprecated. New drivers and applications should use the Camera class controls V4L2_CID_PAN_ABSOLUTE, V4L2_CID_PAN_RELATIVE and V4L2_CID_PAN_RESET instead.
V4L2_CID_VCENTER_DEPRECATED (formerly V4L2_CID_VCENTER)integerVertical image centering. Centering is intended to physically adjust cameras. For image cropping see the section called “Image Cropping, Insertion and Scaling”, for clipping the section called “Video Overlay Interface”. This control is deprecated. New drivers and applications should use the Camera class controls V4L2_CID_TILT_ABSOLUTE, V4L2_CID_TILT_RELATIVE and V4L2_CID_TILT_RESET instead.
V4L2_CID_POWER_LINE_FREQUENCYenumEnables 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) and V4L2_CID_POWER_LINE_FREQUENCY_AUTO (3).
V4L2_CID_HUE_AUTObooleanEnables 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_TEMPERATUREintegerThis 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.
V4L2_CID_SHARPNESSintegerAdjusts the sharpness filters in a camera. The minimum value disables the filters, higher values give a sharper picture.
V4L2_CID_BACKLIGHT_COMPENSATIONintegerAdjusts the backlight compensation in a camera. The minimum value disables backlight compensation.
V4L2_CID_CHROMA_AGCbooleanChroma automatic gain control.
V4L2_CID_CHROMA_GAINintegerAdjusts the Chroma gain control (for use when chroma AGC is disabled).
V4L2_CID_COLOR_KILLERbooleanEnable the color killer (i. e. force a black & white image in case of a weak video signal).
V4L2_CID_COLORFXenumSelects a color effect. Possible values for enum v4l2_colorfx are: V4L2_COLORFX_NONE (0), V4L2_COLORFX_BW (1), V4L2_COLORFX_SEPIA (2), V4L2_COLORFX_NEGATIVE (3), V4L2_COLORFX_EMBOSS (4), V4L2_COLORFX_SKETCH (5), V4L2_COLORFX_SKY_BLUE (6), V4L2_COLORFX_GRASS_GREEN (7), V4L2_COLORFX_SKIN_WHITEN (8) and V4L2_COLORFX_VIVID (9).
V4L2_CID_ROTATEintegerRotates 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 VIDIOC_S_FMT ioctl according to the rotation angle selected.
V4L2_CID_BG_COLORintegerSets 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_2booleanSwitch on or off the illuminator 1 or 2 of the device (usually a microscope).
V4L2_CID_MIN_BUFFERS_FOR_CAPTUREintegerThis 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.
V4L2_CID_MIN_BUFFERS_FOR_OUTPUTintegerThis 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.
V4L2_CID_ALPHA_COMPONENTinteger Sets the alpha color component on the capture device or on the capture buffer queue of a mem-to-mem device. When a mem-to-mem device produces frame format that includes an alpha component (e.g. packed RGB image formats) and the alpha value is not defined by the mem-to-mem input data this control lets you select the alpha component value of all pixels. It is applicable to any pixel format that contains an alpha component.
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 the section called “Querying Capabilities”.

Applications can enumerate the available controls with the VIDIOC_QUERYCTRL and VIDIOC_QUERYMENU ioctls, get and set a control value with the VIDIOC_G_CTRL and VIDIOC_S_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.

Example 1.8. Enumerating all controls

struct v4l2_queryctrl queryctrl;
struct v4l2_querymenu querymenu;

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

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

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 ();
	} 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 ();
	} else {
		if (errno == EINVAL)
			break;

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

Example 1.9. Changing controls

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 = TRUE; /* silence */

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



[9] It will be more convenient for applications if drivers make use of the V4L2_CTRL_FLAG_DISABLED flag, but that was never required.

[10] Applications could call an ioctl to request events. After another process called VIDIOC_S_CTRL or another ioctl changing shared properties the select() function would indicate readability until any ioctl (querying the properties) is called.