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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>User Controls</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="LINUX MEDIA INFRASTRUCTURE API" /><link rel="up" href="ch01.html" title="Chapter 1. Common API Elements" /><link rel="prev" href="ch01s07.html" title="Video Standards" /><link rel="next" href="ch01s09.html" title="Extended Controls" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">User Controls</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch01s07.html">Prev</a> </td><th width="60%" align="center">Chapter 1. Common API Elements</th><td width="20%" align="right"> <a accesskey="n" href="ch01s09.html">Next</a></td></tr></table><hr /></div><div class="section" title="User Controls"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="control"></a>User Controls</h2></div></div></div><p>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.</p><p>All controls are accessed using an ID value. V4L2 defines
several IDs for specific purposes. Drivers can also implement their
own custom controls using <code class="constant">V4L2_CID_PRIVATE_BASE</code>
and higher values. The pre-defined control IDs have the prefix
<code class="constant">V4L2_CID_</code>, and are listed in <a class="xref" href="ch01s08.html#control-id" title="Table 1.1. Control IDs">Table 1.1, “Control IDs”</a>. The ID is used when querying the attributes of
a control, and when getting or setting the current value.</p><p>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.</p><p>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
<span class="emphasis"><em>custom</em></span> ID can also change name and
type.<sup>[<a id="id2579118" href="#ftn.id2579118" class="footnote">9</a>]</sup> 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.<sup>[<a id="id2578169" href="#ftn.id2578169" class="footnote">10</a>]</sup></p><div class="table"><a id="control-id"></a><p class="title"><b>Table 1.1. Control IDs</b></p><div class="table-contents"><table summary="Control IDs" width="100%" border="0"><colgroup><col /><col /><col /></colgroup><thead><tr><th>ID</th><th>Type</th><th>Description</th></tr></thead><tbody valign="top"><tr><td valign="top"><code class="constant">V4L2_CID_BASE</code></td><td valign="top"> </td><td valign="top">First predefined ID, equal to
<code class="constant">V4L2_CID_BRIGHTNESS</code>.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_USER_BASE</code></td><td valign="top"> </td><td valign="top">Synonym of <code class="constant">V4L2_CID_BASE</code>.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_BRIGHTNESS</code></td><td valign="top">integer</td><td valign="top">Picture brightness, or more precisely, the black
level.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_CONTRAST</code></td><td valign="top">integer</td><td valign="top">Picture contrast or luma gain.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_SATURATION</code></td><td valign="top">integer</td><td valign="top">Picture color saturation or chroma gain.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_HUE</code></td><td valign="top">integer</td><td valign="top">Hue or color balance.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_AUDIO_VOLUME</code></td><td valign="top">integer</td><td valign="top">Overall audio volume. Note some drivers also
provide an OSS or ALSA mixer interface.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_AUDIO_BALANCE</code></td><td valign="top">integer</td><td valign="top">Audio stereo balance. Minimum corresponds to all
the way left, maximum to right.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_AUDIO_BASS</code></td><td valign="top">integer</td><td valign="top">Audio bass adjustment.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_AUDIO_TREBLE</code></td><td valign="top">integer</td><td valign="top">Audio treble adjustment.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_AUDIO_MUTE</code></td><td valign="top">boolean</td><td valign="top">Mute audio, i. e. set the volume to zero, however
without affecting <code class="constant">V4L2_CID_AUDIO_VOLUME</code>. 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.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_AUDIO_LOUDNESS</code></td><td valign="top">boolean</td><td valign="top">Loudness mode (bass boost).</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_BLACK_LEVEL</code></td><td valign="top">integer</td><td valign="top">Another name for brightness (not a synonym of
<code class="constant">V4L2_CID_BRIGHTNESS</code>). This control is deprecated
and should not be used in new drivers and applications.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_AUTO_WHITE_BALANCE</code></td><td valign="top">boolean</td><td valign="top">Automatic white balance (cameras).</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_DO_WHITE_BALANCE</code></td><td valign="top">button</td><td valign="top">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
<code class="constant">V4L2_CID_AUTO_WHITE_BALANCE</code>, which, when
activated, keeps adjusting the white balance.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_RED_BALANCE</code></td><td valign="top">integer</td><td valign="top">Red chroma balance.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_BLUE_BALANCE</code></td><td valign="top">integer</td><td valign="top">Blue chroma balance.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_GAMMA</code></td><td valign="top">integer</td><td valign="top">Gamma adjust.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_WHITENESS</code></td><td valign="top">integer</td><td valign="top">Whiteness for grey-scale devices. This is a synonym
for <code class="constant">V4L2_CID_GAMMA</code>. This control is deprecated
and should not be used in new drivers and applications.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_EXPOSURE</code></td><td valign="top">integer</td><td valign="top">Exposure (cameras). [Unit?]</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_AUTOGAIN</code></td><td valign="top">boolean</td><td valign="top">Automatic gain/exposure control.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_GAIN</code></td><td valign="top">integer</td><td valign="top">Gain control.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_HFLIP</code></td><td valign="top">boolean</td><td valign="top">Mirror the picture horizontally.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_VFLIP</code></td><td valign="top">boolean</td><td valign="top">Mirror the picture vertically.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_HCENTER_DEPRECATED</code> (formerly <code class="constant">V4L2_CID_HCENTER</code>)</td><td valign="top">integer</td><td valign="top">Horizontal image centering. This control is
deprecated. New drivers and applications should use the <a class="link" href="ch01s09.html#camera-controls" title="Camera Control Reference">Camera class controls</a>
<code class="constant">V4L2_CID_PAN_ABSOLUTE</code>,
<code class="constant">V4L2_CID_PAN_RELATIVE</code> and
<code class="constant">V4L2_CID_PAN_RESET</code> instead.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_VCENTER_DEPRECATED</code>
(formerly <code class="constant">V4L2_CID_VCENTER</code>)</td><td valign="top">integer</td><td valign="top">Vertical image centering. Centering is intended to
<span class="emphasis"><em>physically</em></span> adjust cameras. For image cropping see
<a class="xref" href="ch01s11.html" title="Image Cropping, Insertion and Scaling">the section called “Image Cropping, Insertion and Scaling”</a>, for clipping <a class="xref" href="ch04s02.html" title="Video Overlay Interface">the section called “Video Overlay Interface”</a>. This
control is deprecated. New drivers and applications should use the
<a class="link" href="ch01s09.html#camera-controls" title="Camera Control Reference">Camera class controls</a>
<code class="constant">V4L2_CID_TILT_ABSOLUTE</code>,
<code class="constant">V4L2_CID_TILT_RELATIVE</code> and
<code class="constant">V4L2_CID_TILT_RESET</code> instead.</td></tr><tr><td valign="top"><a id="v4l2-power-line-frequency"></a><code class="constant">V4L2_CID_POWER_LINE_FREQUENCY</code></td><td valign="top">enum</td><td valign="top">Enables a power line frequency filter to avoid
flicker. Possible values for <code class="constant">enum v4l2_power_line_frequency</code> are:
<code class="constant">V4L2_CID_POWER_LINE_FREQUENCY_DISABLED</code> (0),
<code class="constant">V4L2_CID_POWER_LINE_FREQUENCY_50HZ</code> (1) and
<code class="constant">V4L2_CID_POWER_LINE_FREQUENCY_60HZ</code> (2).</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_HUE_AUTO</code></td><td valign="top">boolean</td><td valign="top">Enables automatic hue control by the device. The
effect of setting <code class="constant">V4L2_CID_HUE</code> while automatic
hue control is enabled is undefined, drivers should ignore such
request.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_WHITE_BALANCE_TEMPERATURE</code></td><td valign="top">integer</td><td valign="top">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 <a class="ulink" href="http://en.wikipedia.org/wiki/Color_temperature" target="_top">Wikipedia</a>.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_SHARPNESS</code></td><td valign="top">integer</td><td valign="top">Adjusts the sharpness filters in a camera. The
minimum value disables the filters, higher values give a sharper
picture.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_BACKLIGHT_COMPENSATION</code></td><td valign="top">integer</td><td valign="top">Adjusts the backlight compensation in a camera. The
minimum value disables backlight compensation.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_CHROMA_AGC</code></td><td valign="top">boolean</td><td valign="top">Chroma automatic gain control.</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_COLOR_KILLER</code></td><td valign="top">boolean</td><td valign="top">Enable the color killer (i. e. force a black & white image in case of a weak video signal).</td></tr><tr><td valign="top"><a id="v4l2-colorfx"></a><code class="constant">V4L2_CID_COLORFX</code></td><td valign="top">enum</td><td valign="top">Selects a color effect. Possible values for
<code class="constant">enum v4l2_colorfx</code> are:
<code class="constant">V4L2_COLORFX_NONE</code> (0),
<code class="constant">V4L2_COLORFX_BW</code> (1) and
<code class="constant">V4L2_COLORFX_SEPIA</code> (2).</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_LASTP1</code></td><td valign="top"> </td><td valign="top">End of the predefined control IDs (currently
<code class="constant">V4L2_CID_COLORFX</code> + 1).</td></tr><tr><td valign="top"><code class="constant">V4L2_CID_PRIVATE_BASE</code></td><td valign="top"> </td><td valign="top">ID of the first custom (driver specific) control.
Applications depending on particular custom controls should check the
driver name and version, see <a class="xref" href="ch01s02.html" title="Querying Capabilities">the section called “Querying Capabilities”</a>.</td></tr></tbody></table></div></div><br class="table-break" /><p>Applications can enumerate the available controls with the
<a class="link" href="re57.html" title="ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU"><code class="constant">VIDIOC_QUERYCTRL</code></a> and <a class="link" href="re57.html" title="ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU"><code class="constant">VIDIOC_QUERYMENU</code></a> ioctls, get and set a
control value with the <a class="link" href="re37.html" title="ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL"><code class="constant">VIDIOC_G_CTRL</code></a> and <a class="link" href="re37.html" title="ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL"><code class="constant">VIDIOC_S_CTRL</code></a> ioctls.
Drivers must implement <code class="constant">VIDIOC_QUERYCTRL</code>,
<code class="constant">VIDIOC_G_CTRL</code> and
<code class="constant">VIDIOC_S_CTRL</code> when the device has one or more
controls, <code class="constant">VIDIOC_QUERYMENU</code> when it has one or
more menu type controls.</p><div class="example"><a id="id2579946"></a><p class="title"><b>Example 1.8. Enumerating all controls</b></p><div class="example-contents"><pre class="programlisting">
struct <a class="link" href="re57.html#v4l2-queryctrl" title="Table A.68. struct v4l2_queryctrl">v4l2_queryctrl</a> queryctrl;
struct <a class="link" href="re57.html#v4l2-querymenu" title="Table A.69. struct v4l2_querymenu">v4l2_querymenu</a> 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, <a class="link" href="re57.html" title="ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU"><code class="constant">VIDIOC_QUERYMENU</code></a>, &querymenu)) {
printf (" %s\n", querymenu.name);
} else {
perror ("VIDIOC_QUERYMENU");
exit (EXIT_FAILURE);
}
}
}
memset (&queryctrl, 0, sizeof (queryctrl));
for (queryctrl.id = V4L2_CID_BASE;
queryctrl.id < V4L2_CID_LASTP1;
queryctrl.id++) {
if (0 == ioctl (fd, <a class="link" href="re57.html" title="ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU"><code class="constant">VIDIOC_QUERYCTRL</code></a>, &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, <a class="link" href="re57.html" title="ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU"><code class="constant">VIDIOC_QUERYCTRL</code></a>, &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);
}
}
</pre></div></div><br class="example-break" /><div class="example"><a id="id2580045"></a><p class="title"><b>Example 1.9. Changing controls</b></p><div class="example-contents"><pre class="programlisting">
struct <a class="link" href="re57.html#v4l2-queryctrl" title="Table A.68. struct v4l2_queryctrl">v4l2_queryctrl</a> queryctrl;
struct <a class="link" href="re37.html#v4l2-control" title="Table A.36. struct v4l2_control">v4l2_control</a> control;
memset (&queryctrl, 0, sizeof (queryctrl));
queryctrl.id = V4L2_CID_BRIGHTNESS;
if (-1 == ioctl (fd, <a class="link" href="re57.html" title="ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU"><code class="constant">VIDIOC_QUERYCTRL</code></a>, &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, <a class="link" href="re37.html" title="ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL"><code class="constant">VIDIOC_S_CTRL</code></a>, &control)) {
perror ("VIDIOC_S_CTRL");
exit (EXIT_FAILURE);
}
}
memset (&control, 0, sizeof (control));
control.id = V4L2_CID_CONTRAST;
if (0 == ioctl (fd, <a class="link" href="re37.html" title="ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL"><code class="constant">VIDIOC_G_CTRL</code></a>, &control)) {
control.value += 1;
/* The driver may clamp the value or return ERANGE, ignored here */
if (-1 == ioctl (fd, <a class="link" href="re37.html" title="ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL"><code class="constant">VIDIOC_S_CTRL</code></a>, &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);
</pre></div></div><br class="example-break" /><div class="footnotes"><br /><hr width="100" align="left" /><div class="footnote"><p><sup>[<a id="ftn.id2579118" href="#id2579118" class="para">9</a>] </sup>It will be more convenient for applications if drivers
make use of the <code class="constant">V4L2_CTRL_FLAG_DISABLED</code> flag, but
that was never required.</p></div><div class="footnote"><p><sup>[<a id="ftn.id2578169" href="#id2578169" class="para">10</a>] </sup>Applications could call an ioctl to request events.
After another process called <a class="link" href="re37.html" title="ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL"><code class="constant">VIDIOC_S_CTRL</code></a> or another ioctl changing
shared properties the <a class="link" href="re67.html" title="V4L2 select()"><code class="function">select()</code></a> function would indicate
readability until any ioctl (querying the properties) is
called.</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch01s07.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch01.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch01s09.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Video Standards </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Extended Controls</td></tr></table></div></body></html>
