2009-09-14 09:16:04 +08:00
<title > Common API Elements</title>
<para > Programming a V4L2 device consists of these
steps:</para>
<itemizedlist >
<listitem >
<para > Opening the device</para>
</listitem>
<listitem >
<para > Changing device properties, selecting a video and audio
input, video standard, picture brightness a. o.</para>
</listitem>
<listitem >
<para > Negotiating a data format</para>
</listitem>
<listitem >
<para > Negotiating an input/output method</para>
</listitem>
<listitem >
<para > The actual input/output loop</para>
</listitem>
<listitem >
<para > Closing the device</para>
</listitem>
</itemizedlist>
<para > In practice most steps are optional and can be executed out of
order. It depends on the V4L2 device type, you can read about the
details in <xref linkend= "devices" /> . In this chapter we will discuss
the basic concepts applicable to all devices.</para>
<section id= "open" >
<title > Opening and Closing Devices</title>
<section >
<title > Device Naming</title>
<para > V4L2 drivers are implemented as kernel modules, loaded
manually by the system administrator or automatically when a device is
first opened. The driver modules plug into the "videodev" kernel
module. It provides helper functions and a common application
interface specified in this document.</para>
<para > Each driver thus loaded registers one or more device nodes
with major number 81 and a minor number between 0 and 255. Assigning
minor numbers to V4L2 devices is entirely up to the system administrator,
this is primarily intended to solve conflicts between devices.<footnote >
<para > Access permissions are associated with character
device special files, hence we must ensure device numbers cannot
change with the module load order. To this end minor numbers are no
longer automatically assigned by the "videodev" module as in V4L but
requested by the driver. The defaults will suffice for most people
unless two drivers compete for the same minor numbers.</para>
</footnote> The module options to select minor numbers are named
after the device special file with a "_nr" suffix. For example "video_nr"
for <filename > /dev/video</filename> video capture devices. The number is
an offset to the base minor number associated with the device type.
<footnote >
<para > In earlier versions of the V4L2 API the module options
where named after the device special file with a "unit_" prefix, expressing
the minor number itself, not an offset. Rationale for this change is unknown.
Lastly the naming and semantics are just a convention among driver writers,
the point to note is that minor numbers are not supposed to be hardcoded
into drivers.</para>
</footnote> When the driver supports multiple devices of the same
type more than one minor number can be assigned, separated by commas:
<informalexample >
<screen >
> insmod mydriver.o video_nr=0,1 radio_nr=0,1</screen>
</informalexample> </para>
<para > In <filename > /etc/modules.conf</filename> this may be
written as: <informalexample >
<screen >
alias char-major-81-0 mydriver
alias char-major-81-1 mydriver
alias char-major-81-64 mydriver <co id= "alias" />
options mydriver video_nr=0,1 radio_nr=0,1 <co id= "options" />
</screen>
<calloutlist >
<callout arearefs= "alias" >
<para > When an application attempts to open a device
special file with major number 81 and minor number 0, 1, or 64, load
"mydriver" (and the "videodev" module it depends upon).</para>
</callout>
<callout arearefs= "options" >
<para > Register the first two video capture devices with
minor number 0 and 1 (base number is 0), the first two radio device
with minor number 64 and 65 (base 64).</para>
</callout>
</calloutlist>
</informalexample> When no minor number is given as module
option the driver supplies a default. <xref linkend= "devices" />
recommends the base minor numbers to be used for the various device
types. Obviously minor numbers must be unique. When the number is
already in use the <emphasis > offending device</emphasis> will not be
registered. <!-- Blessed by Linus Torvalds on
linux-kernel@vger.kernel.org, 2002-11-20. --></para>
<para > By convention system administrators create various
character device special files with these major and minor numbers in
the <filename > /dev</filename> directory. The names recomended for the
different V4L2 device types are listed in <xref linkend= "devices" /> .
</para>
<para > The creation of character special files (with
<application > mknod</application> ) is a privileged operation and
devices cannot be opened by major and minor number. That means
applications cannot <emphasis > reliable</emphasis> scan for loaded or
installed drivers. The user must enter a device name, or the
application can try the conventional device names.</para>
<para > Under the device filesystem (devfs) the minor number
options are ignored. V4L2 drivers (or by proxy the "videodev" module)
automatically create the required device files in the
<filename > /dev/v4l</filename> directory using the conventional device
names above.</para>
</section>
<section id= "related" >
<title > Related Devices</title>
<para > Devices can support several related functions. For example
video capturing, video overlay and VBI capturing are related because
these functions share, amongst other, the same video input and tuner
frequency. V4L and earlier versions of V4L2 used the same device name
and minor number for video capturing and overlay, but different ones
for VBI. Experience showed this approach has several problems<footnote >
<para > Given a device file name one cannot reliable find
related devices. For once names are arbitrary and in a system with
multiple devices, where only some support VBI capturing, a
<filename > /dev/video2</filename> is not necessarily related to
<filename > /dev/vbi2</filename> . The V4L
<constant > VIDIOCGUNIT</constant> ioctl would require a search for a
device file with a particular major and minor number.</para>
</footnote> , and to make things worse the V4L videodev module
used to prohibit multiple opens of a device.</para>
<para > As a remedy the present version of the V4L2 API relaxed the
concept of device types with specific names and minor numbers. For
compatibility with old applications drivers must still register different
minor numbers to assign a default function to the device. But if related
functions are supported by the driver they must be available under all
registered minor numbers. The desired function can be selected after
opening the device as described in <xref linkend= "devices" /> .</para>
<para > Imagine a driver supporting video capturing, video
overlay, raw VBI capturing, and FM radio reception. It registers three
devices with minor number 0, 64 and 224 (this numbering scheme is
inherited from the V4L API). Regardless if
<filename > /dev/video</filename> (81, 0) or
<filename > /dev/vbi</filename> (81, 224) is opened the application can
select any one of the video capturing, overlay or VBI capturing
functions. Without programming (e. g. reading from the device
with <application > dd</application> or <application > cat</application> )
<filename > /dev/video</filename> captures video images, while
<filename > /dev/vbi</filename> captures raw VBI data.
<filename > /dev/radio</filename> (81, 64) is invariable a radio device,
unrelated to the video functions. Being unrelated does not imply the
devices can be used at the same time, however. The &func-open;
function may very well return an &EBUSY; .</para>
<para > Besides video input or output the hardware may also
support audio sampling or playback. If so, these functions are
implemented as OSS or ALSA PCM devices and eventually OSS or ALSA
audio mixer. The V4L2 API makes no provisions yet to find these
related devices. If you have an idea please write to the linux-media
mailing list: &v4l-ml; .</para>
</section>
<section >
<title > Multiple Opens</title>
<para > In general, V4L2 devices can be opened more than once.
When this is supported by the driver, users can for example start a
"panel" application to change controls like brightness or audio
volume, while another application captures video and audio. In other words, panel
applications are comparable to an OSS or ALSA audio mixer application.
When a device supports multiple functions like capturing and overlay
<emphasis > simultaneously</emphasis> , multiple opens allow concurrent
use of the device by forked processes or specialized applications.</para>
<para > Multiple opens are optional, although drivers should
permit at least concurrent accesses without data exchange, &ie; panel
applications. This implies &func-open; can return an &EBUSY; when the
device is already in use, as well as &func-ioctl; functions initiating
data exchange (namely the &VIDIOC-S-FMT; ioctl), and the &func-read;
and &func-write; functions.</para>
<para > Mere opening a V4L2 device does not grant exclusive
access.<footnote >
<para > Drivers could recognize the
<constant > O_EXCL</constant> open flag. Presently this is not required,
so applications cannot know if it really works.</para>
</footnote> Initiating data exchange however assigns the right
to read or write the requested type of data, and to change related
properties, to this file descriptor. Applications can request
additional access privileges using the priority mechanism described in
<xref linkend= "app-pri" /> .</para>
</section>
<section >
<title > Shared Data Streams</title>
<para > V4L2 drivers should not support multiple applications
reading or writing the same data stream on a device by copying
buffers, time multiplexing or similar means. This is better handled by
a proxy application in user space. When the driver supports stream
sharing anyway it must be implemented transparently. The V4L2 API does
not specify how conflicts are solved. <!-- For example O_EXCL when the
application does not want to be preempted, PROT_READ mmapped buffers
which can be mapped twice, what happens when image formats do not
match etc.--></para>
</section>
<section >
<title > Functions</title>
<para > To open and close V4L2 devices applications use the
&func-open; and &func-close; function, respectively. Devices are
programmed using the &func-ioctl; function as explained in the
following sections.</para>
</section>
</section>
<section id= "querycap" >
<title > Querying Capabilities</title>
<para > Because V4L2 covers a wide variety of devices not all
aspects of the API are equally applicable to all types of devices.
Furthermore devices of the same type have different capabilities and
this specification permits the omission of a few complicated and less
important parts of the API.</para>
<para > The &VIDIOC-QUERYCAP; ioctl is available to check if the kernel
device is compatible with this specification, and to query the <link
linkend="devices">functions</link> and <link linkend= "io" > I/O
methods</link> supported by the device. Other features can be queried
by calling the respective ioctl, for example &VIDIOC-ENUMINPUT;
to learn about the number, types and names of video connectors on the
device. Although abstraction is a major objective of this API, the
ioctl also allows driver specific applications to reliable identify
the driver.</para>
<para > All V4L2 drivers must support
<constant > VIDIOC_QUERYCAP</constant> . Applications should always call
this ioctl after opening the device.</para>
</section>
<section id= "app-pri" >
<title > Application Priority</title>
<para > When multiple applications share a device it may be
desirable to assign them different priorities. Contrary to the
traditional "rm -rf /" school of thought a video recording application
could for example block other applications from changing video
controls or switching the current TV channel. Another objective is to
permit low priority applications working in background, which can be
preempted by user controlled applications and automatically regain
control of the device at a later time.</para>
<para > Since these features cannot be implemented entirely in user
space V4L2 defines the &VIDIOC-G-PRIORITY; and &VIDIOC-S-PRIORITY;
ioctls to request and query the access priority associate with a file
descriptor. Opening a device assigns a medium priority, compatible
with earlier versions of V4L2 and drivers not supporting these ioctls.
Applications requiring a different priority will usually call
<constant > VIDIOC_S_PRIORITY</constant> after verifying the device with
the &VIDIOC-QUERYCAP; ioctl.</para>
<para > Ioctls changing driver properties, such as &VIDIOC-S-INPUT; ,
return an &EBUSY; after another application obtained higher priority.
An event mechanism to notify applications about asynchronous property
changes has been proposed but not added yet.</para>
</section>
<section id= "video" >
<title > Video Inputs and Outputs</title>
<para > Video inputs and outputs are physical connectors of a
device. These can be for example RF connectors (antenna/cable), CVBS
a.k.a. Composite Video, S-Video or RGB connectors. Only video and VBI
capture devices have inputs, output devices have outputs, at least one
each. Radio devices have no video inputs or outputs.</para>
<para > To learn about the number and attributes of the
available inputs and outputs applications can enumerate them with the
&VIDIOC-ENUMINPUT; and &VIDIOC-ENUMOUTPUT; ioctl, respectively. The
&v4l2-input; returned by the <constant > VIDIOC_ENUMINPUT</constant>
ioctl also contains signal status information applicable when the
current video input is queried.</para>
<para > The &VIDIOC-G-INPUT; and &VIDIOC-G-OUTPUT; ioctl return the
index of the current video input or output. To select a different
input or output applications call the &VIDIOC-S-INPUT; and
&VIDIOC-S-OUTPUT; ioctl. Drivers must implement all the input ioctls
when the device has one or more inputs, all the output ioctls when the
device has one or more outputs.</para>
<!--
<figure id= io-tree >
<title > Input and output enumeration is the root of most device properties.</title>
<mediaobject >
<imageobject >
<imagedata fileref= "links.pdf" format= "ps" />
</imageobject>
<imageobject >
<imagedata fileref= "links.gif" format= "gif" />
</imageobject>
<textobject >
<phrase > Links between various device property structures.</phrase>
</textobject>
</mediaobject>
</figure>
-->
<example >
<title > Information about the current video input</title>
<programlisting >
&v4l2-input; input;
int index;
if (-1 == ioctl (fd, &VIDIOC-G-INPUT; , & index)) {
perror ("VIDIOC_G_INPUT");
exit (EXIT_FAILURE);
}
memset (& input, 0, sizeof (input));
input.index = index;
if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT; , & input)) {
perror ("VIDIOC_ENUMINPUT");
exit (EXIT_FAILURE);
}
printf ("Current input: %s\n", input.name);
</programlisting>
</example>
<example >
<title > Switching to the first video input</title>
<programlisting >
int index;
index = 0;
if (-1 == ioctl (fd, &VIDIOC-S-INPUT; , & index)) {
perror ("VIDIOC_S_INPUT");
exit (EXIT_FAILURE);
}
</programlisting>
</example>
</section>
<section id= "audio" >
<title > Audio Inputs and Outputs</title>
<para > Audio inputs and outputs are physical connectors of a
device. Video capture devices have inputs, output devices have
outputs, zero or more each. Radio devices have no audio inputs or
outputs. They have exactly one tuner which in fact
<emphasis > is</emphasis> an audio source, but this API associates
tuners with video inputs or outputs only, and radio devices have
none of these.<footnote >
<para > Actually &v4l2-audio; ought to have a
<structfield > tuner</structfield> field like &v4l2-input; , not only
making the API more consistent but also permitting radio devices with
multiple tuners.</para>
</footnote> A connector on a TV card to loop back the received
audio signal to a sound card is not considered an audio output.</para>
<para > Audio and video inputs and outputs are associated. Selecting
a video source also selects an audio source. This is most evident when
the video and audio source is a tuner. Further audio connectors can
combine with more than one video input or output. Assumed two
composite video inputs and two audio inputs exist, there may be up to
four valid combinations. The relation of video and audio connectors
is defined in the <structfield > audioset</structfield> field of the
respective &v4l2-input; or &v4l2-output; , where each bit represents
the index number, starting at zero, of one audio input or output.</para>
<para > To learn about the number and attributes of the
available inputs and outputs applications can enumerate them with the
&VIDIOC-ENUMAUDIO; and &VIDIOC-ENUMAUDOUT; ioctl, respectively. The
&v4l2-audio; returned by the <constant > VIDIOC_ENUMAUDIO</constant> ioctl
also contains signal status information applicable when the current
audio input is queried.</para>
<para > The &VIDIOC-G-AUDIO; and &VIDIOC-G-AUDOUT; ioctl report
the current audio input and output, respectively. Note that, unlike
&VIDIOC-G-INPUT; and &VIDIOC-G-OUTPUT; these ioctls return a structure
as <constant > VIDIOC_ENUMAUDIO</constant> and
<constant > VIDIOC_ENUMAUDOUT</constant> do, not just an index.</para>
<para > To select an audio input and change its properties
applications call the &VIDIOC-S-AUDIO; ioctl. To select an audio
output (which presently has no changeable properties) applications
call the &VIDIOC-S-AUDOUT; ioctl.</para>
<para > Drivers must implement all input ioctls when the device
has one or more inputs, all output ioctls when the device has one
or more outputs. When the device has any audio inputs or outputs the
driver must set the <constant > V4L2_CAP_AUDIO</constant> flag in the
&v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl.</para>
<example >
<title > Information about the current audio input</title>
<programlisting >
&v4l2-audio; audio;
memset (& audio, 0, sizeof (audio));
if (-1 == ioctl (fd, &VIDIOC-G-AUDIO; , & audio)) {
perror ("VIDIOC_G_AUDIO");
exit (EXIT_FAILURE);
}
printf ("Current input: %s\n", audio.name);
</programlisting>
</example>
<example >
<title > Switching to the first audio input</title>
<programlisting >
&v4l2-audio; audio;
memset (& audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */
audio.index = 0;
if (-1 == ioctl (fd, &VIDIOC-S-AUDIO; , & audio)) {
perror ("VIDIOC_S_AUDIO");
exit (EXIT_FAILURE);
}
</programlisting>
</example>
</section>
<section id= "tuner" >
<title > Tuners and Modulators</title>
<section >
<title > Tuners</title>
<para > Video input devices can have one or more tuners
demodulating a RF signal. Each tuner is associated with one or more
video inputs, depending on the number of RF connectors on the tuner.
The <structfield > type</structfield> field of the respective
&v4l2-input; returned by the &VIDIOC-ENUMINPUT; ioctl is set to
<constant > V4L2_INPUT_TYPE_TUNER</constant> and its
<structfield > tuner</structfield> field contains the index number of
the tuner.</para>
<para > Radio devices have exactly one tuner with index zero, no
video inputs.</para>
<para > To query and change tuner properties applications use the
&VIDIOC-G-TUNER; and &VIDIOC-S-TUNER; ioctl, respectively. The
&v4l2-tuner; returned by <constant > VIDIOC_G_TUNER</constant> also
contains signal status information applicable when the tuner of the
current video input, or a radio tuner is queried. Note that
<constant > VIDIOC_S_TUNER</constant> does not switch the current tuner,
when there is more than one at all. The tuner is solely determined by
the current video input. Drivers must support both ioctls and set the
<constant > V4L2_CAP_TUNER</constant> flag in the &v4l2-capability;
returned by the &VIDIOC-QUERYCAP; ioctl when the device has one or
more tuners.</para>
</section>
<section >
<title > Modulators</title>
<para > Video output devices can have one or more modulators, uh,
modulating a video signal for radiation or connection to the antenna
input of a TV set or video recorder. Each modulator is associated with
one or more video outputs, depending on the number of RF connectors on
the modulator. The <structfield > type</structfield> field of the
respective &v4l2-output; returned by the &VIDIOC-ENUMOUTPUT; ioctl is
set to <constant > V4L2_OUTPUT_TYPE_MODULATOR</constant> and its
<structfield > modulator</structfield> field contains the index number
of the modulator. This specification does not define radio output
devices.</para>
<para > To query and change modulator properties applications use
the &VIDIOC-G-MODULATOR; and &VIDIOC-S-MODULATOR; ioctl. Note that
<constant > VIDIOC_S_MODULATOR</constant> does not switch the current
modulator, when there is more than one at all. The modulator is solely
determined by the current video output. Drivers must support both
ioctls and set the <constant > V4L2_CAP_MODULATOR</constant> flag in
the &v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl when the
device has one or more modulators.</para>
</section>
<section >
<title > Radio Frequency</title>
<para > To get and set the tuner or modulator radio frequency
applications use the &VIDIOC-G-FREQUENCY; and &VIDIOC-S-FREQUENCY;
ioctl which both take a pointer to a &v4l2-frequency; . These ioctls
are used for TV and radio devices alike. Drivers must support both
ioctls when the tuner or modulator ioctls are supported, or
when the device is a radio device.</para>
</section>
</section>
<section id= "standard" >
<title > Video Standards</title>
<para > Video devices typically support one or more different video
standards or variations of standards. Each video input and output may
support another set of standards. This set is reported by the
<structfield > std</structfield> field of &v4l2-input; and
&v4l2-output; returned by the &VIDIOC-ENUMINPUT; and
&VIDIOC-ENUMOUTPUT; ioctl, respectively.</para>
<para > V4L2 defines one bit for each analog video standard
currently in use worldwide, and sets aside bits for driver defined
standards, ⪚ hybrid standards to watch NTSC video tapes on PAL TVs
and vice versa. Applications can use the predefined bits to select a
particular standard, although presenting the user a menu of supported
standards is preferred. To enumerate and query the attributes of the
supported standards applications use the &VIDIOC-ENUMSTD; ioctl.</para>
<para > Many of the defined standards are actually just variations
of a few major standards. The hardware may in fact not distinguish
between them, or do so internal and switch automatically. Therefore
enumerated standards also contain sets of one or more standard
bits.</para>
<para > Assume a hypothetic tuner capable of demodulating B/PAL,
G/PAL and I/PAL signals. The first enumerated standard is a set of B
and G/PAL, switched automatically depending on the selected radio
frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I"
choice. Similar a Composite input may collapse standards, enumerating
"PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".<footnote >
<para > Some users are already confused by technical terms PAL,
NTSC and SECAM. There is no point asking them to distinguish between
B, G, D, or K when the software or hardware can do that
automatically.</para>
</footnote> </para>
<para > To query and select the standard used by the current video
input or output applications call the &VIDIOC-G-STD; and
&VIDIOC-S-STD; ioctl, respectively. The <emphasis > received</emphasis>
standard can be sensed with the &VIDIOC-QUERYSTD; ioctl. Note parameter of all these ioctls is a pointer to a &v4l2-std-id; type (a standard set), <emphasis > not</emphasis> an index into the standard enumeration.<footnote >
<para > An alternative to the current scheme is to use pointers
to indices as arguments of <constant > VIDIOC_G_STD</constant> and
<constant > VIDIOC_S_STD</constant> , the &v4l2-input; and
&v4l2-output; <structfield > std</structfield> field would be a set of
indices like <structfield > audioset</structfield> .</para>
<para > Indices are consistent with the rest of the API
and identify the standard unambiguously. In the present scheme of
things an enumerated standard is looked up by &v4l2-std-id; . Now the
standards supported by the inputs of a device can overlap. Just
assume the tuner and composite input in the example above both
exist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggests
a choice which does not exist. We cannot merge or omit sets, because
applications would be unable to find the standards reported by
<constant > VIDIOC_G_STD</constant> . That leaves separate enumerations
for each input. Also selecting a standard by &v4l2-std-id; can be
ambiguous. Advantage of this method is that applications need not
identify the standard indirectly, after enumerating.</para> <para > So in
summary, the lookup itself is unavoidable. The difference is only
whether the lookup is necessary to find an enumerated standard or to
switch to a standard by &v4l2-std-id; .</para>
</footnote> Drivers must implement all video standard ioctls
when the device has one or more video inputs or outputs.</para>
<para > Special rules apply to USB cameras where the notion of video
standards makes little sense. More generally any capture device,
output devices accordingly, which is <itemizedlist >
<listitem >
<para > incapable of capturing fields or frames at the nominal
rate of the video standard, or</para>
</listitem>
<listitem >
<para > where <link linkend= "buffer" > timestamps</link> refer
to the instant the field or frame was received by the driver, not the
capture time, or</para>
</listitem>
<listitem >
<para > where <link linkend= "buffer" > sequence numbers</link>
refer to the frames received by the driver, not the captured
frames.</para>
</listitem>
</itemizedlist> Here the driver shall set the
<structfield > std</structfield> field of &v4l2-input; and &v4l2-output;
to zero, the <constant > VIDIOC_G_STD</constant> ,
<constant > VIDIOC_S_STD</constant> ,
<constant > VIDIOC_QUERYSTD</constant> and
<constant > VIDIOC_ENUMSTD</constant> ioctls shall return the
&EINVAL; .<footnote >
<para > See <xref linkend= "buffer" /> for a rationale. Probably
even USB cameras follow some well known video standard. It might have
been better to explicitly indicate elsewhere if a device cannot live
up to normal expectations, instead of this exception.</para>
</footnote> </para>
<example >
<title > Information about the current video standard</title>
<programlisting >
&v4l2-std-id; std_id;
&v4l2-standard; standard;
if (-1 == ioctl (fd, &VIDIOC-G-STD; , & std_id)) {
/* Note when VIDIOC_ENUMSTD always returns EINVAL this
is no video device or it falls under the USB exception,
and VIDIOC_G_STD returning EINVAL is no error. */
perror ("VIDIOC_G_STD");
exit (EXIT_FAILURE);
}
memset (& standard, 0, sizeof (standard));
standard.index = 0;
while (0 == ioctl (fd, &VIDIOC-ENUMSTD; , & standard)) {
if (standard.id & std_id) {
printf ("Current video standard: %s\n", standard.name);
exit (EXIT_SUCCESS);
}
standard.index++;
}
/* EINVAL indicates the end of the enumeration, which cannot be
empty unless this device falls under the USB exception. */
if (errno == EINVAL || standard.index == 0) {
perror ("VIDIOC_ENUMSTD");
exit (EXIT_FAILURE);
}
</programlisting>
</example>
<example >
<title > Listing the video standards supported by the current
input</title>
<programlisting >
&v4l2-input; input;
&v4l2-standard; standard;
memset (& input, 0, sizeof (input));
if (-1 == ioctl (fd, &VIDIOC-G-INPUT; , & input.index)) {
perror ("VIDIOC_G_INPUT");
exit (EXIT_FAILURE);
}
if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT; , & input)) {
perror ("VIDIOC_ENUM_INPUT");
exit (EXIT_FAILURE);
}
printf ("Current input %s supports:\n", input.name);
memset (& standard, 0, sizeof (standard));
standard.index = 0;
while (0 == ioctl (fd, &VIDIOC-ENUMSTD; , & standard)) {
if (standard.id & input.std)
printf ("%s\n", standard.name);
standard.index++;
}
/* EINVAL indicates the end of the enumeration, which cannot be
empty unless this device falls under the USB exception. */
if (errno != EINVAL || standard.index == 0) {
perror ("VIDIOC_ENUMSTD");
exit (EXIT_FAILURE);
}
</programlisting>
</example>
<example >
<title > Selecting a new video standard</title>
<programlisting >
&v4l2-input; input;
&v4l2-std-id; std_id;
memset (& input, 0, sizeof (input));
if (-1 == ioctl (fd, &VIDIOC-G-INPUT; , & input.index)) {
perror ("VIDIOC_G_INPUT");
exit (EXIT_FAILURE);
}
if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT; , & input)) {
perror ("VIDIOC_ENUM_INPUT");
exit (EXIT_FAILURE);
}
if (0 == (input.std & V4L2_STD_PAL_BG)) {
fprintf (stderr, "Oops. B/G PAL is not supported.\n");
exit (EXIT_FAILURE);
}
/* Note this is also supposed to work when only B
<emphasis > or</emphasis> G/PAL is supported. */
std_id = V4L2_STD_PAL_BG;
if (-1 == ioctl (fd, &VIDIOC-S-STD; , & std_id)) {
perror ("VIDIOC_S_STD");
exit (EXIT_FAILURE);
}
</programlisting>
</example>
</section>
&sub-controls;
<section id= "format" >
<title > Data Formats</title>
<section >
<title > Data Format Negotiation</title>
<para > Different devices exchange different kinds of data with
applications, for example video images, raw or sliced VBI data, RDS
datagrams. Even within one kind many different formats are possible,
in particular an abundance of image formats. Although drivers must
provide a default and the selection persists across closing and
reopening a device, applications should always negotiate a data format
before engaging in data exchange. Negotiation means the application
asks for a particular format and the driver selects and reports the
best the hardware can do to satisfy the request. Of course
applications can also just query the current selection.</para>
<para > A single mechanism exists to negotiate all data formats
using the aggregate &v4l2-format; and the &VIDIOC-G-FMT; and
&VIDIOC-S-FMT; ioctls. Additionally the &VIDIOC-TRY-FMT; ioctl can be
used to examine what the hardware <emphasis > could</emphasis> do,
without actually selecting a new data format. The data formats
supported by the V4L2 API are covered in the respective device section
in <xref linkend= "devices" /> . For a closer look at image formats see
<xref linkend= "pixfmt" /> .</para>
<para > The <constant > VIDIOC_S_FMT</constant> ioctl is a major
turning-point in the initialization sequence. Prior to this point
multiple panel applications can access the same device concurrently to
select the current input, change controls or modify other properties.
The first <constant > VIDIOC_S_FMT</constant> assigns a logical stream
(video data, VBI data etc.) exclusively to one file descriptor.</para>
<para > Exclusive means no other application, more precisely no
other file descriptor, can grab this stream or change device
properties inconsistent with the negotiated parameters. A video
standard change for example, when the new standard uses a different
number of scan lines, can invalidate the selected image format.
Therefore only the file descriptor owning the stream can make
invalidating changes. Accordingly multiple file descriptors which
grabbed different logical streams prevent each other from interfering
with their settings. When for example video overlay is about to start
or already in progress, simultaneous video capturing may be restricted
to the same cropping and image size.</para>
<para > When applications omit the
<constant > VIDIOC_S_FMT</constant> ioctl its locking side effects are
implied by the next step, the selection of an I/O method with the
&VIDIOC-REQBUFS; ioctl or implicit with the first &func-read; or
&func-write; call.</para>
<para > Generally only one logical stream can be assigned to a
file descriptor, the exception being drivers permitting simultaneous
video capturing and overlay using the same file descriptor for
compatibility with V4L and earlier versions of V4L2. Switching the
logical stream or returning into "panel mode" is possible by closing
and reopening the device. Drivers <emphasis > may</emphasis> support a
switch using <constant > VIDIOC_S_FMT</constant> .</para>
<para > All drivers exchanging data with
applications must support the <constant > VIDIOC_G_FMT</constant> and
<constant > VIDIOC_S_FMT</constant> ioctl. Implementation of the
<constant > VIDIOC_TRY_FMT</constant> is highly recommended but
optional.</para>
</section>
<section >
<title > Image Format Enumeration</title>
<para > Apart of the generic format negotiation functions
a special ioctl to enumerate all image formats supported by video
capture, overlay or output devices is available.<footnote >
<para > Enumerating formats an application has no a-priori
2009-09-16 07:27:18 +08:00
knowledge of (otherwise it could explicitly ask for them and need not
2009-09-14 09:16:04 +08:00
enumerate) seems useless, but there are applications serving as proxy
between drivers and the actual video applications for which this is
useful.</para>
</footnote> </para>
<para > The &VIDIOC-ENUM-FMT; ioctl must be supported
by all drivers exchanging image data with applications.</para>
<important >
<para > Drivers are not supposed to convert image formats in
kernel space. They must enumerate only formats directly supported by
the hardware. If necessary driver writers should publish an example
conversion routine or library for integration into applications.</para>
</important>
</section>
</section>
<section id= "crop" >
<title > Image Cropping, Insertion and Scaling</title>
<para > Some video capture devices can sample a subsection of the
picture and shrink or enlarge it to an image of arbitrary size. We
call these abilities cropping and scaling. Some video output devices
can scale an image up or down and insert it at an arbitrary scan line
and horizontal offset into a video signal.</para>
<para > Applications can use the following API to select an area in
the video signal, query the default area and the hardware limits.
<emphasis > Despite their name, the &VIDIOC-CROPCAP; , &VIDIOC-G-CROP;
and &VIDIOC-S-CROP; ioctls apply to input as well as output
devices.</emphasis> </para>
<para > Scaling requires a source and a target. On a video capture
or overlay device the source is the video signal, and the cropping
ioctls determine the area actually sampled. The target are images
read by the application or overlaid onto the graphics screen. Their
size (and position for an overlay) is negotiated with the
&VIDIOC-G-FMT; and &VIDIOC-S-FMT; ioctls.</para>
<para > On a video output device the source are the images passed in
by the application, and their size is again negotiated with the
<constant > VIDIOC_G/S_FMT</constant> ioctls, or may be encoded in a
compressed video stream. The target is the video signal, and the
cropping ioctls determine the area where the images are
inserted.</para>
<para > Source and target rectangles are defined even if the device
does not support scaling or the <constant > VIDIOC_G/S_CROP</constant>
ioctls. Their size (and position where applicable) will be fixed in
this case. <emphasis > All capture and output device must support the
<constant > VIDIOC_CROPCAP</constant> ioctl such that applications can
determine if scaling takes place.</emphasis> </para>
<section >
<title > Cropping Structures</title>
<figure id= "crop-scale" >
<title > Image Cropping, Insertion and Scaling</title>
<mediaobject >
<imageobject >
<imagedata fileref= "crop.pdf" format= "PS" />
</imageobject>
<imageobject >
<imagedata fileref= "crop.gif" format= "GIF" />
</imageobject>
<textobject >
<phrase > The cropping, insertion and scaling process</phrase>
</textobject>
</mediaobject>
</figure>
<para > For capture devices the coordinates of the top left
corner, width and height of the area which can be sampled is given by
the <structfield > bounds</structfield> substructure of the
&v4l2-cropcap; returned by the <constant > VIDIOC_CROPCAP</constant>
ioctl. To support a wide range of hardware this specification does not
define an origin or units. However by convention drivers should
horizontally count unscaled samples relative to 0H (the leading edge
of the horizontal sync pulse, see <xref linkend= "vbi-hsync" /> ).
Vertically ITU-R line
numbers of the first field (<xref linkend= "vbi-525" /> , <xref
linkend="vbi-625" />), multiplied by two if the driver can capture both
fields.</para>
<para > The top left corner, width and height of the source
rectangle, that is the area actually sampled, is given by &v4l2-crop;
using the same coordinate system as &v4l2-cropcap; . Applications can
use the <constant > VIDIOC_G_CROP</constant> and
<constant > VIDIOC_S_CROP</constant> ioctls to get and set this
rectangle. It must lie completely within the capture boundaries and
the driver may further adjust the requested size and/or position
according to hardware limitations.</para>
<para > Each capture device has a default source rectangle, given
by the <structfield > defrect</structfield> substructure of
&v4l2-cropcap; . The center of this rectangle shall align with the
center of the active picture area of the video signal, and cover what
the driver writer considers the complete picture. Drivers shall reset
the source rectangle to the default when the driver is first loaded,
but not later.</para>
<para > For output devices these structures and ioctls are used
accordingly, defining the <emphasis > target</emphasis> rectangle where
the images will be inserted into the video signal.</para>
</section>
<section >
<title > Scaling Adjustments</title>
<para > Video hardware can have various cropping, insertion and
scaling limitations. It may only scale up or down, support only
discrete scaling factors, or have different scaling abilities in
horizontal and vertical direction. Also it may not support scaling at
all. At the same time the &v4l2-crop; rectangle may have to be
aligned, and both the source and target rectangles may have arbitrary
upper and lower size limits. In particular the maximum
<structfield > width</structfield> and <structfield > height</structfield>
in &v4l2-crop; may be smaller than the
&v4l2-cropcap; .<structfield > bounds</structfield> area. Therefore, as
usual, drivers are expected to adjust the requested parameters and
return the actual values selected.</para>
<para > Applications can change the source or the target rectangle
first, as they may prefer a particular image size or a certain area in
the video signal. If the driver has to adjust both to satisfy hardware
limitations, the last requested rectangle shall take priority, and the
driver should preferably adjust the opposite one. The &VIDIOC-TRY-FMT;
ioctl however shall not change the driver state and therefore only
adjust the requested rectangle.</para>
<para > Suppose scaling on a video capture device is restricted to
a factor 1:1 or 2:1 in either direction and the target image size must
be a multiple of 16 × 16 pixels. The source cropping
rectangle is set to defaults, which are also the upper limit in this
example, of 640 × 400 pixels at offset 0, 0. An
application requests an image size of 300 × 225
pixels, assuming video will be scaled down from the "full picture"
accordingly. The driver sets the image size to the closest possible
values 304 × 224, then chooses the cropping rectangle
closest to the requested size, that is 608 × 224
(224 × 2:1 would exceed the limit 400). The offset
0, 0 is still valid, thus unmodified. Given the default cropping
rectangle reported by <constant > VIDIOC_CROPCAP</constant> the
application can easily propose another offset to center the cropping
rectangle.</para>
<para > Now the application may insist on covering an area using a
picture aspect ratio closer to the original request, so it asks for a
cropping rectangle of 608 × 456 pixels. The present
scaling factors limit cropping to 640 × 384, so the
driver returns the cropping size 608 × 384 and adjusts
the image size to closest possible 304 × 192.</para>
</section>
<section >
<title > Examples</title>
<para > Source and target rectangles shall remain unchanged across
closing and reopening a device, such that piping data into or out of a
device will work without special preparations. More advanced
applications should ensure the parameters are suitable before starting
I/O.</para>
<example >
<title > Resetting the cropping parameters</title>
<para > (A video capture device is assumed; change
<constant > V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> for other
devices.)</para>
<programlisting >
&v4l2-cropcap; cropcap;
&v4l2-crop; crop;
memset (& cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl (fd, &VIDIOC-CROPCAP; , & cropcap)) {
perror ("VIDIOC_CROPCAP");
exit (EXIT_FAILURE);
}
memset (& crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
crop.c = cropcap.defrect;
/* Ignore if cropping is not supported (EINVAL). */
if (-1 == ioctl (fd, &VIDIOC-S-CROP; , & crop)
& & errno != EINVAL) {
perror ("VIDIOC_S_CROP");
exit (EXIT_FAILURE);
}
</programlisting>
</example>
<example >
<title > Simple downscaling</title>
<para > (A video capture device is assumed.)</para>
<programlisting >
&v4l2-cropcap; cropcap;
&v4l2-format; format;
reset_cropping_parameters ();
/* Scale down to 1/4 size of full picture. */
memset (& format, 0, sizeof (format)); /* defaults */
format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
format.fmt.pix.width = cropcap.defrect.width > > 1;
format.fmt.pix.height = cropcap.defrect.height > > 1;
format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
if (-1 == ioctl (fd, &VIDIOC-S-FMT; , & format)) {
perror ("VIDIOC_S_FORMAT");
exit (EXIT_FAILURE);
}
/* We could check the actual image size now, the actual scaling factor
or if the driver can scale at all. */
</programlisting>
</example>
<example >
<title > Selecting an output area</title>
<programlisting >
&v4l2-cropcap; cropcap;
&v4l2-crop; crop;
memset (& cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
if (-1 == ioctl (fd, VIDIOC_CROPCAP;, & cropcap)) {
perror ("VIDIOC_CROPCAP");
exit (EXIT_FAILURE);
}
memset (& crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
crop.c = cropcap.defrect;
/* Scale the width and height to 50 % of their original size
and center the output. */
crop.c.width /= 2;
crop.c.height /= 2;
crop.c.left += crop.c.width / 2;
crop.c.top += crop.c.height / 2;
/* Ignore if cropping is not supported (EINVAL). */
if (-1 == ioctl (fd, VIDIOC_S_CROP, & crop)
& & errno != EINVAL) {
perror ("VIDIOC_S_CROP");
exit (EXIT_FAILURE);
}
</programlisting>
</example>
<example >
<title > Current scaling factor and pixel aspect</title>
<para > (A video capture device is assumed.)</para>
<programlisting >
&v4l2-cropcap; cropcap;
&v4l2-crop; crop;
&v4l2-format; format;
double hscale, vscale;
double aspect;
int dwidth, dheight;
memset (& cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl (fd, &VIDIOC-CROPCAP; , & cropcap)) {
perror ("VIDIOC_CROPCAP");
exit (EXIT_FAILURE);
}
memset (& crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl (fd, &VIDIOC-G-CROP; , & crop)) {
if (errno != EINVAL) {
perror ("VIDIOC_G_CROP");
exit (EXIT_FAILURE);
}
/* Cropping not supported. */
crop.c = cropcap.defrect;
}
memset (& format, 0, sizeof (format));
format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl (fd, &VIDIOC-G-FMT; , & format)) {
perror ("VIDIOC_G_FMT");
exit (EXIT_FAILURE);
}
/* The scaling applied by the driver. */
hscale = format.fmt.pix.width / (double) crop.c.width;
vscale = format.fmt.pix.height / (double) crop.c.height;
aspect = cropcap.pixelaspect.numerator /
(double) cropcap.pixelaspect.denominator;
aspect = aspect * hscale / vscale;
/* Devices following ITU-R BT.601 do not capture
square pixels. For playback on a computer monitor
we should scale the images to this size. */
dwidth = format.fmt.pix.width / aspect;
dheight = format.fmt.pix.height;
</programlisting>
</example>
</section>
</section>
<section id= "streaming-par" >
<title > Streaming Parameters</title>
<para > Streaming parameters are intended to optimize the video
capture process as well as I/O. Presently applications can request a
high quality capture mode with the &VIDIOC-S-PARM; ioctl.</para>
<para > The current video standard determines a nominal number of
frames per second. If less than this number of frames is to be
captured or output, applications can request frame skipping or
duplicating on the driver side. This is especially useful when using
the &func-read; or &func-write; , which are not augmented by timestamps
or sequence counters, and to avoid unneccessary data copying.</para>
<para > Finally these ioctls can be used to determine the number of
buffers used internally by a driver in read/write mode. For
implications see the section discussing the &func-read;
function.</para>
<para > To get and set the streaming parameters applications call
the &VIDIOC-G-PARM; and &VIDIOC-S-PARM; ioctl, respectively. They take
a pointer to a &v4l2-streamparm; , which contains a union holding
separate parameters for input and output devices.</para>
<para > These ioctls are optional, drivers need not implement
them. If so, they return the &EINVAL; .</para>
</section>
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