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+  <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.&nbsp;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>
+&gt; 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.&nbsp;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;, &amp;index)) {
+        perror ("VIDIOC_G_INPUT");
+        exit (EXIT_FAILURE);
+}
+
+memset (&amp;input, 0, sizeof (input));
+input.index = index;
+
+if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &amp;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;, &amp;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 (&amp;audio, 0, sizeof (audio));
+
+if (-1 == ioctl (fd, &VIDIOC-G-AUDIO;, &amp;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 (&amp;audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */
+
+audio.index = 0;
+
+if (-1 == ioctl (fd, &VIDIOC-S-AUDIO;, &amp;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, &eg; 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;, &amp;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 (&amp;standard, 0, sizeof (standard));
+standard.index = 0;
+
+while (0 == ioctl (fd, &VIDIOC-ENUMSTD;, &amp;standard)) {
+        if (standard.id &amp; 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 (&amp;input, 0, sizeof (input));
+
+if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &amp;input.index)) {
+        perror ("VIDIOC_G_INPUT");
+        exit (EXIT_FAILURE);
+}
+
+if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &amp;input)) {
+        perror ("VIDIOC_ENUM_INPUT");
+        exit (EXIT_FAILURE);
+}
+
+printf ("Current input %s supports:\n", input.name);
+
+memset (&amp;standard, 0, sizeof (standard));
+standard.index = 0;
+
+while (0 == ioctl (fd, &VIDIOC-ENUMSTD;, &amp;standard)) {
+        if (standard.id &amp; 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 (&amp;input, 0, sizeof (input));
+
+if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &amp;input.index)) {
+        perror ("VIDIOC_G_INPUT");
+        exit (EXIT_FAILURE);
+}
+
+if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &amp;input)) {
+        perror ("VIDIOC_ENUM_INPUT");
+        exit (EXIT_FAILURE);
+}
+
+if (0 == (input.std &amp; 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;, &amp;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
+knowledge of (otherwise it could explicitly ask for them and need not
+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&nbsp;&times;&nbsp;16 pixels. The source cropping
+rectangle is set to defaults, which are also the upper limit in this
+example, of 640&nbsp;&times;&nbsp;400 pixels at offset 0,&nbsp;0. An
+application requests an image size of 300&nbsp;&times;&nbsp;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&nbsp;&times;&nbsp;224, then chooses the cropping rectangle
+closest to the requested size, that is 608&nbsp;&times;&nbsp;224
+(224&nbsp;&times;&nbsp;2:1 would exceed the limit 400). The offset
+0,&nbsp;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&nbsp;&times;&nbsp;456 pixels. The present
+scaling factors limit cropping to 640&nbsp;&times;&nbsp;384, so the
+driver returns the cropping size 608&nbsp;&times;&nbsp;384 and adjusts
+the image size to closest possible 304&nbsp;&times;&nbsp;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 (&amp;cropcap, 0, sizeof (cropcap));
+cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+if (-1 == ioctl (fd, &VIDIOC-CROPCAP;, &amp;cropcap)) {
+        perror ("VIDIOC_CROPCAP");
+        exit (EXIT_FAILURE);
+}
+
+memset (&amp;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;, &amp;crop)
+    &amp;&amp; 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 (&amp;format, 0, sizeof (format)); /* defaults */
+
+format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+format.fmt.pix.width = cropcap.defrect.width &gt;&gt; 1;
+format.fmt.pix.height = cropcap.defrect.height &gt;&gt; 1;
+format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
+
+if (-1 == ioctl (fd, &VIDIOC-S-FMT;, &amp;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 (&amp;cropcap, 0, sizeof (cropcap));
+cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
+
+if (-1 == ioctl (fd, VIDIOC_CROPCAP;, &amp;cropcap)) {
+        perror ("VIDIOC_CROPCAP");
+        exit (EXIT_FAILURE);
+}
+
+memset (&amp;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, &amp;crop)
+    &amp;&amp; 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 (&amp;cropcap, 0, sizeof (cropcap));
+cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+if (-1 == ioctl (fd, &VIDIOC-CROPCAP;, &amp;cropcap)) {
+        perror ("VIDIOC_CROPCAP");
+        exit (EXIT_FAILURE);
+}
+
+memset (&amp;crop, 0, sizeof (crop));
+crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+if (-1 == ioctl (fd, &VIDIOC-G-CROP;, &amp;crop)) {
+        if (errno != EINVAL) {
+                perror ("VIDIOC_G_CROP");
+                exit (EXIT_FAILURE);
+        }
+
+        /* Cropping not supported. */
+        crop.c = cropcap.defrect;
+}
+
+memset (&amp;format, 0, sizeof (format));
+format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+
+if (-1 == ioctl (fd, &VIDIOC-G-FMT;, &amp;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>
+
+  <!--
+Local Variables:
+mode: sgml
+sgml-parent-document: "v4l2.sgml"
+indent-tabs-mode: nil
+End:
+  -->