linux-vybrid_public/Documentation/DocBook/v4l/compat.xml

  <title>Changes</title>

  <para>The following chapters document the evolution of the V4L2 API,
errata or extensions. They are also intended to help application and
driver writers to port or update their code.</para>

  <section id="diff-v4l">
    <title>Differences between V4L and V4L2</title>

    <para>The Video For Linux API was first introduced in Linux 2.1 to
unify and replace various TV and radio device related interfaces,
developed independently by driver writers in prior years. Starting
with Linux 2.5 the much improved V4L2 API replaces the V4L API,
although existing drivers will continue to support V4L applications in
the future, either directly or through the V4L2 compatibility layer in
the <filename>videodev</filename> kernel module translating ioctls on
the fly. For a transition period not all drivers will support the V4L2
API.</para>

    <section>
      <title>Opening and Closing Devices</title>

      <para>For compatibility reasons the character device file names
recommended for V4L2 video capture, overlay, radio and raw
vbi capture devices did not change from those used by V4L. They are
listed in <xref linkend="devices" /> and below in <xref
	  linkend="v4l-dev" />.</para>

      <para>The teletext devices (minor range 192-223) have been removed in
V4L2 and no longer exist. There is no hardware available anymore for handling
pure teletext. Instead raw or sliced VBI is used.</para>

      <para>The V4L <filename>videodev</filename> module automatically
assigns minor numbers to drivers in load order, depending on the
registered device type. We recommend that V4L2 drivers by default
register devices with the same numbers, but the system administrator
can assign arbitrary minor numbers using driver module options. The
major device number remains 81.</para>

      <table id="v4l-dev">
	<title>V4L Device Types, Names and Numbers</title>
	<tgroup cols="3">
	  <thead>
	    <row>
	      <entry>Device Type</entry>
	      <entry>File Name</entry>
	      <entry>Minor Numbers</entry>
	    </row>
	  </thead>
	  <tbody valign="top">
	    <row>
	      <entry>Video capture and overlay</entry>
	      <entry><para><filename>/dev/video</filename> and
<filename>/dev/bttv0</filename><footnote> <para>According to
Documentation/devices.txt these should be symbolic links to
<filename>/dev/video0</filename>. Note the original bttv interface is
not compatible with V4L or V4L2.</para> </footnote>,
<filename>/dev/video0</filename> to
<filename>/dev/video63</filename></para></entry>
	      <entry>0-63</entry>
	    </row>
	    <row>
	      <entry>Radio receiver</entry>
	      <entry><para><filename>/dev/radio</filename><footnote>
		    <para>According to
<filename>Documentation/devices.txt</filename> a symbolic link to
<filename>/dev/radio0</filename>.</para>
		  </footnote>, <filename>/dev/radio0</filename> to
<filename>/dev/radio63</filename></para></entry>
	      <entry>64-127</entry>
	    </row>
	    <row>
	      <entry>Raw VBI capture</entry>
	      <entry><para><filename>/dev/vbi</filename>,
<filename>/dev/vbi0</filename> to
<filename>/dev/vbi31</filename></para></entry>
	      <entry>224-255</entry>
	    </row>
	  </tbody>
	</tgroup>
      </table>

      <para>V4L prohibits (or used to prohibit) multiple opens of a
device file. V4L2 drivers <emphasis>may</emphasis> support multiple
opens, see <xref linkend="open" /> for details and consequences.</para>

      <para>V4L drivers respond to V4L2 ioctls with an &EINVAL;. The
compatibility layer in the V4L2 <filename>videodev</filename> module
can translate V4L ioctl requests to their V4L2 counterpart, however a
V4L2 driver usually needs more preparation to become fully V4L
compatible. This is covered in more detail in <xref
	  linkend="driver" />.</para>
    </section>

    <section>
      <title>Querying Capabilities</title>

      <para>The V4L <constant>VIDIOCGCAP</constant> ioctl is
equivalent to V4L2's &VIDIOC-QUERYCAP;.</para>

      <para>The <structfield>name</structfield> field in struct
<structname>video_capability</structname> became
<structfield>card</structfield> in &v4l2-capability;,
<structfield>type</structfield> was replaced by
<structfield>capabilities</structfield>. Note V4L2 does not
distinguish between device types like this, better think of basic
video input, video output and radio devices supporting a set of
related functions like video capturing, video overlay and VBI
capturing. See <xref linkend="open" /> for an
introduction.<informaltable>
	  <tgroup cols="3">
	    <thead>
	      <row>
		<entry>struct
<structname>video_capability</structname>
<structfield>type</structfield></entry>
		<entry>&v4l2-capability;
<structfield>capabilities</structfield> flags</entry>
		<entry>Purpose</entry>
	      </row>
	    </thead>
	    <tbody valign="top">
	      <row>
		<entry><constant>VID_TYPE_CAPTURE</constant></entry>
		<entry><constant>V4L2_CAP_VIDEO_CAPTURE</constant></entry>
		<entry>The <link linkend="capture">video
capture</link> interface is supported.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_TUNER</constant></entry>
		<entry><constant>V4L2_CAP_TUNER</constant></entry>
		<entry>The device has a <link linkend="tuner">tuner or
modulator</link>.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_TELETEXT</constant></entry>
		<entry><constant>V4L2_CAP_VBI_CAPTURE</constant></entry>
		<entry>The <link linkend="raw-vbi">raw VBI
capture</link> interface is supported.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_OVERLAY</constant></entry>
		<entry><constant>V4L2_CAP_VIDEO_OVERLAY</constant></entry>
		<entry>The <link linkend="overlay">video
overlay</link> interface is supported.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_CHROMAKEY</constant></entry>
		<entry><constant>V4L2_FBUF_CAP_CHROMAKEY</constant> in
field <structfield>capability</structfield> of
&v4l2-framebuffer;</entry>
		<entry>Whether chromakey overlay is supported. For
more information on overlay see
<xref linkend="overlay" />.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_CLIPPING</constant></entry>
		<entry><constant>V4L2_FBUF_CAP_LIST_CLIPPING</constant>
and <constant>V4L2_FBUF_CAP_BITMAP_CLIPPING</constant> in field
<structfield>capability</structfield> of &v4l2-framebuffer;</entry>
		<entry>Whether clipping the overlaid image is
supported, see <xref linkend="overlay" />.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_FRAMERAM</constant></entry>
		<entry><constant>V4L2_FBUF_CAP_EXTERNOVERLAY</constant>
<emphasis>not set</emphasis> in field
<structfield>capability</structfield> of &v4l2-framebuffer;</entry>
		<entry>Whether overlay overwrites frame buffer memory,
see <xref linkend="overlay" />.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_SCALES</constant></entry>
		<entry><constant>-</constant></entry>
		<entry>This flag indicates if the hardware can scale
images. The V4L2 API implies the scale factor by setting the cropping
dimensions and image size with the &VIDIOC-S-CROP; and &VIDIOC-S-FMT;
ioctl, respectively. The driver returns the closest sizes possible.
For more information on cropping and scaling see <xref
		    linkend="crop" />.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_MONOCHROME</constant></entry>
		<entry><constant>-</constant></entry>
		<entry>Applications can enumerate the supported image
formats with the &VIDIOC-ENUM-FMT; ioctl to determine if the device
supports grey scale capturing only. For more information on image
formats see <xref linkend="pixfmt" />.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_SUBCAPTURE</constant></entry>
		<entry><constant>-</constant></entry>
		<entry>Applications can call the &VIDIOC-G-CROP; ioctl
to determine if the device supports capturing a subsection of the full
picture ("cropping" in V4L2). If not, the ioctl returns the &EINVAL;.
For more information on cropping and scaling see <xref
		    linkend="crop" />.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_MPEG_DECODER</constant></entry>
		<entry><constant>-</constant></entry>
		<entry>Applications can enumerate the supported image
formats with the &VIDIOC-ENUM-FMT; ioctl to determine if the device
supports MPEG streams.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_MPEG_ENCODER</constant></entry>
		<entry><constant>-</constant></entry>
		<entry>See above.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_MJPEG_DECODER</constant></entry>
		<entry><constant>-</constant></entry>
		<entry>See above.</entry>
	      </row>
	      <row>
		<entry><constant>VID_TYPE_MJPEG_ENCODER</constant></entry>
		<entry><constant>-</constant></entry>
		<entry>See above.</entry>
	      </row>
	    </tbody>
	  </tgroup>
	</informaltable></para>

      <para>The <structfield>audios</structfield> field was replaced
by <structfield>capabilities</structfield> flag
<constant>V4L2_CAP_AUDIO</constant>, indicating
<emphasis>if</emphasis> the device has any audio inputs or outputs. To
determine their number applications can enumerate audio inputs with
the &VIDIOC-G-AUDIO; ioctl. The audio ioctls are described in <xref
	  linkend="audio" />.</para>

      <para>The <structfield>maxwidth</structfield>,
<structfield>maxheight</structfield>,
<structfield>minwidth</structfield> and
<structfield>minheight</structfield> fields were removed. Calling the
&VIDIOC-S-FMT; or &VIDIOC-TRY-FMT; ioctl with the desired dimensions
returns the closest size possible, taking into account the current
video standard, cropping and scaling limitations.</para>
    </section>

    <section>
      <title>Video Sources</title>

      <para>V4L provides the <constant>VIDIOCGCHAN</constant> and
<constant>VIDIOCSCHAN</constant> ioctl using struct
<structname>video_channel</structname> to enumerate
the video inputs of a V4L device. The equivalent V4L2 ioctls
are &VIDIOC-ENUMINPUT;, &VIDIOC-G-INPUT; and &VIDIOC-S-INPUT;
using &v4l2-input; as discussed in <xref linkend="video" />.</para>

      <para>The <structfield>channel</structfield> field counting
inputs was renamed to <structfield>index</structfield>, the video
input types were renamed as follows: <informaltable>
	  <tgroup cols="2">
	    <thead>
	      <row>
		<entry>struct <structname>video_channel</structname>
<structfield>type</structfield></entry>
		<entry>&v4l2-input;
<structfield>type</structfield></entry>
	      </row>
	    </thead>
	    <tbody valign="top">
	      <row>
		<entry><constant>VIDEO_TYPE_TV</constant></entry>
		<entry><constant>V4L2_INPUT_TYPE_TUNER</constant></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_TYPE_CAMERA</constant></entry>
		<entry><constant>V4L2_INPUT_TYPE_CAMERA</constant></entry>
	      </row>
	    </tbody>
	  </tgroup>
	</informaltable></para>

      <para>Unlike the <structfield>tuners</structfield> field
expressing the number of tuners of this input, V4L2 assumes each video
input is connected to at most one tuner. However a tuner can have more
than one input, &ie; RF connectors, and a device can have multiple
tuners. The index number of the tuner associated with the input, if
any, is stored in field <structfield>tuner</structfield> of
&v4l2-input;. Enumeration of tuners is discussed in <xref
	  linkend="tuner" />.</para>

      <para>The redundant <constant>VIDEO_VC_TUNER</constant> flag was
dropped. Video inputs associated with a tuner are of type
<constant>V4L2_INPUT_TYPE_TUNER</constant>. The
<constant>VIDEO_VC_AUDIO</constant> flag was replaced by the
<structfield>audioset</structfield> field. V4L2 considers devices with
up to 32 audio inputs. Each set bit in the
<structfield>audioset</structfield> field represents one audio input
this video input combines with. For information about audio inputs and
how to switch between them see <xref linkend="audio" />.</para>

      <para>The <structfield>norm</structfield> field describing the
supported video standards was replaced by
<structfield>std</structfield>. The V4L specification mentions a flag
<constant>VIDEO_VC_NORM</constant> indicating whether the standard can
be changed. This flag was a later addition together with the
<structfield>norm</structfield> field and has been removed in the
meantime. V4L2 has a similar, albeit more comprehensive approach
to video standards, see <xref linkend="standard" /> for more
information.</para>
    </section>

    <section>
      <title>Tuning</title>

      <para>The V4L <constant>VIDIOCGTUNER</constant> and
<constant>VIDIOCSTUNER</constant> ioctl and struct
<structname>video_tuner</structname> can be used to enumerate the
tuners of a V4L TV or radio device. The equivalent V4L2 ioctls are
&VIDIOC-G-TUNER; and &VIDIOC-S-TUNER; using &v4l2-tuner;. Tuners are
covered in <xref linkend="tuner" />.</para>

      <para>The <structfield>tuner</structfield> field counting tuners
was renamed to <structfield>index</structfield>. The fields
<structfield>name</structfield>, <structfield>rangelow</structfield>
and <structfield>rangehigh</structfield> remained unchanged.</para>

      <para>The <constant>VIDEO_TUNER_PAL</constant>,
<constant>VIDEO_TUNER_NTSC</constant> and
<constant>VIDEO_TUNER_SECAM</constant> flags indicating the supported
video standards were dropped. This information is now contained in the
associated &v4l2-input;. No replacement exists for the
<constant>VIDEO_TUNER_NORM</constant> flag indicating whether the
video standard can be switched. The <structfield>mode</structfield>
field to select a different video standard was replaced by a whole new
set of ioctls and structures described in <xref linkend="standard" />.
Due to its ubiquity it should be mentioned the BTTV driver supports
several standards in addition to the regular
<constant>VIDEO_MODE_PAL</constant> (0),
<constant>VIDEO_MODE_NTSC</constant>,
<constant>VIDEO_MODE_SECAM</constant> and
<constant>VIDEO_MODE_AUTO</constant> (3). Namely N/PAL Argentina,
M/PAL, N/PAL, and NTSC Japan with numbers 3-6 (sic).</para>

      <para>The <constant>VIDEO_TUNER_STEREO_ON</constant> flag
indicating stereo reception became
<constant>V4L2_TUNER_SUB_STEREO</constant> in field
<structfield>rxsubchans</structfield>. This field also permits the
detection of monaural and bilingual audio, see the definition of
&v4l2-tuner; for details. Presently no replacement exists for the
<constant>VIDEO_TUNER_RDS_ON</constant> and
<constant>VIDEO_TUNER_MBS_ON</constant> flags.</para>

      <para> The <constant>VIDEO_TUNER_LOW</constant> flag was renamed
to <constant>V4L2_TUNER_CAP_LOW</constant> in the &v4l2-tuner;
<structfield>capability</structfield> field.</para>

      <para>The <constant>VIDIOCGFREQ</constant> and
<constant>VIDIOCSFREQ</constant> ioctl to change the tuner frequency
where renamed to &VIDIOC-G-FREQUENCY; and  &VIDIOC-S-FREQUENCY;. They
take a pointer to a &v4l2-frequency; instead of an unsigned long
integer.</para>
    </section>

    <section id="v4l-image-properties">
      <title>Image Properties</title>

      <para>V4L2 has no equivalent of the
<constant>VIDIOCGPICT</constant> and <constant>VIDIOCSPICT</constant>
ioctl and struct <structname>video_picture</structname>. The following
fields where replaced by V4L2 controls accessible with the
&VIDIOC-QUERYCTRL;, &VIDIOC-G-CTRL; and &VIDIOC-S-CTRL; ioctls:<informaltable>
	  <tgroup cols="2">
	    <thead>
	      <row>
		<entry>struct <structname>video_picture</structname></entry>
		<entry>V4L2 Control ID</entry>
	      </row>
	    </thead>
	    <tbody valign="top">
	      <row>
		<entry><structfield>brightness</structfield></entry>
		<entry><constant>V4L2_CID_BRIGHTNESS</constant></entry>
	      </row>
	      <row>
		<entry><structfield>hue</structfield></entry>
		<entry><constant>V4L2_CID_HUE</constant></entry>
	      </row>
	      <row>
		<entry><structfield>colour</structfield></entry>
		<entry><constant>V4L2_CID_SATURATION</constant></entry>
	      </row>
	      <row>
		<entry><structfield>contrast</structfield></entry>
		<entry><constant>V4L2_CID_CONTRAST</constant></entry>
	      </row>
	      <row>
		<entry><structfield>whiteness</structfield></entry>
		<entry><constant>V4L2_CID_WHITENESS</constant></entry>
	      </row>
	    </tbody>
	  </tgroup>
	</informaltable></para>

      <para>The V4L picture controls are assumed to range from 0 to
65535 with no particular reset value. The V4L2 API permits arbitrary
limits and defaults which can be queried with the &VIDIOC-QUERYCTRL;
ioctl. For general information about controls see <xref
linkend="control" />.</para>

      <para>The <structfield>depth</structfield> (average number of
bits per pixel) of a video image is implied by the selected image
format. V4L2 does not explicitely provide such information assuming
applications recognizing the format are aware of the image depth and
others need not know. The <structfield>palette</structfield> field
moved into the &v4l2-pix-format;:<informaltable>
	  <tgroup cols="2">
	    <thead>
	      <row>
		<entry>struct <structname>video_picture</structname>
<structfield>palette</structfield></entry>
		<entry>&v4l2-pix-format;
<structfield>pixfmt</structfield></entry>
	      </row>
	    </thead>
	    <tbody valign="top">
	      <row>
		<entry><constant>VIDEO_PALETTE_GREY</constant></entry>
		<entry><para><link
linkend="V4L2-PIX-FMT-GREY"><constant>V4L2_PIX_FMT_GREY</constant></link></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_HI240</constant></entry>
		<entry><para><link
linkend="pixfmt-reserved"><constant>V4L2_PIX_FMT_HI240</constant></link><footnote>
		      <para>This is a custom format used by the BTTV
driver, not one of the V4L2 standard formats.</para>
		    </footnote></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_RGB565</constant></entry>
		<entry><para><link
linkend="pixfmt-rgb"><constant>V4L2_PIX_FMT_RGB565</constant></link></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_RGB555</constant></entry>
		<entry><para><link
linkend="pixfmt-rgb"><constant>V4L2_PIX_FMT_RGB555</constant></link></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_RGB24</constant></entry>
		<entry><para><link
linkend="pixfmt-rgb"><constant>V4L2_PIX_FMT_BGR24</constant></link></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_RGB32</constant></entry>
		<entry><para><link
linkend="pixfmt-rgb"><constant>V4L2_PIX_FMT_BGR32</constant></link><footnote>
		      <para>Presumably all V4L RGB formats are
little-endian, although some drivers might interpret them according to machine endianess. V4L2 defines little-endian, big-endian and red/blue
swapped variants. For details see <xref linkend="pixfmt-rgb" />.</para>
		    </footnote></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_YUV422</constant></entry>
		<entry><para><link
linkend="V4L2-PIX-FMT-YUYV"><constant>V4L2_PIX_FMT_YUYV</constant></link></para></entry>
	      </row>
	      <row>
		<entry><para><constant>VIDEO_PALETTE_YUYV</constant><footnote>
		      <para><constant>VIDEO_PALETTE_YUV422</constant>
and <constant>VIDEO_PALETTE_YUYV</constant> are the same formats. Some
V4L drivers respond to one, some to the other.</para>
		    </footnote></para></entry>
		<entry><para><link
linkend="V4L2-PIX-FMT-YUYV"><constant>V4L2_PIX_FMT_YUYV</constant></link></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_UYVY</constant></entry>
		<entry><para><link
linkend="V4L2-PIX-FMT-UYVY"><constant>V4L2_PIX_FMT_UYVY</constant></link></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_YUV420</constant></entry>
		<entry>None</entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_YUV411</constant></entry>
		<entry><para><link
linkend="V4L2-PIX-FMT-Y41P"><constant>V4L2_PIX_FMT_Y41P</constant></link><footnote>
		      <para>Not to be confused with
<constant>V4L2_PIX_FMT_YUV411P</constant>, which is a planar
format.</para> </footnote></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_RAW</constant></entry>
		<entry><para>None<footnote> <para>V4L explains this
as: "RAW capture (BT848)"</para> </footnote></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_YUV422P</constant></entry>
		<entry><para><link
linkend="V4L2-PIX-FMT-YUV422P"><constant>V4L2_PIX_FMT_YUV422P</constant></link></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_YUV411P</constant></entry>
		<entry><para><link
linkend="V4L2-PIX-FMT-YUV411P"><constant>V4L2_PIX_FMT_YUV411P</constant></link><footnote>
		      <para>Not to be confused with
<constant>V4L2_PIX_FMT_Y41P</constant>, which is a packed
format.</para> </footnote></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_YUV420P</constant></entry>
		<entry><para><link
linkend="V4L2-PIX-FMT-YVU420"><constant>V4L2_PIX_FMT_YVU420</constant></link></para></entry>
	      </row>
	      <row>
		<entry><constant>VIDEO_PALETTE_YUV410P</constant></entry>
		<entry><para><link
linkend="V4L2-PIX-FMT-YVU410"><constant>V4L2_PIX_FMT_YVU410</constant></link></para></entry>
	      </row>
	    </tbody>
	  </tgroup>
	</informaltable></para>

      <para>V4L2 image formats are defined in <xref
linkend="pixfmt" />. The image format can be selected with the
&VIDIOC-S-FMT; ioctl.</para>
    </section>

    <section>
      <title>Audio</title>

      <para>The <constant>VIDIOCGAUDIO</constant> and
<constant>VIDIOCSAUDIO</constant> ioctl and struct
<structname>video_audio</structname> are used to enumerate the
audio inputs of a V4L device. The equivalent V4L2 ioctls are
&VIDIOC-G-AUDIO; and &VIDIOC-S-AUDIO; using &v4l2-audio; as
discussed in <xref linkend="audio" />.</para>

      <para>The <structfield>audio</structfield> "channel number"
field counting audio inputs was renamed to
<structfield>index</structfield>.</para>

      <para>On <constant>VIDIOCSAUDIO</constant> the
<structfield>mode</structfield> field selects <emphasis>one</emphasis>
of the <constant>VIDEO_SOUND_MONO</constant>,
<constant>VIDEO_SOUND_STEREO</constant>,
<constant>VIDEO_SOUND_LANG1</constant> or
<constant>VIDEO_SOUND_LANG2</constant> audio demodulation modes. When
the current audio standard is BTSC
<constant>VIDEO_SOUND_LANG2</constant> refers to SAP and
<constant>VIDEO_SOUND_LANG1</constant> is meaningless. Also
undocumented in the V4L specification, there is no way to query the
selected mode. On <constant>VIDIOCGAUDIO</constant> the driver returns
the <emphasis>actually received</emphasis> audio programmes in this
field. In the V4L2 API this information is stored in the &v4l2-tuner;
<structfield>rxsubchans</structfield> and
<structfield>audmode</structfield> fields, respectively. See <xref
linkend="tuner" /> for more information on tuners. Related to audio
modes &v4l2-audio; also reports if this is a mono or stereo
input, regardless if the source is a tuner.</para>

      <para>The following fields where replaced by V4L2 controls
accessible with the &VIDIOC-QUERYCTRL;, &VIDIOC-G-CTRL; and
&VIDIOC-S-CTRL; ioctls:<informaltable>
	  <tgroup cols="2">
	    <thead>
	      <row>
		<entry>struct
<structname>video_audio</structname></entry>
		<entry>V4L2 Control ID</entry>
	      </row>
	    </thead>
	    <tbody valign="top">
	      <row>
		<entry><structfield>volume</structfield></entry>
		<entry><constant>V4L2_CID_AUDIO_VOLUME</constant></entry>
	      </row>
	      <row>
		<entry><structfield>bass</structfield></entry>
		<entry><constant>V4L2_CID_AUDIO_BASS</constant></entry>
	      </row>
	      <row>
		<entry><structfield>treble</structfield></entry>
		<entry><constant>V4L2_CID_AUDIO_TREBLE</constant></entry>
	      </row>
	      <row>
		<entry><structfield>balance</structfield></entry>
		<entry><constant>V4L2_CID_AUDIO_BALANCE</constant></entry>
	      </row>
	    </tbody>
	  </tgroup>
	</informaltable></para>

      <para>To determine which of these controls are supported by a
driver V4L provides the <structfield>flags</structfield>
<constant>VIDEO_AUDIO_VOLUME</constant>,
<constant>VIDEO_AUDIO_BASS</constant>,
<constant>VIDEO_AUDIO_TREBLE</constant> and
<constant>VIDEO_AUDIO_BALANCE</constant>. In the V4L2 API the
&VIDIOC-QUERYCTRL; ioctl reports if the respective control is
supported. Accordingly the <constant>VIDEO_AUDIO_MUTABLE</constant>
and <constant>VIDEO_AUDIO_MUTE</constant> flags where replaced by the
boolean <constant>V4L2_CID_AUDIO_MUTE</constant> control.</para>

      <para>All V4L2 controls have a <structfield>step</structfield>
attribute replacing the struct <structname>video_audio</structname>
<structfield>step</structfield> field. The V4L audio controls are
assumed to range from 0 to 65535 with no particular reset value. The
V4L2 API permits arbitrary limits and defaults which can be queried
with the &VIDIOC-QUERYCTRL; ioctl. For general information about
controls see <xref linkend="control" />.</para>
    </section>

    <section>
      <title>Frame Buffer Overlay</title>

      <para>The V4L2 ioctls equivalent to
<constant>VIDIOCGFBUF</constant> and <constant>VIDIOCSFBUF</constant>
are &VIDIOC-G-FBUF; and &VIDIOC-S-FBUF;. The
<structfield>base</structfield> field of struct
<structname>video_buffer</structname> remained unchanged, except V4L2
defines a flag to indicate non-destructive overlays instead of a
<constant>NULL</constant> pointer. All other fields moved into the
&v4l2-pix-format; <structfield>fmt</structfield> substructure of
&v4l2-framebuffer;. The <structfield>depth</structfield> field was
replaced by <structfield>pixelformat</structfield>. See <xref
	  linkend="pixfmt-rgb" /> for a list of RGB formats and their
respective color depths.</para>

      <para>Instead of the special ioctls
<constant>VIDIOCGWIN</constant> and <constant>VIDIOCSWIN</constant>
V4L2 uses the general-purpose data format negotiation ioctls
&VIDIOC-G-FMT; and &VIDIOC-S-FMT;. They take a pointer to a
&v4l2-format; as argument. Here the <structfield>win</structfield>
member of the <structfield>fmt</structfield> union is used, a
&v4l2-window;.</para>

      <para>The <structfield>x</structfield>,
<structfield>y</structfield>, <structfield>width</structfield> and
<structfield>height</structfield> fields of struct
<structname>video_window</structname> moved into &v4l2-rect;
substructure <structfield>w</structfield> of struct
<structname>v4l2_window</structname>. The
<structfield>chromakey</structfield>,
<structfield>clips</structfield>, and
<structfield>clipcount</structfield> fields remained unchanged. Struct
<structname>video_clip</structname> was renamed to &v4l2-clip;, also
containing a struct <structname>v4l2_rect</structname>, but the
semantics are still the same.</para>

      <para>The <constant>VIDEO_WINDOW_INTERLACE</constant> flag was
dropped. Instead applications must set the
<structfield>field</structfield> field to
<constant>V4L2_FIELD_ANY</constant> or
<constant>V4L2_FIELD_INTERLACED</constant>. The
<constant>VIDEO_WINDOW_CHROMAKEY</constant> flag moved into
&v4l2-framebuffer;, under the new name
<constant>V4L2_FBUF_FLAG_CHROMAKEY</constant>.</para>

      <para>In V4L, storing a bitmap pointer in
<structfield>clips</structfield> and setting
<structfield>clipcount</structfield> to
<constant>VIDEO_CLIP_BITMAP</constant> (-1) requests bitmap
clipping, using a fixed size bitmap of 1024 &times; 625 bits. Struct
<structname>v4l2_window</structname> has a separate
<structfield>bitmap</structfield> pointer field for this purpose and
the bitmap size is determined by <structfield>w.width</structfield> and
<structfield>w.height</structfield>.</para>

      <para>The <constant>VIDIOCCAPTURE</constant> ioctl to enable or
disable overlay was renamed to &VIDIOC-OVERLAY;.</para>
    </section>

    <section>
      <title>Cropping</title>

      <para>To capture only a subsection of the full picture V4L
defines the <constant>VIDIOCGCAPTURE</constant> and
<constant>VIDIOCSCAPTURE</constant> ioctls using struct
<structname>video_capture</structname>. The equivalent V4L2 ioctls are
&VIDIOC-G-CROP; and &VIDIOC-S-CROP; using &v4l2-crop;, and the related
&VIDIOC-CROPCAP; ioctl. This is a rather complex matter, see
<xref linkend="crop" /> for details.</para>

      <para>The <structfield>x</structfield>,
<structfield>y</structfield>, <structfield>width</structfield> and
<structfield>height</structfield> fields moved into &v4l2-rect;
substructure <structfield>c</structfield> of struct
<structname>v4l2_crop</structname>. The
<structfield>decimation</structfield> field was dropped. In the V4L2
API the scaling factor is implied by the size of the cropping
rectangle and the size of the captured or overlaid image.</para>

      <para>The <constant>VIDEO_CAPTURE_ODD</constant>
and <constant>VIDEO_CAPTURE_EVEN</constant> flags to capture only the
odd or even field, respectively, were replaced by
<constant>V4L2_FIELD_TOP</constant> and
<constant>V4L2_FIELD_BOTTOM</constant> in the field named
<structfield>field</structfield> of &v4l2-pix-format; and
&v4l2-window;. These structures are used to select a capture or
overlay format with the &VIDIOC-S-FMT; ioctl.</para>
    </section>

    <section>
      <title>Reading Images, Memory Mapping</title>

      <section>
	<title>Capturing using the read method</title>

	<para>There is no essential difference between reading images
from a V4L or V4L2 device using the &func-read; function, however V4L2
drivers are not required to support this I/O method. Applications can
determine if the function is available with the &VIDIOC-QUERYCAP;
ioctl. All V4L2 devices exchanging data with applications must support
the &func-select; and &func-poll; functions.</para>

	<para>To select an image format and size, V4L provides the
<constant>VIDIOCSPICT</constant> and <constant>VIDIOCSWIN</constant>
ioctls. V4L2 uses the general-purpose data format negotiation ioctls
&VIDIOC-G-FMT; and &VIDIOC-S-FMT;. They take a pointer to a
&v4l2-format; as argument, here the &v4l2-pix-format; named
<structfield>pix</structfield> of its <structfield>fmt</structfield>
union is used.</para>

	<para>For more information about the V4L2 read interface see
<xref linkend="rw" />.</para>
      </section>
      <section>
	<title>Capturing using memory mapping</title>

	<para>Applications can read from V4L devices by mapping
buffers in device memory, or more often just buffers allocated in
DMA-able system memory, into their address space. This avoids the data
copying overhead of the read method. V4L2 supports memory mapping as
well, with a few differences.</para>

	<informaltable>
	  <tgroup cols="2">
	    <thead>
	      <row>
		<entry>V4L</entry>
		<entry>V4L2</entry>
	      </row>
	    </thead>
	    <tbody valign="top">
	      <row>
		<entry></entry>
		<entry>The image format must be selected before
buffers are allocated, with the &VIDIOC-S-FMT; ioctl. When no format
is selected the driver may use the last, possibly by another
application requested format.</entry>
	      </row>
	      <row>
		<entry><para>Applications cannot change the number of
buffers. The it is built into the driver, unless it has a module
option to change the number when the driver module is
loaded.</para></entry>
		<entry><para>The &VIDIOC-REQBUFS; ioctl allocates the
desired number of buffers, this is a required step in the initialization
sequence.</para></entry>
	      </row>
	      <row>
		<entry><para>Drivers map all buffers as one contiguous
range of memory. The <constant>VIDIOCGMBUF</constant> ioctl is
available to query the number of buffers, the offset of each buffer
from the start of the virtual file, and the overall amount of memory
used, which can be used as arguments for the &func-mmap;
function.</para></entry>
		<entry><para>Buffers are individually mapped. The
offset and size of each buffer can be determined with the
&VIDIOC-QUERYBUF; ioctl.</para></entry>
	      </row>
	      <row>
		<entry><para>The <constant>VIDIOCMCAPTURE</constant>
ioctl prepares a buffer for capturing. It also determines the image
format for this buffer. The ioctl returns immediately, eventually with
an &EAGAIN; if no video signal had been detected. When the driver
supports more than one buffer applications can call the ioctl multiple
times and thus have multiple outstanding capture
requests.</para><para>The <constant>VIDIOCSYNC</constant> ioctl
suspends execution until a particular buffer has been
filled.</para></entry>
		<entry><para>Drivers maintain an incoming and outgoing
queue. &VIDIOC-QBUF; enqueues any empty buffer into the incoming
queue. Filled buffers are dequeued from the outgoing queue with the
&VIDIOC-DQBUF; ioctl. To wait until filled buffers become available this
function, &func-select; or &func-poll; can be used. The
&VIDIOC-STREAMON; ioctl must be called once after enqueuing one or
more buffers to start capturing. Its counterpart
&VIDIOC-STREAMOFF; stops capturing and dequeues all buffers from both
queues. Applications can query the signal status, if known, with the
&VIDIOC-ENUMINPUT; ioctl.</para></entry>
	      </row>
	    </tbody>
	  </tgroup>
	</informaltable>

	<para>For a more in-depth discussion of memory mapping and
examples, see <xref linkend="mmap" />.</para>
      </section>
    </section>

    <section>
      <title>Reading Raw VBI Data</title>

      <para>Originally the V4L API did not specify a raw VBI capture
interface, only the device file <filename>/dev/vbi</filename> was
reserved for this purpose. The only driver supporting this interface
was the BTTV driver, de-facto defining the V4L VBI interface. Reading
from the device yields a raw VBI image with the following
parameters:<informaltable>
	    <tgroup cols="2">
	      <thead>
		<row>
		  <entry>&v4l2-vbi-format;</entry>
		  <entry>V4L, BTTV driver</entry>
		</row>
	      </thead>
	      <tbody valign="top">
		<row>
		  <entry>sampling_rate</entry>
		  <entry>28636363&nbsp;Hz NTSC (or any other 525-line
standard); 35468950&nbsp;Hz PAL and SECAM (625-line standards)</entry>
		</row>
		<row>
		  <entry>offset</entry>
		  <entry>?</entry>
		</row>
		<row>
		  <entry>samples_per_line</entry>
		  <entry>2048</entry>
		</row>
		<row>
		  <entry>sample_format</entry>
		  <entry>V4L2_PIX_FMT_GREY. The last four bytes (a
machine endianess integer) contain a frame counter.</entry>
		</row>
		<row>
		  <entry>start[]</entry>
		  <entry>10, 273 NTSC; 22, 335 PAL and SECAM</entry>
		</row>
		<row>
		  <entry>count[]</entry>
		  <entry><para>16, 16<footnote><para>Old driver
versions used different values, eventually the custom
<constant>BTTV_VBISIZE</constant> ioctl was added to query the
correct values.</para></footnote></para></entry>
		</row>
		<row>
		  <entry>flags</entry>
		  <entry>0</entry>
		</row>
	      </tbody>
	    </tgroup>
	</informaltable></para>

      <para>Undocumented in the V4L specification, in Linux 2.3 the
<constant>VIDIOCGVBIFMT</constant> and
<constant>VIDIOCSVBIFMT</constant> ioctls using struct
<structname>vbi_format</structname> were added to determine the VBI
image parameters. These ioctls are only partially compatible with the
V4L2 VBI interface specified in <xref linkend="raw-vbi" />.</para>

      <para>An <structfield>offset</structfield> field does not
exist, <structfield>sample_format</structfield> is supposed to be
<constant>VIDEO_PALETTE_RAW</constant>, equivalent to
<constant>V4L2_PIX_FMT_GREY</constant>. The remaining fields are
probably equivalent to &v4l2-vbi-format;.</para>

      <para>Apparently only the Zoran (ZR 36120) driver implements
these ioctls. The semantics differ from those specified for V4L2 in two
ways. The parameters are reset on &func-open; and
<constant>VIDIOCSVBIFMT</constant> always returns an &EINVAL; if the
parameters are invalid.</para>
    </section>

    <section>
      <title>Miscellaneous</title>

      <para>V4L2 has no equivalent of the
<constant>VIDIOCGUNIT</constant> ioctl. Applications can find the VBI
device associated with a video capture device (or vice versa) by
reopening the device and requesting VBI data. For details see
<xref linkend="open" />.</para>

      <para>No replacement exists for <constant>VIDIOCKEY</constant>,
and the V4L functions for microcode programming. A new interface for
MPEG compression and playback devices is documented in <xref
	  linkend="extended-controls" />.</para>
    </section>

  </section>

  <section id="hist-v4l2">
    <title>Changes of the V4L2 API</title>

    <para>Soon after the V4L API was added to the kernel it was
criticised as too inflexible. In August 1998 Bill Dirks proposed a
number of improvements and began to work on documentation, example
drivers and applications. With the help of other volunteers this
eventually became the V4L2 API, not just an extension but a
replacement for the V4L API. However it took another four years and
two stable kernel releases until the new API was finally accepted for
inclusion into the kernel in its present form.</para>

    <section>
      <title>Early Versions</title>
      <para>1998-08-20: First version.</para>

      <para>1998-08-27: The &func-select; function was introduced.</para>

      <para>1998-09-10: New video standard interface.</para>

      <para>1998-09-18: The <constant>VIDIOC_NONCAP</constant> ioctl
was replaced by the otherwise meaningless <constant>O_TRUNC</constant>
&func-open; flag, and the aliases <constant>O_NONCAP</constant> and
<constant>O_NOIO</constant> were defined. Applications can set this
flag if they intend to access controls only, as opposed to capture
applications which need exclusive access. The
<constant>VIDEO_STD_XXX</constant> identifiers are now ordinals
instead of flags, and the <function>video_std_construct()</function>
helper function takes id and transmission arguments.</para>

      <para>1998-09-28: Revamped video standard. Made video controls
individually enumerable.</para>

      <para>1998-10-02: The <structfield>id</structfield> field was
removed from struct <structname>video_standard</structname> and the
color subcarrier fields were renamed. The &VIDIOC-QUERYSTD; ioctl was
renamed to &VIDIOC-ENUMSTD;, &VIDIOC-G-INPUT; to &VIDIOC-ENUMINPUT;. A
first draft of the Codec API was released.</para>

      <para>1998-11-08: Many minor changes. Most symbols have been
renamed. Some material changes to &v4l2-capability;.</para>

      <para>1998-11-12: The read/write directon of some ioctls was misdefined.</para>

      <para>1998-11-14: <constant>V4L2_PIX_FMT_RGB24</constant>
changed to <constant>V4L2_PIX_FMT_BGR24</constant>, and
<constant>V4L2_PIX_FMT_RGB32</constant> changed to
<constant>V4L2_PIX_FMT_BGR32</constant>. Audio controls are now
accessible with the &VIDIOC-G-CTRL; and &VIDIOC-S-CTRL; ioctls under
names starting with <constant>V4L2_CID_AUDIO</constant>. The
<constant>V4L2_MAJOR</constant> define was removed from
<filename>videodev.h</filename> since it was only used once in the
<filename>videodev</filename> kernel module. The
<constant>YUV422</constant> and <constant>YUV411</constant> planar
image formats were added.</para>

      <para>1998-11-28: A few ioctl symbols changed. Interfaces for codecs and
video output devices were added.</para>

      <para>1999-01-14: A raw VBI capture interface was added.</para>

      <para>1999-01-19: The <constant>VIDIOC_NEXTBUF</constant> ioctl
      was removed.</para>
    </section>

    <section>
      <title>V4L2 Version 0.16 1999-01-31</title>
      <para>1999-01-27: There is now one QBUF ioctl, VIDIOC_QWBUF and VIDIOC_QRBUF
are gone. VIDIOC_QBUF takes a v4l2_buffer as a parameter. Added
digital zoom (cropping) controls.</para>
    </section>

    <!-- Where's 0.17? mhs couldn't find that videodev.h, perhaps Bill
	 forgot to bump the version number or never released it. -->

    <section>
      <title>V4L2 Version 0.18 1999-03-16</title>
      <para>Added a v4l to V4L2 ioctl compatibility layer to
videodev.c. Driver writers, this changes how you implement your ioctl
handler. See the Driver Writer's Guide. Added some more control id
codes.</para>
    </section>

    <section>
      <title>V4L2 Version 0.19 1999-06-05</title>
      <para>1999-03-18: Fill in the category and catname fields of
v4l2_queryctrl objects before passing them to the driver. Required a
minor change to the VIDIOC_QUERYCTRL handlers in the sample
drivers.</para>
      <para>1999-03-31: Better compatibility for v4l memory capture
ioctls. Requires changes to drivers to fully support new compatibility
features, see Driver Writer's Guide and v4l2cap.c. Added new control
IDs: V4L2_CID_HFLIP, _VFLIP. Changed V4L2_PIX_FMT_YUV422P to _YUV422P,
and _YUV411P to _YUV411P.</para>
      <para>1999-04-04: Added a few more control IDs.</para>
      <para>1999-04-07: Added the button control type.</para>
      <para>1999-05-02: Fixed a typo in videodev.h, and added the
V4L2_CTRL_FLAG_GRAYED (later V4L2_CTRL_FLAG_GRABBED) flag.</para>
      <para>1999-05-20: Definition of VIDIOC_G_CTRL was wrong causing
a malfunction of this ioctl.</para>
      <para>1999-06-05: Changed the value of
V4L2_CID_WHITENESS.</para>
    </section>

    <section>
      <title>V4L2 Version 0.20 (1999-09-10)</title>

      <para>Version 0.20 introduced a number of changes which were
<emphasis>not backward compatible</emphasis> with 0.19 and earlier
versions. Purpose of these changes was to simplify the API, while
making it more extensible and following common Linux driver API
conventions.</para>

      <orderedlist>
	<listitem>
	  <para>Some typos in <constant>V4L2_FMT_FLAG</constant>
symbols were fixed. &v4l2-clip; was changed for compatibility with
v4l. (1999-08-30)</para>
	</listitem>

	<listitem>
	  <para><constant>V4L2_TUNER_SUB_LANG1</constant> was added.
(1999-09-05)</para>
	</listitem>

	<listitem>
	  <para>All ioctl() commands that used an integer argument now
take a pointer to an integer. Where it makes sense, ioctls will return
the actual new value in the integer pointed to by the argument, a
common convention in the V4L2 API. The affected ioctls are:
VIDIOC_PREVIEW, VIDIOC_STREAMON, VIDIOC_STREAMOFF, VIDIOC_S_FREQ,
VIDIOC_S_INPUT, VIDIOC_S_OUTPUT, VIDIOC_S_EFFECT. For example
<programlisting>
err = ioctl (fd, VIDIOC_XXX, V4L2_XXX);
</programlisting> becomes <programlisting>
int a = V4L2_XXX; err = ioctl(fd, VIDIOC_XXX, &amp;a);
</programlisting>
	  </para>
	</listitem>

	<listitem>
	  <para>All the different get- and set-format commands were
swept into one &VIDIOC-G-FMT; and &VIDIOC-S-FMT; ioctl taking a union
and a type field selecting the union member as parameter. Purpose is to
simplify the API by eliminating several ioctls and to allow new and
driver private data streams without adding new ioctls.</para>

	  <para>This change obsoletes the following ioctls:
<constant>VIDIOC_S_INFMT</constant>,
<constant>VIDIOC_G_INFMT</constant>,
<constant>VIDIOC_S_OUTFMT</constant>,
<constant>VIDIOC_G_OUTFMT</constant>,
<constant>VIDIOC_S_VBIFMT</constant> and
<constant>VIDIOC_G_VBIFMT</constant>. The image format structure
<structname>v4l2_format</structname> was renamed to &v4l2-pix-format;,
while &v4l2-format; is now the envelopping structure for all format
negotiations.</para>
	</listitem>

	<listitem>
	  <para>Similar to the changes above, the
<constant>VIDIOC_G_PARM</constant> and
<constant>VIDIOC_S_PARM</constant> ioctls were merged with
<constant>VIDIOC_G_OUTPARM</constant> and
<constant>VIDIOC_S_OUTPARM</constant>. A
<structfield>type</structfield> field in the new &v4l2-streamparm;
selects the respective union member.</para>

	  <para>This change obsoletes the
<constant>VIDIOC_G_OUTPARM</constant> and
<constant>VIDIOC_S_OUTPARM</constant> ioctls.</para>
	</listitem>

	<listitem>
	  <para>Control enumeration was simplified, and two new
control flags were introduced and one dropped. The
<structfield>catname</structfield> field was replaced by a
<structfield>group</structfield> field.</para>

	  <para>Drivers can now flag unsupported and temporarily
unavailable controls with <constant>V4L2_CTRL_FLAG_DISABLED</constant>
and <constant>V4L2_CTRL_FLAG_GRABBED</constant> respectively. The
<structfield>group</structfield> name indicates a possibly narrower
classification than the <structfield>category</structfield>. In other
words, there may be multiple groups within a category. Controls within
a group would typically be drawn within a group box. Controls in
different categories might have a greater separation, or may even
appear in separate windows.</para>
	</listitem>

	<listitem>
	  <para>The &v4l2-buffer; <structfield>timestamp</structfield>
was changed to a 64 bit integer, containing the sampling or output
time of the frame in nanoseconds. Additionally timestamps will be in
absolute system time, not starting from zero at the beginning of a
stream. The data type name for timestamps is stamp_t, defined as a
signed 64-bit integer. Output devices should not send a buffer out
until the time in the timestamp field has arrived. I would like to
follow SGI's lead, and adopt a multimedia timestamping system like
their UST (Unadjusted System Time). See
http://web.archive.org/web/*/http://reality.sgi.com
/cpirazzi_engr/lg/time/intro.html. 
UST uses timestamps that are 64-bit signed integers
(not struct timeval's) and given in nanosecond units. The UST clock
starts at zero when the system is booted and runs continuously and
uniformly. It takes a little over 292 years for UST to overflow. There
is no way to set the UST clock. The regular Linux time-of-day clock
can be changed periodically, which would cause errors if it were being
used for timestamping a multimedia stream. A real UST style clock will
require some support in the kernel that is not there yet. But in
anticipation, I will change the timestamp field to a 64-bit integer,
and I will change the v4l2_masterclock_gettime() function (used only
by drivers) to return a 64-bit integer.</para>
	</listitem>

	<listitem>
	  <para>A <structfield>sequence</structfield> field was added
to &v4l2-buffer;. The <structfield>sequence</structfield> field counts
captured frames, it is ignored by output devices. When a capture
driver drops a frame, the sequence number of that frame is
skipped.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 Version 0.20 incremental changes</title>
      <!-- Version number didn't change anymore, reason unknown. -->

      <para>1999-12-23: In &v4l2-vbi-format; the
<structfield>reserved1</structfield> field became
<structfield>offset</structfield>. Previously drivers were required to
clear the <structfield>reserved1</structfield> field.</para>

      <para>2000-01-13: The
      <constant>V4L2_FMT_FLAG_NOT_INTERLACED</constant> flag was added.</para>

      <para>2000-07-31: The <filename>linux/poll.h</filename> header
is now included by <filename>videodev.h</filename> for compatibility
with the original <filename>videodev.h</filename> file.</para>

      <para>2000-11-20: <constant>V4L2_TYPE_VBI_OUTPUT</constant> and
<constant>V4L2_PIX_FMT_Y41P</constant> were added.</para>

      <para>2000-11-25: <constant>V4L2_TYPE_VBI_INPUT</constant> was
added.</para>

      <para>2000-12-04: A couple typos in symbol names were fixed.</para>

      <para>2001-01-18: To avoid namespace conflicts the
<constant>fourcc</constant> macro defined in the
<filename>videodev.h</filename> header file was renamed to
<constant>v4l2_fourcc</constant>.</para>

      <para>2001-01-25: A possible driver-level compatibility problem
between the <filename>videodev.h</filename> file in Linux 2.4.0 and
the <filename>videodev.h</filename> file included in the
<filename>videodevX</filename> patch was fixed. Users of an earlier
version of <filename>videodevX</filename> on Linux 2.4.0 should
recompile their V4L and V4L2 drivers.</para>

      <para>2001-01-26: A possible kernel-level incompatibility
between the <filename>videodev.h</filename> file in the
<filename>videodevX</filename> patch and the
<filename>videodev.h</filename> file in Linux 2.2.x with devfs patches
applied was fixed.</para>

      <para>2001-03-02: Certain V4L ioctls which pass data in both
direction although they are defined with read-only parameter, did not
work correctly through the backward compatibility layer.
[Solution?]</para>

      <para>2001-04-13: Big endian 16-bit RGB formats were added.</para>

      <para>2001-09-17: New YUV formats and the &VIDIOC-G-FREQUENCY; and
&VIDIOC-S-FREQUENCY; ioctls were added. (The old
<constant>VIDIOC_G_FREQ</constant> and
<constant>VIDIOC_S_FREQ</constant> ioctls did not take multiple tuners
into account.)</para>

      <para>2000-09-18: <constant>V4L2_BUF_TYPE_VBI</constant> was
added. This may <emphasis>break compatibility</emphasis> as the
&VIDIOC-G-FMT; and &VIDIOC-S-FMT; ioctls may fail now if the struct
<structname>v4l2_fmt</structname> <structfield>type</structfield>
field does not contain <constant>V4L2_BUF_TYPE_VBI</constant>. In the
documentation of the &v4l2-vbi-format;
<structfield>offset</structfield> field the ambiguous phrase "rising
edge" was changed to "leading edge".</para>
    </section>

    <section>
      <title>V4L2 Version 0.20 2000-11-23</title>

      <para>A number of changes were made to the raw VBI
interface.</para>

      <orderedlist>
	<listitem>
	  <para>Figures clarifying the line numbering scheme were
added to the V4L2 API specification. The
<structfield>start</structfield>[0] and
<structfield>start</structfield>[1] fields no longer count line
numbers beginning at zero. Rationale: a) The previous definition was
unclear. b) The <structfield>start</structfield>[] values are ordinal
numbers. c) There is no point in inventing a new line numbering
scheme. We now use line number as defined by ITU-R, period.
Compatibility: Add one to the start values. Applications depending on
the previous semantics may not function correctly.</para>
	</listitem>

	<listitem>
	  <para>The restriction "count[0] &gt; 0 and count[1] &gt; 0"
has been relaxed  to "(count[0] + count[1]) &gt; 0". Rationale:
Drivers may allocate resources at scan line granularity and some data
services are transmitted only on the first field. The comment that
both <structfield>count</structfield> values will usually be equal is
misleading and pointless and has been removed. This change
<emphasis>breaks compatibility</emphasis> with earlier versions:
Drivers may return EINVAL, applications may not function
correctly.</para>
	</listitem>

	<listitem>
	  <para>Drivers are again permitted to return negative
(unknown) start values as proposed earlier. Why this feature was
dropped is unclear. This change may <emphasis>break
compatibility</emphasis> with applications depending on the start
values being positive. The use of <constant>EBUSY</constant> and
<constant>EINVAL</constant> error codes with the &VIDIOC-S-FMT; ioctl
was clarified. The &EBUSY; was finally documented, and the
<structfield>reserved2</structfield> field which was previously
mentioned only in the <filename>videodev.h</filename> header
file.</para>
	</listitem>

	<listitem>
	  <para>New buffer types
<constant>V4L2_TYPE_VBI_INPUT</constant> and
<constant>V4L2_TYPE_VBI_OUTPUT</constant> were added. The former is an
alias for the old <constant>V4L2_TYPE_VBI</constant>, the latter was
missing in the <filename>videodev.h</filename> file.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 Version 0.20 2002-07-25</title>
      <para>Added sliced VBI interface proposal.</para>
    </section>

    <section>
      <title>V4L2 in Linux 2.5.46, 2002-10</title>

      <para>Around October-November 2002, prior to an announced
feature freeze of Linux 2.5, the API was revised, drawing from
experience with V4L2 0.20. This unnamed version was finally merged
into Linux 2.5.46.</para>

      <orderedlist>
	<listitem>
	  <para>As specified in <xref linkend="related" />, drivers
must make related device functions available under all minor device
numbers.</para>
	</listitem>

	<listitem>
	  <para>The &func-open; function requires access mode
<constant>O_RDWR</constant> regardless of the device type. All V4L2
drivers exchanging data with applications must support the
<constant>O_NONBLOCK</constant> flag. The <constant>O_NOIO</constant>
flag, a V4L2 symbol which aliased the meaningless
<constant>O_TRUNC</constant> to indicate accesses without data
exchange (panel applications) was dropped. Drivers must stay in "panel
mode" until the application attempts to initiate a data exchange, see
<xref linkend="open" />.</para>
	</listitem>

	<listitem>
	  <para>The &v4l2-capability; changed dramatically. Note that
also the size of the structure changed, which is encoded in the ioctl
request code, thus older V4L2 devices will respond with an &EINVAL; to
the new &VIDIOC-QUERYCAP; ioctl.</para>

	  <para>There are new fields to identify the driver, a new RDS
device function <constant>V4L2_CAP_RDS_CAPTURE</constant>, the
<constant>V4L2_CAP_AUDIO</constant> flag indicates if the device has
any audio connectors, another I/O capability
<constant>V4L2_CAP_ASYNCIO</constant> can be flagged. In response to
these changes the <structfield>type</structfield> field became a bit
set and was merged into the <structfield>flags</structfield> field.
<constant>V4L2_FLAG_TUNER</constant> was renamed to
<constant>V4L2_CAP_TUNER</constant>,
<constant>V4L2_CAP_VIDEO_OVERLAY</constant> replaced
<constant>V4L2_FLAG_PREVIEW</constant> and
<constant>V4L2_CAP_VBI_CAPTURE</constant> and
<constant>V4L2_CAP_VBI_OUTPUT</constant> replaced
<constant>V4L2_FLAG_DATA_SERVICE</constant>.
<constant>V4L2_FLAG_READ</constant> and
<constant>V4L2_FLAG_WRITE</constant> were merged into
<constant>V4L2_CAP_READWRITE</constant>.</para>

	  <para>The redundant fields
<structfield>inputs</structfield>, <structfield>outputs</structfield>
and <structfield>audios</structfield> were removed. These properties
can be determined as described in <xref linkend="video" /> and <xref
linkend="audio" />.</para>

	  <para>The somewhat volatile and therefore barely useful
fields <structfield>maxwidth</structfield>,
<structfield>maxheight</structfield>,
<structfield>minwidth</structfield>,
<structfield>minheight</structfield>,
<structfield>maxframerate</structfield> were removed. This information
is available as described in <xref linkend="format" /> and
<xref linkend="standard" />.</para>

	  <para><constant>V4L2_FLAG_SELECT</constant> was removed. We
believe the select() function is important enough to require support
of it in all V4L2 drivers exchanging data with applications. The
redundant <constant>V4L2_FLAG_MONOCHROME</constant> flag was removed,
this information is available as described in <xref
	      linkend="format" />.</para>
	</listitem>

	<listitem>
	  <para>In &v4l2-input; the
<structfield>assoc_audio</structfield> field and the
<structfield>capability</structfield> field and its only flag
<constant>V4L2_INPUT_CAP_AUDIO</constant> was replaced by the new
<structfield>audioset</structfield> field. Instead of linking one
video input to one audio input this field reports all audio inputs
this video input combines with.</para>

	  <para>New fields are <structfield>tuner</structfield>
(reversing the former link from tuners to video inputs),
<structfield>std</structfield> and
<structfield>status</structfield>.</para>

	  <para>Accordingly &v4l2-output; lost its
<structfield>capability</structfield> and
<structfield>assoc_audio</structfield> fields.
<structfield>audioset</structfield>,
<structfield>modulator</structfield> and
<structfield>std</structfield> where added instead.</para>
	</listitem>

	<listitem>
	  <para>The &v4l2-audio; field
<structfield>audio</structfield> was renamed to
<structfield>index</structfield>, for consistency with other
structures. A new capability flag
<constant>V4L2_AUDCAP_STEREO</constant> was added to indicated if the
audio input in question supports stereo sound.
<constant>V4L2_AUDCAP_EFFECTS</constant> and the corresponding
<constant>V4L2_AUDMODE</constant> flags where removed. This can be
easily implemented using controls. (However the same applies to AVL
which is still there.)</para>

	  <para>Again for consistency the &v4l2-audioout; field
<structfield>audio</structfield> was renamed to
<structfield>index</structfield>.</para>
	</listitem>

	<listitem>
	  <para>The &v4l2-tuner;
<structfield>input</structfield> field was replaced by an
<structfield>index</structfield> field, permitting devices with
multiple tuners. The link between video inputs and tuners is now
reversed, inputs point to their tuner. The
<structfield>std</structfield> substructure became a
simple set (more about this below) and moved into &v4l2-input;. A
<structfield>type</structfield> field was added.</para>

	  <para>Accordingly in &v4l2-modulator; the
<structfield>output</structfield> was replaced by an
<structfield>index</structfield> field.</para>

	  <para>In &v4l2-frequency; the
<structfield>port</structfield> field was replaced by a
<structfield>tuner</structfield> field containing the respective tuner
or modulator index number. A tuner <structfield>type</structfield>
field was added and the <structfield>reserved</structfield> field
became larger for future extensions (satellite tuners in
particular).</para>
	</listitem>

	<listitem>
	  <para>The idea of completely transparent video standards was
dropped. Experience showed that applications must be able to work with
video standards beyond presenting the user a menu. Instead of
enumerating supported standards with an ioctl applications can now
refer to standards by &v4l2-std-id; and symbols defined in the
<filename>videodev2.h</filename> header file. For details see <xref
	      linkend="standard" />. The &VIDIOC-G-STD; and
&VIDIOC-S-STD; now take a pointer to this type as argument.
&VIDIOC-QUERYSTD; was added to autodetect the received standard, if
the hardware has this capability. In &v4l2-standard; an
<structfield>index</structfield> field was added for &VIDIOC-ENUMSTD;.
A &v4l2-std-id; field named <structfield>id</structfield> was added as
machine readable identifier, also replacing the
<structfield>transmission</structfield> field. The misleading
<structfield>framerate</structfield> field was renamed
to <structfield>frameperiod</structfield>. The now obsolete
<structfield>colorstandard</structfield> information, originally
needed to distguish between variations of standards, were
removed.</para>

	  <para>Struct <structname>v4l2_enumstd</structname> ceased to
be. &VIDIOC-ENUMSTD; now takes a pointer to a &v4l2-standard;
directly. The information which standards are supported by a
particular video input or output moved into &v4l2-input; and
&v4l2-output; fields named <structfield>std</structfield>,
respectively.</para>
	</listitem>

	<listitem>
	  <para>The &v4l2-queryctrl; fields
<structfield>category</structfield> and
<structfield>group</structfield> did not catch on and/or were not
implemented as expected and therefore removed.</para>
	</listitem>

	<listitem>
	  <para>The &VIDIOC-TRY-FMT; ioctl was added to negotiate data
formats as with &VIDIOC-S-FMT;, but without the overhead of
programming the hardware and regardless of I/O in progress.</para>

	  <para>In &v4l2-format; the <structfield>fmt</structfield>
union was extended to contain &v4l2-window;. All image format
negotiations are now possible with <constant>VIDIOC_G_FMT</constant>,
<constant>VIDIOC_S_FMT</constant> and
<constant>VIDIOC_TRY_FMT</constant>; ioctl. The
<constant>VIDIOC_G_WIN</constant> and
<constant>VIDIOC_S_WIN</constant> ioctls to prepare for a video
overlay were removed. The <structfield>type</structfield> field
changed to type &v4l2-buf-type; and the buffer type names changed as
follows.<informaltable>
	      <tgroup cols="2">
		<thead>
		  <row>
		    <entry>Old defines</entry>
		    <entry>&v4l2-buf-type;</entry>
		  </row>
		</thead>
		<tbody valign="top">
		  <row>
		    <entry><constant>V4L2_BUF_TYPE_CAPTURE</constant></entry>
		    <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant></entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_BUF_TYPE_CODECIN</constant></entry>
		    <entry>Omitted for now</entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_BUF_TYPE_CODECOUT</constant></entry>
		    <entry>Omitted for now</entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_BUF_TYPE_EFFECTSIN</constant></entry>
		    <entry>Omitted for now</entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_BUF_TYPE_EFFECTSIN2</constant></entry>
		    <entry>Omitted for now</entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_BUF_TYPE_EFFECTSOUT</constant></entry>
		    <entry>Omitted for now</entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_BUF_TYPE_VIDEOOUT</constant></entry>
		    <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant></entry>
		  </row>
		  <row>
		    <entry><constant>-</constant></entry>
		    <entry><constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant></entry>
		  </row>
		  <row>
		    <entry><constant>-</constant></entry>
		    <entry><constant>V4L2_BUF_TYPE_VBI_CAPTURE</constant></entry>
		  </row>
		  <row>
		    <entry><constant>-</constant></entry>
		    <entry><constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant></entry>
		  </row>
		  <row>
		    <entry><constant>-</constant></entry>
		    <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_CAPTURE</constant></entry>
		  </row>
		  <row>
		    <entry><constant>-</constant></entry>
		    <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_OUTPUT</constant></entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_BUF_TYPE_PRIVATE_BASE</constant></entry>
		    <entry><constant>V4L2_BUF_TYPE_PRIVATE</constant></entry>
		  </row>
		</tbody>
	      </tgroup>
	    </informaltable></para>
	</listitem>

	<listitem>
	  <para>In &v4l2-fmtdesc; a &v4l2-buf-type; field named
<structfield>type</structfield> was added as in &v4l2-format;. The
<constant>VIDIOC_ENUM_FBUFFMT</constant> ioctl is no longer needed and
was removed. These calls can be replaced by &VIDIOC-ENUM-FMT; with
type <constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant>.</para>
	</listitem>

	<listitem>
	  <para>In &v4l2-pix-format; the
<structfield>depth</structfield> field was removed, assuming
applications which recognize the format by its four-character-code
already know the color depth, and others do not care about it. The
same rationale lead to the removal of the
<constant>V4L2_FMT_FLAG_COMPRESSED</constant> flag. The
<constant>V4L2_FMT_FLAG_SWCONVECOMPRESSED</constant> flag was removed
because drivers are not supposed to convert images in kernel space. A
user library of conversion functions should be provided instead. The
<constant>V4L2_FMT_FLAG_BYTESPERLINE</constant> flag was redundant.
Applications can set the <structfield>bytesperline</structfield> field
to zero to get a reasonable default. Since the remaining flags were
replaced as well, the <structfield>flags</structfield> field itself
was removed.</para>
	  <para>The interlace flags were replaced by a &v4l2-field;
value in a newly added <structfield>field</structfield>
field.<informaltable>
	      <tgroup cols="2">
		<thead>
		  <row>
		    <entry>Old flag</entry>
		    <entry>&v4l2-field;</entry>
		  </row>
		</thead>
		<tbody valign="top">
		  <row>
		    <entry><constant>V4L2_FMT_FLAG_NOT_INTERLACED</constant></entry>
		    <entry>?</entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_FMT_FLAG_INTERLACED</constant>
= <constant>V4L2_FMT_FLAG_COMBINED</constant></entry>
		    <entry><constant>V4L2_FIELD_INTERLACED</constant></entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_FMT_FLAG_TOPFIELD</constant>
= <constant>V4L2_FMT_FLAG_ODDFIELD</constant></entry>
		    <entry><constant>V4L2_FIELD_TOP</constant></entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_FMT_FLAG_BOTFIELD</constant>
= <constant>V4L2_FMT_FLAG_EVENFIELD</constant></entry>
		    <entry><constant>V4L2_FIELD_BOTTOM</constant></entry>
		  </row>
		  <row>
		    <entry><constant>-</constant></entry>
		    <entry><constant>V4L2_FIELD_SEQ_TB</constant></entry>
		  </row>
		  <row>
		    <entry><constant>-</constant></entry>
		    <entry><constant>V4L2_FIELD_SEQ_BT</constant></entry>
		  </row>
		  <row>
		    <entry><constant>-</constant></entry>
		    <entry><constant>V4L2_FIELD_ALTERNATE</constant></entry>
		  </row>
		</tbody>
	      </tgroup>
	    </informaltable></para>

	  <para>The color space flags were replaced by a
&v4l2-colorspace; value in a newly added
<structfield>colorspace</structfield> field, where one of
<constant>V4L2_COLORSPACE_SMPTE170M</constant>,
<constant>V4L2_COLORSPACE_BT878</constant>,
<constant>V4L2_COLORSPACE_470_SYSTEM_M</constant> or
<constant>V4L2_COLORSPACE_470_SYSTEM_BG</constant> replaces
<constant>V4L2_FMT_CS_601YUV</constant>.</para>
	</listitem>

	<listitem>
	  <para>In &v4l2-requestbuffers; the
<structfield>type</structfield> field was properly defined as
&v4l2-buf-type;. Buffer types changed as mentioned above. A new
<structfield>memory</structfield> field of type &v4l2-memory; was
added to distinguish between I/O methods using buffers allocated
by the driver or the application. See <xref linkend="io" /> for
details.</para>
	</listitem>

	<listitem>
	  <para>In &v4l2-buffer; the <structfield>type</structfield>
field was properly defined as &v4l2-buf-type;. Buffer types changed as
mentioned above. A <structfield>field</structfield> field of type
&v4l2-field; was added to indicate if a buffer contains a top or
bottom field. The old field flags were removed. Since no unadjusted
system time clock was added to the kernel as planned, the
<structfield>timestamp</structfield> field changed back from type
stamp_t, an unsigned 64 bit integer expressing the sample time in
nanoseconds, to struct <structname>timeval</structname>. With the
addition of a second memory mapping method the
<structfield>offset</structfield> field moved into union
<structfield>m</structfield>, and a new
<structfield>memory</structfield> field of type &v4l2-memory; was
added to distinguish between I/O methods. See <xref linkend="io" />
for details.</para>

	  <para>The <constant>V4L2_BUF_REQ_CONTIG</constant>
flag was used by the V4L compatibility layer, after changes to this
code it was no longer needed. The
<constant>V4L2_BUF_ATTR_DEVICEMEM</constant> flag would indicate if
the buffer was indeed allocated in device memory rather than DMA-able
system memory. It was barely useful and so was removed.</para>
	</listitem>

	<listitem>
	  <para>In &v4l2-framebuffer; the
<structfield>base[3]</structfield> array anticipating double- and
triple-buffering in off-screen video memory, however without defining
a synchronization mechanism, was replaced by a single pointer. The
<constant>V4L2_FBUF_CAP_SCALEUP</constant> and
<constant>V4L2_FBUF_CAP_SCALEDOWN</constant> flags were removed.
Applications can determine this capability more accurately using the
new cropping and scaling interface. The
<constant>V4L2_FBUF_CAP_CLIPPING</constant> flag was replaced by
<constant>V4L2_FBUF_CAP_LIST_CLIPPING</constant> and
<constant>V4L2_FBUF_CAP_BITMAP_CLIPPING</constant>.</para>
	</listitem>

	<listitem>
	  <para>In &v4l2-clip; the <structfield>x</structfield>,
<structfield>y</structfield>, <structfield>width</structfield> and
<structfield>height</structfield> field moved into a
<structfield>c</structfield> substructure of type &v4l2-rect;. The
<structfield>x</structfield> and <structfield>y</structfield> fields
were renamed to <structfield>left</structfield> and
<structfield>top</structfield>, &ie; offsets to a context dependent
origin.</para>
	</listitem>

	<listitem>
	  <para>In &v4l2-window; the <structfield>x</structfield>,
<structfield>y</structfield>, <structfield>width</structfield> and
<structfield>height</structfield> field moved into a
<structfield>w</structfield> substructure as above. A
<structfield>field</structfield> field of type %v4l2-field; was added
to distinguish between field and frame (interlaced) overlay.</para>
	</listitem>

	<listitem>
	  <para>The digital zoom interface, including struct
<structname>v4l2_zoomcap</structname>, struct
<structname>v4l2_zoom</structname>,
<constant>V4L2_ZOOM_NONCAP</constant> and
<constant>V4L2_ZOOM_WHILESTREAMING</constant> was replaced by a new
cropping and scaling interface. The previously unused struct
<structname>v4l2_cropcap</structname> and
<structname>v4l2_crop</structname> where redefined for this purpose.
See <xref linkend="crop" /> for details.</para>
	</listitem>

	<listitem>
	  <para>In &v4l2-vbi-format; the
<structfield>SAMPLE_FORMAT</structfield> field now contains a
four-character-code as used to identify video image formats and
<constant>V4L2_PIX_FMT_GREY</constant> replaces the
<constant>V4L2_VBI_SF_UBYTE</constant> define. The
<structfield>reserved</structfield> field was extended.</para>
	</listitem>

	<listitem>
	  <para>In &v4l2-captureparm; the type of the
<structfield>timeperframe</structfield> field changed from unsigned
long to &v4l2-fract;. This allows the accurate expression of multiples
of the NTSC-M frame rate 30000 / 1001. A new field
<structfield>readbuffers</structfield> was added to control the driver
behaviour in read I/O mode.</para>

	  <para>Similar changes were made to &v4l2-outputparm;.</para>
	</listitem>

	<listitem>
	  <para>The struct <structname>v4l2_performance</structname>
and <constant>VIDIOC_G_PERF</constant> ioctl were dropped. Except when
using the <link linkend="rw">read/write I/O method</link>, which is
limited anyway, this information is already available to
applications.</para>
	</listitem>

	<listitem>
	  <para>The example transformation from RGB to YCbCr color
space in the old V4L2 documentation was inaccurate, this has been
corrected in <xref linkend="pixfmt" />.<!-- 0.5670G should be
0.587, and 127/112 != 255/224 --></para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 2003-06-19</title>

      <orderedlist>
	<listitem>
	  <para>A new capability flag
<constant>V4L2_CAP_RADIO</constant> was added for radio devices. Prior
to this change radio devices would identify solely by having exactly one
tuner whose type field reads <constant>V4L2_TUNER_RADIO</constant>.</para>
	</listitem>

	<listitem>
	  <para>An optional driver access priority mechanism was
added, see <xref linkend="app-pri" /> for details.</para>
	</listitem>

	<listitem>
	  <para>The audio input and output interface was found to be
incomplete.</para>
	  <para>Previously the &VIDIOC-G-AUDIO;
ioctl would enumerate the available audio inputs. An ioctl to
determine the current audio input, if more than one combines with the
current video input, did not exist. So
<constant>VIDIOC_G_AUDIO</constant> was renamed to
<constant>VIDIOC_G_AUDIO_OLD</constant>, this ioctl will be removed in
the future. The &VIDIOC-ENUMAUDIO; ioctl was added to enumerate
audio inputs, while &VIDIOC-G-AUDIO; now reports the current audio
input.</para>
	  <para>The same changes were made to &VIDIOC-G-AUDOUT; and
&VIDIOC-ENUMAUDOUT;.</para>
	  <para>Until further the "videodev" module will automatically
translate between the old and new ioctls, but drivers and applications
must be updated to successfully compile again.</para>
	</listitem>

	<listitem>
	  <para>The &VIDIOC-OVERLAY; ioctl was incorrectly defined with
write-read parameter. It was changed to write-only, while the write-read
version was renamed to <constant>VIDIOC_OVERLAY_OLD</constant>. The old
ioctl will be removed in the future. Until further the "videodev"
kernel module will automatically translate to the new version, so drivers
must be recompiled, but not applications.</para>
	</listitem>

	<listitem>
	  <para><xref linkend="overlay" /> incorrectly stated that
clipping rectangles define regions where the video can be seen.
Correct is that clipping rectangles define regions where
<emphasis>no</emphasis> video shall be displayed and so the graphics
surface can be seen.</para>
	</listitem>

	<listitem>
	  <para>The &VIDIOC-S-PARM; and &VIDIOC-S-CTRL; ioctls were
defined with write-only parameter, inconsistent with other ioctls
modifying their argument. They were changed to write-read, while a
<constant>_OLD</constant> suffix was added to the write-only versions.
The old ioctls will be removed in the future. Drivers and
applications assuming a constant parameter need an update.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 2003-11-05</title>
      <orderedlist>
	<listitem>
	  <para>In <xref linkend="pixfmt-rgb" /> the following pixel
formats were incorrectly transferred from Bill Dirks' V4L2
specification. Descriptions below refer to bytes in memory, in
ascending address order.<informaltable>
	      <tgroup cols="3">
		<thead>
		  <row>
		    <entry>Symbol</entry>
		    <entry>In this document prior to revision
0.5</entry>
		    <entry>Corrected</entry>
		  </row>
		</thead>
		<tbody valign="top">
		  <row>
		    <entry><constant>V4L2_PIX_FMT_RGB24</constant></entry>
		    <entry>B, G, R</entry>
		    <entry>R, G, B</entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_PIX_FMT_BGR24</constant></entry>
		    <entry>R, G, B</entry>
		    <entry>B, G, R</entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_PIX_FMT_RGB32</constant></entry>
		    <entry>B, G, R, X</entry>
		    <entry>R, G, B, X</entry>
		  </row>
		  <row>
		    <entry><constant>V4L2_PIX_FMT_BGR32</constant></entry>
		    <entry>R, G, B, X</entry>
		    <entry>B, G, R, X</entry>
		  </row>
		</tbody>
	      </tgroup>
	    </informaltable> The
<constant>V4L2_PIX_FMT_BGR24</constant> example was always
correct.</para>
	  <para>In <xref linkend="v4l-image-properties" /> the mapping
of the V4L <constant>VIDEO_PALETTE_RGB24</constant> and
<constant>VIDEO_PALETTE_RGB32</constant> formats to V4L2 pixel formats
was accordingly corrected.</para>
	</listitem>

	<listitem>
	  <para>Unrelated to the fixes above, drivers may still
interpret some V4L2 RGB pixel formats differently. These issues have
yet to be addressed, for details see <xref
	      linkend="pixfmt-rgb" />.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.6, 2004-05-09</title>
      <orderedlist>
	<listitem>
	  <para>The &VIDIOC-CROPCAP; ioctl was incorrectly defined
with read-only parameter. It is now defined as write-read ioctl, while
the read-only version was renamed to
<constant>VIDIOC_CROPCAP_OLD</constant>. The old ioctl will be removed
in the future.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.8</title>
      <orderedlist>
	<listitem>
	  <para>A new field <structfield>input</structfield> (former
<structfield>reserved[0]</structfield>) was added to the &v4l2-buffer;
structure. Purpose of this field is to alternate between video inputs
(&eg; cameras) in step with the video capturing process. This function
must be enabled with the new <constant>V4L2_BUF_FLAG_INPUT</constant>
flag. The <structfield>flags</structfield> field is no longer
read-only.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 spec erratum 2004-08-01</title>

      <orderedlist>
	<listitem>
	  <para>The return value of the
<xref linkend="func-open" /> function was incorrectly documented.</para>
	</listitem>

	<listitem>
	  <para>Audio output ioctls end in -AUDOUT, not -AUDIOOUT.</para>
	</listitem>

	<listitem>
	  <para>In the Current Audio Input example the
<constant>VIDIOC_G_AUDIO</constant> ioctl took the wrong
argument.</para>
	</listitem>

	<listitem>
	  <para>The documentation of the &VIDIOC-QBUF; and
&VIDIOC-DQBUF; ioctls did not mention the &v4l2-buffer;
<structfield>memory</structfield> field. It was also missing from
examples. Also on the <constant>VIDIOC_DQBUF</constant> page the &EIO;
was not documented.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.14</title>
      <orderedlist>
	<listitem>
	  <para>A new sliced VBI interface was added. It is documented
in <xref linkend="sliced" /> and replaces the interface first
proposed in V4L2 specification 0.8.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.15</title>
      <orderedlist>
	<listitem>
	  <para>The &VIDIOC-LOG-STATUS; ioctl was added.</para>
	</listitem>

	<listitem>
	  <para>New video standards
<constant>V4L2_STD_NTSC_443</constant>,
<constant>V4L2_STD_SECAM_LC</constant>,
<constant>V4L2_STD_SECAM_DK</constant> (a set of SECAM D, K and K1),
and <constant>V4L2_STD_ATSC</constant> (a set of
<constant>V4L2_STD_ATSC_8_VSB</constant> and
<constant>V4L2_STD_ATSC_16_VSB</constant>) were defined. Note the
<constant>V4L2_STD_525_60</constant> set now includes
<constant>V4L2_STD_NTSC_443</constant>. See also <xref
	      linkend="v4l2-std-id" />.</para>
	</listitem>

	<listitem>
	  <para>The <constant>VIDIOC_G_COMP</constant> and
<constant>VIDIOC_S_COMP</constant> ioctl were renamed to
<constant>VIDIOC_G_MPEGCOMP</constant> and
<constant>VIDIOC_S_MPEGCOMP</constant> respectively. Their argument
was replaced by a struct
<structname>v4l2_mpeg_compression</structname> pointer. (The
<constant>VIDIOC_G_MPEGCOMP</constant> and
<constant>VIDIOC_S_MPEGCOMP</constant> ioctls where removed in Linux
2.6.25.)</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 spec erratum 2005-11-27</title>
      <para>The capture example in <xref linkend="capture-example" />
called the &VIDIOC-S-CROP; ioctl without checking if cropping is
supported. In the video standard selection example in
<xref linkend="standard" /> the &VIDIOC-S-STD; call used the wrong
argument type.</para>
    </section>

    <section>
      <title>V4L2 spec erratum 2006-01-10</title>
      <orderedlist>
	<listitem>
	  <para>The <constant>V4L2_IN_ST_COLOR_KILL</constant> flag in
&v4l2-input; not only indicates if the color killer is enabled, but
also if it is active. (The color killer disables color decoding when
it detects no color in the video signal to improve the image
quality.)</para>
	</listitem>

	<listitem>
	  <para>&VIDIOC-S-PARM; is a write-read ioctl, not write-only as
stated on its reference page. The ioctl changed in 2003 as noted above.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 spec erratum 2006-02-03</title>
      <orderedlist>
	<listitem>
	  <para>In &v4l2-captureparm; and &v4l2-outputparm; the
<structfield>timeperframe</structfield> field gives the time in
seconds, not microseconds.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 spec erratum 2006-02-04</title>
      <orderedlist>
	<listitem>
	  <para>The <structfield>clips</structfield> field in
&v4l2-window; must point to an array of &v4l2-clip;, not a linked
list, because drivers ignore the struct
<structname>v4l2_clip</structname>.<structfield>next</structfield>
pointer.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.17</title>
      <orderedlist>
	<listitem>
	  <para>New video standard macros were added:
<constant>V4L2_STD_NTSC_M_KR</constant> (NTSC M South Korea), and the
sets <constant>V4L2_STD_MN</constant>,
<constant>V4L2_STD_B</constant>, <constant>V4L2_STD_GH</constant> and
<constant>V4L2_STD_DK</constant>. The
<constant>V4L2_STD_NTSC</constant> and
<constant>V4L2_STD_SECAM</constant> sets now include
<constant>V4L2_STD_NTSC_M_KR</constant> and
<constant>V4L2_STD_SECAM_LC</constant> respectively.</para>
	</listitem>

	<listitem>
	  <para>A new <constant>V4L2_TUNER_MODE_LANG1_LANG2</constant>
was defined to record both languages of a bilingual program. The
use of <constant>V4L2_TUNER_MODE_STEREO</constant> for this purpose
is deprecated now. See the &VIDIOC-G-TUNER; section for
details.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 spec erratum 2006-09-23 (Draft 0.15)</title>
      <orderedlist>
	<listitem>
	  <para>In various places
<constant>V4L2_BUF_TYPE_SLICED_VBI_CAPTURE</constant> and
<constant>V4L2_BUF_TYPE_SLICED_VBI_OUTPUT</constant> of the sliced VBI
interface were not mentioned along with other buffer types.</para>
	</listitem>

	<listitem>
	  <para>In <xref linkend="vidioc-g-audio" /> it was clarified
that the &v4l2-audio; <structfield>mode</structfield> field is a flags
field.</para>
	</listitem>

	<listitem>
	  <para><xref linkend="vidioc-querycap" /> did not mention the
sliced VBI and radio capability flags.</para>
	</listitem>

	<listitem>
	  <para>In <xref linkend="vidioc-g-frequency" /> it was
clarified that applications must initialize the tuner
<structfield>type</structfield> field of &v4l2-frequency; before
calling &VIDIOC-S-FREQUENCY;.</para>
	</listitem>

	<listitem>
	  <para>The <structfield>reserved</structfield> array
in &v4l2-requestbuffers; has 2 elements, not 32.</para>
	</listitem>

	<listitem>
	  <para>In <xref linkend="output" /> and <xref
	      linkend="raw-vbi" /> the device file names
<filename>/dev/vout</filename> which never caught on were replaced
by <filename>/dev/video</filename>.</para>
	</listitem>

	<listitem>
	  <para>With Linux 2.6.15 the possible range for VBI device minor
numbers was extended from 224-239 to 224-255. Accordingly device file names
<filename>/dev/vbi0</filename> to <filename>/dev/vbi31</filename> are
possible now.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.18</title>
      <orderedlist>
	<listitem>
	  <para>New ioctls &VIDIOC-G-EXT-CTRLS;, &VIDIOC-S-EXT-CTRLS;
and &VIDIOC-TRY-EXT-CTRLS; were added, a flag to skip unsupported
controls with &VIDIOC-QUERYCTRL;, new control types
<constant>V4L2_CTRL_TYPE_INTEGER64</constant> and
<constant>V4L2_CTRL_TYPE_CTRL_CLASS</constant> (<xref
	      linkend="v4l2-ctrl-type" />), and new control flags
<constant>V4L2_CTRL_FLAG_READ_ONLY</constant>,
<constant>V4L2_CTRL_FLAG_UPDATE</constant>,
<constant>V4L2_CTRL_FLAG_INACTIVE</constant> and
<constant>V4L2_CTRL_FLAG_SLIDER</constant> (<xref
	      linkend="control-flags" />). See <xref
	      linkend="extended-controls" /> for details.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.19</title>
      <orderedlist>
	<listitem>
	  <para>In &v4l2-sliced-vbi-cap; a buffer type field was added
replacing a reserved field. Note on architectures where the size of
enum types differs from int types the size of the structure changed.
The &VIDIOC-G-SLICED-VBI-CAP; ioctl was redefined from being read-only
to write-read. Applications must initialize the type field and clear
the reserved fields now. These changes may <emphasis>break the
compatibility</emphasis> with older drivers and applications.</para>
	</listitem>

	<listitem>
	  <para>The ioctls &VIDIOC-ENUM-FRAMESIZES; and
&VIDIOC-ENUM-FRAMEINTERVALS; were added.</para>
	</listitem>

	<listitem>
	  <para>A new pixel format <constant>V4L2_PIX_FMT_RGB444</constant> (<xref
linkend="rgb-formats" />) was added.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 spec erratum 2006-10-12 (Draft 0.17)</title>
      <orderedlist>
	<listitem>
	  <para><constant>V4L2_PIX_FMT_HM12</constant> (<xref
linkend="reserved-formats" />) is a YUV 4:2:0, not 4:2:2 format.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.21</title>
      <orderedlist>
	<listitem>
	  <para>The <filename>videodev2.h</filename> header file is
now dual licensed under GNU General Public License version two or
later, and under a 3-clause BSD-style license.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.22</title>
      <orderedlist>
	<listitem>
	  <para>Two new field orders
	  <constant>V4L2_FIELD_INTERLACED_TB</constant> and
	  <constant>V4L2_FIELD_INTERLACED_BT</constant> were
	  added. See <xref linkend="v4l2-field" /> for details.</para>
	</listitem>

	<listitem>
	  <para>Three new clipping/blending methods with a global or
straight or inverted local alpha value were added to the video overlay
interface. See the description of the &VIDIOC-G-FBUF; and
&VIDIOC-S-FBUF; ioctls for details.</para>
	  <para>A new <structfield>global_alpha</structfield> field
was added to <link
linkend="v4l2-window"><structname>v4l2_window</structname></link>,
extending the structure. This may <emphasis>break
compatibility</emphasis> with applications using a struct
<structname>v4l2_window</structname> directly. However the <link
linkend="vidioc-g-fmt">VIDIOC_G/S/TRY_FMT</link> ioctls, which take a
pointer to a <link linkend="v4l2-format">v4l2_format</link> parent
structure with padding bytes at the end, are not affected.</para>
	</listitem>

	<listitem>
	  <para>The format of the <structfield>chromakey</structfield>
field in &v4l2-window; changed from "host order RGB32" to a pixel
value in the same format as the framebuffer. This may <emphasis>break
compatibility</emphasis> with existing applications. Drivers
supporting the "host order RGB32" format are not known.</para>
	</listitem>

      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.24</title>
      <orderedlist>
	<listitem>
	  <para>The pixel formats
<constant>V4L2_PIX_FMT_PAL8</constant>,
<constant>V4L2_PIX_FMT_YUV444</constant>,
<constant>V4L2_PIX_FMT_YUV555</constant>,
<constant>V4L2_PIX_FMT_YUV565</constant> and
<constant>V4L2_PIX_FMT_YUV32</constant> were added.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.25</title>
      <orderedlist>
	<listitem>
	  <para>The pixel formats <link linkend="V4L2-PIX-FMT-Y16">
<constant>V4L2_PIX_FMT_Y16</constant></link> and <link
linkend="V4L2-PIX-FMT-SBGGR16">
<constant>V4L2_PIX_FMT_SBGGR16</constant></link> were added.</para>
	</listitem>
	<listitem>
	  <para>New <link linkend="control">controls</link>
<constant>V4L2_CID_POWER_LINE_FREQUENCY</constant>,
<constant>V4L2_CID_HUE_AUTO</constant>,
<constant>V4L2_CID_WHITE_BALANCE_TEMPERATURE</constant>,
<constant>V4L2_CID_SHARPNESS</constant> and
<constant>V4L2_CID_BACKLIGHT_COMPENSATION</constant> were added. The
controls <constant>V4L2_CID_BLACK_LEVEL</constant>,
<constant>V4L2_CID_WHITENESS</constant>,
<constant>V4L2_CID_HCENTER</constant> and
<constant>V4L2_CID_VCENTER</constant> were deprecated.
</para>
	</listitem>
	<listitem>
	  <para>A <link linkend="camera-controls">Camera controls
class</link> was added, with the new controls
<constant>V4L2_CID_EXPOSURE_AUTO</constant>,
<constant>V4L2_CID_EXPOSURE_ABSOLUTE</constant>,
<constant>V4L2_CID_EXPOSURE_AUTO_PRIORITY</constant>,
<constant>V4L2_CID_PAN_RELATIVE</constant>,
<constant>V4L2_CID_TILT_RELATIVE</constant>,
<constant>V4L2_CID_PAN_RESET</constant>,
<constant>V4L2_CID_TILT_RESET</constant>,
<constant>V4L2_CID_PAN_ABSOLUTE</constant>,
<constant>V4L2_CID_TILT_ABSOLUTE</constant>,
<constant>V4L2_CID_FOCUS_ABSOLUTE</constant>,
<constant>V4L2_CID_FOCUS_RELATIVE</constant> and
<constant>V4L2_CID_FOCUS_AUTO</constant>.</para>
	</listitem>
	<listitem>
	  <para>The <constant>VIDIOC_G_MPEGCOMP</constant> and
<constant>VIDIOC_S_MPEGCOMP</constant> ioctls, which were superseded
by the <link linkend="extended-controls">extended controls</link>
interface in Linux 2.6.18, where finally removed from the
<filename>videodev2.h</filename> header file.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.26</title>
      <orderedlist>
	<listitem>
	  <para>The pixel formats
<constant>V4L2_PIX_FMT_Y16</constant> and
<constant>V4L2_PIX_FMT_SBGGR16</constant> were added.</para>
	</listitem>
	<listitem>
	  <para>Added user controls
<constant>V4L2_CID_CHROMA_AGC</constant> and
<constant>V4L2_CID_COLOR_KILLER</constant>.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.27</title>
      <orderedlist>
	<listitem>
	  <para>The &VIDIOC-S-HW-FREQ-SEEK; ioctl and the
<constant>V4L2_CAP_HW_FREQ_SEEK</constant> capability were added.</para>
	</listitem>
	<listitem>
	  <para>The pixel formats
<constant>V4L2_PIX_FMT_YVYU</constant>,
<constant>V4L2_PIX_FMT_PCA501</constant>,
<constant>V4L2_PIX_FMT_PCA505</constant>,
<constant>V4L2_PIX_FMT_PCA508</constant>,
<constant>V4L2_PIX_FMT_PCA561</constant>,
<constant>V4L2_PIX_FMT_SGBRG8</constant>,
<constant>V4L2_PIX_FMT_PAC207</constant> and
<constant>V4L2_PIX_FMT_PJPG</constant> were added.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.28</title>
      <orderedlist>
	<listitem>
	  <para>Added <constant>V4L2_MPEG_AUDIO_ENCODING_AAC</constant> and
<constant>V4L2_MPEG_AUDIO_ENCODING_AC3</constant> MPEG audio encodings.</para>
	</listitem>
	<listitem>
	  <para>Added <constant>V4L2_MPEG_VIDEO_ENCODING_MPEG_4_AVC</constant> MPEG
video encoding.</para>
	</listitem>
	<listitem>
	  <para>The pixel formats
<constant>V4L2_PIX_FMT_SGRBG10</constant> and
<constant>V4L2_PIX_FMT_SGRBG10DPCM8</constant> were added.</para>
	</listitem>
      </orderedlist>
    </section>

    <section>
      <title>V4L2 in Linux 2.6.29</title>
      <orderedlist>
	<listitem>
	  <para>The <constant>VIDIOC_G_CHIP_IDENT</constant> ioctl was renamed
to <constant>VIDIOC_G_CHIP_IDENT_OLD</constant> and &VIDIOC-DBG-G-CHIP-IDENT;
was introduced in its place. The old struct <structname>v4l2_chip_ident</structname>
was renamed to <structname id="v4l2-chip-ident-old">v4l2_chip_ident_old</structname>.</para>
	</listitem>
	<listitem>
	  <para>The pixel formats
<constant>V4L2_PIX_FMT_VYUY</constant>,
<constant>V4L2_PIX_FMT_NV16</constant> and
<constant>V4L2_PIX_FMT_NV61</constant> were added.</para>
	</listitem>
	<listitem>
	  <para>Added camera controls
<constant>V4L2_CID_ZOOM_ABSOLUTE</constant>,
<constant>V4L2_CID_ZOOM_RELATIVE</constant>,
<constant>V4L2_CID_ZOOM_CONTINUOUS</constant> and
<constant>V4L2_CID_PRIVACY</constant>.</para>
	</listitem>
      </orderedlist>
    </section>
    <section>
      <title>V4L2 in Linux 2.6.30</title>
      <orderedlist>
	<listitem>
	  <para>New control flag <constant>V4L2_CTRL_FLAG_WRITE_ONLY</constant> was added.</para>
	</listitem>
	<listitem>
	  <para>New control <constant>V4L2_CID_COLORFX</constant> was added.</para>
	</listitem>
      </orderedlist>
    </section>
    <section>
      <title>V4L2 in Linux 2.6.32</title>
      <orderedlist>
	<listitem>
	  <para>In order to be easier to compare a V4L2 API and a kernel
version, now V4L2 API is numbered using the Linux Kernel version numeration.</para>
	</listitem>
	<listitem>
	  <para>Finalized the RDS capture API. See <xref linkend="rds" /> for
more information.</para>
	</listitem>
	<listitem>
	  <para>Added new capabilities for modulators and RDS encoders.</para>
	</listitem>
	<listitem>
	  <para>Add description for libv4l API.</para>
	</listitem>
	<listitem>
	  <para>Added support for string controls via new type <constant>V4L2_CTRL_TYPE_STRING</constant>.</para>
	</listitem>
	<listitem>
	  <para>Added <constant>V4L2_CID_BAND_STOP_FILTER</constant> documentation.</para>
	</listitem>
	<listitem>
	  <para>Added FM Modulator (FM TX) Extended Control Class: <constant>V4L2_CTRL_CLASS_FM_TX</constant> and their Control IDs.</para>
	</listitem>
	<listitem>
	  <para>Added Remote Controller chapter, describing the default Remote Controller mapping for media devices.</para>
	</listitem>
      </orderedlist>
    </section>
    <section>
      <title>V4L2 in Linux 2.6.33</title>
      <orderedlist>
	<listitem>
	  <para>Added support for Digital Video timings in order to support HDTV receivers and transmitters.</para>
	</listitem>
      </orderedlist>
    </section>
    <section>
      <title>V4L2 in Linux 2.6.34</title>
      <orderedlist>
	<listitem>
	  <para>Added
<constant>V4L2_CID_IRIS_ABSOLUTE</constant> and
<constant>V4L2_CID_IRIS_RELATIVE</constant> controls to the
	    <link linkend="camera-controls">Camera controls class</link>.
	  </para>
	</listitem>
      </orderedlist>
    </section>
    <section>
      <title>V4L2 in Linux 2.6.37</title>
      <orderedlist>
	<listitem>
	  <para>Remove the vtx (videotext/teletext) API. This API was no longer
used and no hardware exists to verify the API. Nor were any userspace applications found
that used it. It was originally scheduled for removal in 2.6.35.
	  </para>
	</listitem>
      </orderedlist>
    </section>

    <section id="other">
      <title>Relation of V4L2 to other Linux multimedia APIs</title>

      <section id="xvideo">
        <title>X Video Extension</title>

        <para>The X Video Extension (abbreviated XVideo or just Xv) is
an extension of the X Window system, implemented for example by the
XFree86 project. Its scope is similar to V4L2, an API to video capture
and output devices for X clients. Xv allows applications to display
live video in a window, send window contents to a TV output, and
capture or output still images in XPixmaps<footnote>
	  <para>This is not implemented in XFree86.</para>
	</footnote>. With their implementation XFree86 makes the
extension available across many operating systems and
architectures.</para>

        <para>Because the driver is embedded into the X server Xv has a
number of advantages over the V4L2 <link linkend="overlay">video
overlay interface</link>. The driver can easily determine the overlay
target, &ie; visible graphics memory or off-screen buffers for a
destructive overlay. It can program the RAMDAC for a non-destructive
overlay, scaling or color-keying, or the clipping functions of the
video capture hardware, always in sync with drawing operations or
windows moving or changing their stacking order.</para>

        <para>To combine the advantages of Xv and V4L a special Xv
driver exists in XFree86 and XOrg, just programming any overlay capable
Video4Linux device it finds. To enable it
<filename>/etc/X11/XF86Config</filename> must contain these lines:</para>
        <para><screen>
Section "Module"
    Load "v4l"
EndSection</screen></para>

        <para>As of XFree86 4.2 this driver still supports only V4L
ioctls, however it should work just fine with all V4L2 devices through
the V4L2 backward-compatibility layer. Since V4L2 permits multiple
opens it is possible (if supported by the V4L2 driver) to capture
video while an X client requested video overlay. Restrictions of
simultaneous capturing and overlay are discussed in <xref
	  linkend="overlay" /> apply.</para>

        <para>Only marginally related to V4L2, XFree86 extended Xv to
support hardware YUV to RGB conversion and scaling for faster video
playback, and added an interface to MPEG-2 decoding hardware. This API
is useful to display images captured with V4L2 devices.</para>
      </section>

      <section>
        <title>Digital Video</title>

        <para>V4L2 does not support digital terrestrial, cable or
satellite broadcast. A separate project aiming at digital receivers
exists. You can find its homepage at <ulink
url="http://linuxtv.org">http://linuxtv.org</ulink>. The Linux DVB API
has no connection to the V4L2 API except that drivers for hybrid
hardware may support both.</para>
      </section>

      <section>
        <title>Audio Interfaces</title>

        <para>[to do - OSS/ALSA]</para>
      </section>
    </section>

    <section id="experimental">
      <title>Experimental API Elements</title>

      <para>The following V4L2 API elements are currently experimental
and may change in the future.</para>

      <itemizedlist>
        <listitem>
	  <para>Video Output Overlay (OSD) Interface, <xref
	    linkend="osd" />.</para>
        </listitem>
	<listitem>
	  <para><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant>,
	&v4l2-buf-type;, <xref linkend="v4l2-buf-type" />.</para>
        </listitem>
        <listitem>
	  <para><constant>V4L2_CAP_VIDEO_OUTPUT_OVERLAY</constant>,
&VIDIOC-QUERYCAP; ioctl, <xref linkend="device-capabilities" />.</para>
        </listitem>
        <listitem>
	  <para>&VIDIOC-ENUM-FRAMESIZES; and
&VIDIOC-ENUM-FRAMEINTERVALS; ioctls.</para>
        </listitem>
        <listitem>
	  <para>&VIDIOC-G-ENC-INDEX; ioctl.</para>
        </listitem>
        <listitem>
	  <para>&VIDIOC-ENCODER-CMD; and &VIDIOC-TRY-ENCODER-CMD;
ioctls.</para>
        </listitem>
        <listitem>
	  <para>&VIDIOC-DBG-G-REGISTER; and &VIDIOC-DBG-S-REGISTER;
ioctls.</para>
        </listitem>
        <listitem>
	  <para>&VIDIOC-DBG-G-CHIP-IDENT; ioctl.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section id="obsolete">
      <title>Obsolete API Elements</title>

      <para>The following V4L2 API elements were superseded by new
interfaces and should not be implemented in new drivers.</para>

      <itemizedlist>
        <listitem>
	  <para><constant>VIDIOC_G_MPEGCOMP</constant> and
<constant>VIDIOC_S_MPEGCOMP</constant> ioctls. Use Extended Controls,
<xref linkend="extended-controls" />.</para>
        </listitem>
      </itemizedlist>
    </section>
  </section>

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