dev-capture.xml

  <title>Video Capture Interface</title>

  <para>Video capture devices sample an analog video signal and store
the digitized images in memory. Today nearly all devices can capture
at full 25 or 30 frames/second. With this interface applications can
control the capture process and move images from the driver into user
space.</para>

  <para>Conventionally V4L2 video capture devices are accessed through
character device special files named <filename>/dev/video</filename>
and <filename>/dev/video0</filename> to
<filename>/dev/video63</filename> with major number 81 and minor
numbers 0 to 63. <filename>/dev/video</filename> is typically a
symbolic link to the preferred video device. Note the same device
files are used for video output devices.</para>

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

    <para>Devices supporting the video capture interface set the
<constant>V4L2_CAP_VIDEO_CAPTURE</constant> or
<constant>V4L2_CAP_VIDEO_CAPTURE_MPLANE</constant> flag in the
<structfield>capabilities</structfield> field of &v4l2-capability;
returned by the &VIDIOC-QUERYCAP; ioctl. As secondary device functions
they may also support the <link linkend="overlay">video overlay</link>
(<constant>V4L2_CAP_VIDEO_OVERLAY</constant>) and the <link
linkend="raw-vbi">raw VBI capture</link>
(<constant>V4L2_CAP_VBI_CAPTURE</constant>) interface. At least one of
the read/write or streaming I/O methods must be supported. Tuners and
audio inputs are optional.</para>
  </section>

  <section>
    <title>Supplemental Functions</title>

    <para>Video capture devices shall support <link
linkend="audio">audio input</link>, <link
linkend="tuner">tuner</link>, <link linkend="control">controls</link>,
<link linkend="crop">cropping and scaling</link> and <link
linkend="streaming-par">streaming parameter</link> ioctls as needed.
The <link linkend="video">video input</link> and <link
linkend="standard">video standard</link> ioctls must be supported by
all video capture devices.</para>
  </section>

  <section>
    <title>Image Format Negotiation</title>

    <para>The result of a capture operation is determined by
cropping and image format parameters. The former select an area of the
video picture to capture, the latter how images are stored in memory,
&ie; in RGB or YUV format, the number of bits per pixel or width and
height. Together they also define how images are scaled in the
process.</para>

    <para>As usual these parameters are <emphasis>not</emphasis> reset
at &func-open; time to permit Unix tool chains, programming a device
and then reading from it as if it was a plain file. Well written V4L2
applications ensure they really get what they want, including cropping
and scaling.</para>

    <para>Cropping initialization at minimum requires to reset the
parameters to defaults. An example is given in <xref
linkend="crop" />.</para>

    <para>To query the current image format applications set the
<structfield>type</structfield> field of a &v4l2-format; to
<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> or
<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE</constant> and call the
&VIDIOC-G-FMT; ioctl with a pointer to this structure. Drivers fill
the &v4l2-pix-format; <structfield>pix</structfield> or the
&v4l2-pix-format-mplane; <structfield>pix_mp</structfield> member of the
<structfield>fmt</structfield> union.</para>

    <para>To request different parameters applications set the
<structfield>type</structfield> field of a &v4l2-format; as above and
initialize all fields of the &v4l2-pix-format;
<structfield>vbi</structfield> member of the
<structfield>fmt</structfield> union, or better just modify the
results of <constant>VIDIOC_G_FMT</constant>, and call the
&VIDIOC-S-FMT; ioctl with a pointer to this structure. Drivers may
adjust the parameters and finally return the actual parameters as
<constant>VIDIOC_G_FMT</constant> does.</para>

    <para>Like <constant>VIDIOC_S_FMT</constant> the
&VIDIOC-TRY-FMT; ioctl can be used to learn about hardware limitations
without disabling I/O or possibly time consuming hardware
preparations.</para>

    <para>The contents of &v4l2-pix-format; and &v4l2-pix-format-mplane;
are discussed in <xref linkend="pixfmt" />. See also the specification of the
<constant>VIDIOC_G_FMT</constant>, <constant>VIDIOC_S_FMT</constant>
and <constant>VIDIOC_TRY_FMT</constant> ioctls for details. Video
capture devices must implement both the
<constant>VIDIOC_G_FMT</constant> and
<constant>VIDIOC_S_FMT</constant> ioctl, even if
<constant>VIDIOC_S_FMT</constant> ignores all requests and always
returns default parameters as <constant>VIDIOC_G_FMT</constant> does.
<constant>VIDIOC_TRY_FMT</constant> is optional.</para>
  </section>

  <section>
    <title>Reading Images</title>

    <para>A video capture device may support the <link
linkend="rw">read() function</link> and/or streaming (<link
linkend="mmap">memory mapping</link> or <link
linkend="userp">user pointer</link>) I/O. See <xref
linkend="io" /> for details.</para>
  </section>

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