111 lines
4.9 KiB
XML
111 lines
4.9 KiB
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>
|