From 849369d6c66d3054688672f97d31fceb8e8230fb Mon Sep 17 00:00:00 2001
From: root <root@artemis.panaceas.org>
Date: Fri, 25 Dec 2015 04:40:36 +0000
Subject: initial_commit

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+  <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>
+
+  <!--
+Local Variables:
+mode: sgml
+sgml-parent-document: "v4l2.sgml"
+indent-tabs-mode: nil
+End:
+  -->
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