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+ <title>Video Output Interface</title>
+ <para>Video output devices encode stills or image sequences as
+analog video signal. With this interface applications can
+control the encoding process and move images from user space to
+the driver.</para>
+ <para>Conventionally V4L2 video output 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 capture devices.</para>
+ <section>
+ <title>Querying Capabilities</title>
+ <para>Devices supporting the video output interface set the
+<constant>V4L2_CAP_VIDEO_OUTPUT</constant> or
+<constant>V4L2_CAP_VIDEO_OUTPUT_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="raw-vbi">raw VBI
+output</link> (<constant>V4L2_CAP_VBI_OUTPUT</constant>) interface. At
+least one of the read/write or streaming I/O methods must be
+supported. Modulators and audio outputs are optional.</para>
+ </section>
+ <section>
+ <title>Supplemental Functions</title>
+ <para>Video output devices shall support <link
+linkend="audio">audio output</link>, <link
+linkend="tuner">modulator</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 output</link> and <link
+linkend="standard">video standard</link> ioctls must be supported by
+all video output devices.</para>
+ </section>
+ <section>
+ <title>Image Format Negotiation</title>
+ <para>The output is determined by cropping and image format
+parameters. The former select an area of the video picture where the
+image will appear, 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 writing to 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_OUTPUT</constant> or
+<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_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
+ <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
+output 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>Writing Images</title>
+ <para>A video output device may support the <link
+linkend="rw">write() 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>