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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>