112 lines
4.6 KiB
XML
112 lines
4.6 KiB
XML
<title>Video Output Interface</title>
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<para>Video output devices encode stills or image sequences as
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analog video signal. With this interface applications can
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control the encoding process and move images from user space to
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the driver.</para>
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<para>Conventionally V4L2 video output devices are accessed through
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character device special files named <filename>/dev/video</filename>
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and <filename>/dev/video0</filename> to
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<filename>/dev/video63</filename> with major number 81 and minor
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numbers 0 to 63. <filename>/dev/video</filename> is typically a
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symbolic link to the preferred video device. Note the same device
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files are used for video capture devices.</para>
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<section>
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<title>Querying Capabilities</title>
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<para>Devices supporting the video output interface set the
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<constant>V4L2_CAP_VIDEO_OUTPUT</constant> flag in the
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<structfield>capabilities</structfield> field of &v4l2-capability;
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returned by the &VIDIOC-QUERYCAP; ioctl. As secondary device functions
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they may also support the <link linkend="raw-vbi">raw VBI
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output</link> (<constant>V4L2_CAP_VBI_OUTPUT</constant>) interface. At
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least one of the read/write or streaming I/O methods must be
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supported. Modulators and audio outputs are optional.</para>
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</section>
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<section>
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<title>Supplemental Functions</title>
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<para>Video output devices shall support <link
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linkend="audio">audio output</link>, <link
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linkend="tuner">modulator</link>, <link linkend="control">controls</link>,
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<link linkend="crop">cropping and scaling</link> and <link
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linkend="streaming-par">streaming parameter</link> ioctls as needed.
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The <link linkend="video">video output</link> and <link
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linkend="standard">video standard</link> ioctls must be supported by
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all video output devices.</para>
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</section>
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<section>
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<title>Image Format Negotiation</title>
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<para>The output is determined by cropping and image format
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parameters. The former select an area of the video picture where the
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image will appear, the latter how images are stored in memory, &ie; in
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RGB or YUV format, the number of bits per pixel or width and height.
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Together they also define how images are scaled in the process.</para>
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<para>As usual these parameters are <emphasis>not</emphasis> reset
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at &func-open; time to permit Unix tool chains, programming a device
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and then writing to it as if it was a plain file. Well written V4L2
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applications ensure they really get what they want, including cropping
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and scaling.</para>
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<para>Cropping initialization at minimum requires to reset the
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parameters to defaults. An example is given in <xref
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linkend="crop" />.</para>
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<para>To query the current image format applications set the
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<structfield>type</structfield> field of a &v4l2-format; to
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<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant> and call the
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&VIDIOC-G-FMT; ioctl with a pointer to this structure. Drivers fill
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the &v4l2-pix-format; <structfield>pix</structfield> member of the
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<structfield>fmt</structfield> union.</para>
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<para>To request different parameters applications set the
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<structfield>type</structfield> field of a &v4l2-format; as above and
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initialize all fields of the &v4l2-pix-format;
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<structfield>vbi</structfield> member of the
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<structfield>fmt</structfield> union, or better just modify the
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results of <constant>VIDIOC_G_FMT</constant>, and call the
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&VIDIOC-S-FMT; ioctl with a pointer to this structure. Drivers may
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adjust the parameters and finally return the actual parameters as
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<constant>VIDIOC_G_FMT</constant> does.</para>
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<para>Like <constant>VIDIOC_S_FMT</constant> the
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&VIDIOC-TRY-FMT; ioctl can be used to learn about hardware limitations
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without disabling I/O or possibly time consuming hardware
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preparations.</para>
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<para>The contents of &v4l2-pix-format; are discussed in <xref
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linkend="pixfmt" />. See also the specification of the
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<constant>VIDIOC_G_FMT</constant>, <constant>VIDIOC_S_FMT</constant>
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and <constant>VIDIOC_TRY_FMT</constant> ioctls for details. Video
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output devices must implement both the
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<constant>VIDIOC_G_FMT</constant> and
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<constant>VIDIOC_S_FMT</constant> ioctl, even if
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<constant>VIDIOC_S_FMT</constant> ignores all requests and always
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returns default parameters as <constant>VIDIOC_G_FMT</constant> does.
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<constant>VIDIOC_TRY_FMT</constant> is optional.</para>
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</section>
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<section>
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<title>Writing Images</title>
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<para>A video output device may support the <link
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linkend="rw">write() function</link> and/or streaming (<link
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linkend="mmap">memory mapping</link> or <link
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linkend="userp">user pointer</link>) I/O. See <xref
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linkend="io" /> for details.</para>
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</section>
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