c6401af669
This removes comment tags intended for emacs configuration from 67 files in the Media API DocBook. Such comments are not really helpful and violate the coding style rules. Signed-off-by: Sylwester Nawrocki <snjw23@gmail.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
110 lines
4.9 KiB
XML
110 lines
4.9 KiB
XML
<title>Video Capture Interface</title>
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<para>Video capture devices sample an analog video signal and store
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the digitized images in memory. Today nearly all devices can capture
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at full 25 or 30 frames/second. With this interface applications can
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control the capture process and move images from the driver into user
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space.</para>
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<para>Conventionally V4L2 video capture 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 output devices.</para>
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<section>
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<title>Querying Capabilities</title>
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<para>Devices supporting the video capture interface set the
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<constant>V4L2_CAP_VIDEO_CAPTURE</constant> or
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<constant>V4L2_CAP_VIDEO_CAPTURE_MPLANE</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="overlay">video overlay</link>
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(<constant>V4L2_CAP_VIDEO_OVERLAY</constant>) and the <link
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linkend="raw-vbi">raw VBI capture</link>
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(<constant>V4L2_CAP_VBI_CAPTURE</constant>) interface. At least one of
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the read/write or streaming I/O methods must be supported. Tuners and
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audio inputs 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 capture devices shall support <link
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linkend="audio">audio input</link>, <link
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linkend="tuner">tuner</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 input</link> and <link
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linkend="standard">video standard</link> ioctls must be supported by
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all video capture 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 result of a capture operation is determined by
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cropping and image format parameters. The former select an area of the
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video picture to capture, the latter how images are stored in memory,
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&ie; in RGB or YUV format, the number of bits per pixel or width and
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height. Together they also define how images are scaled in the
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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 reading from 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_CAPTURE</constant> or
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<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE</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> or the
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&v4l2-pix-format-mplane; <structfield>pix_mp</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; and &v4l2-pix-format-mplane;
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are discussed in <xref 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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capture 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>Reading Images</title>
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<para>A video capture device may support the <link
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linkend="rw">read() 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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