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authorLinus Torvalds <torvalds@linux-foundation.org>2009-09-21 09:03:10 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2009-09-21 09:03:10 -0700
commitc720f5655df159a630fa0290a0bd67c93e92b0bf (patch)
tree940d139d0ec1ff5201efddef6cc663166a8a2df3 /Documentation/DocBook/v4l/dev-capture.xml
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Merge branch 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-2.6
* 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-2.6: (222 commits) V4L/DVB (13033): pt1: Don't use a deprecated DMA_BIT_MASK macro V4L/DVB (13029): radio-si4713: remove #include <linux/version.h> V4L/DVB (13027): go7007: convert printks to v4l2_info V4L/DVB (13026): s2250-board: Implement brightness and contrast controls V4L/DVB (13025): s2250-board: Fix memory leaks V4L/DVB (13024): go7007: Implement vidioc_g_std and vidioc_querystd V4L/DVB (13023): go7007: Merge struct gofh and go declarations V4L/DVB (13022): go7007: Fix mpeg controls V4L/DVB (13021): go7007: Fix whitespace and line lengths V4L/DVB (13020): go7007: Updates to Kconfig and Makefile V4L/DVB (13019): video: initial support for ADV7180 V4L/DVB (13018): kzalloc failure ignored in au8522_probe() V4L/DVB (13017): gspca: kmalloc failure ignored in sd_start() V4L/DVB (13016): kmalloc failure ignored in lgdt3304_attach() and s921_attach() V4L/DVB (13015): kmalloc failure ignored in m920x_firmware_download() V4L/DVB (13014): Add support for Compro VideoMate E800 (DVB-T part only) V4L/DVB (13013): FM TX: si4713: Kconfig: Fixed two typos. V4L/DVB (13012): uvc: introduce missing kfree V4L/DVB (13011): Change tuner type of BeholdTV cards V4L/DVB (13009): gspca - stv06xx-hdcs: Reduce exposure range ...
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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> 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> and call the
+&VIDIOC-G-FMT; ioctl with a pointer to this structure. Drivers fill
+the &v4l2-pix-format; <structfield>pix</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; 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:
+ -->