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379 lines
16 KiB
379 lines
16 KiB
<title>Video Overlay Interface</title>
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<subtitle>Also known as Framebuffer Overlay or Previewing</subtitle>
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<para>Video overlay devices have the ability to genlock (TV-)video
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into the (VGA-)video signal of a graphics card, or to store captured
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images directly in video memory of a graphics card, typically with
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clipping. This can be considerable more efficient than capturing
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images and displaying them by other means. In the old days when only
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nuclear power plants needed cooling towers this used to be the only
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way to put live video into a window.</para>
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<para>Video overlay devices are accessed through the same character
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special files as <link linkend="capture">video capture</link> devices.
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Note the default function of a <filename>/dev/video</filename> device
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is video capturing. The overlay function is only available after
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calling the &VIDIOC-S-FMT; ioctl.</para>
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<para>The driver may support simultaneous overlay and capturing
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using the read/write and streaming I/O methods. If so, operation at
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the nominal frame rate of the video standard is not guaranteed. Frames
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may be directed away from overlay to capture, or one field may be used
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for overlay and the other for capture if the capture parameters permit
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this.</para>
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<para>Applications should use different file descriptors for
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capturing and overlay. This must be supported by all drivers capable
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of simultaneous capturing and overlay. Optionally these drivers may
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also permit capturing and overlay with a single file descriptor for
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compatibility with V4L and earlier versions of V4L2.<footnote>
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<para>A common application of two file descriptors is the
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XFree86 <link linkend="xvideo">Xv/V4L</link> interface driver and
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a V4L2 application. While the X server controls video overlay, the
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application can take advantage of memory mapping and DMA.</para>
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<para>In the opinion of the designers of this API, no driver
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writer taking the efforts to support simultaneous capturing and
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overlay will restrict this ability by requiring a single file
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descriptor, as in V4L and earlier versions of V4L2. Making this
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optional means applications depending on two file descriptors need
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backup routines to be compatible with all drivers, which is
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considerable more work than using two fds in applications which do
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not. Also two fd's fit the general concept of one file descriptor for
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each logical stream. Hence as a complexity trade-off drivers
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<emphasis>must</emphasis> support two file descriptors and
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<emphasis>may</emphasis> support single fd operation.</para>
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</footnote></para>
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<section>
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<title>Querying Capabilities</title>
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<para>Devices supporting the video overlay interface set the
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<constant>V4L2_CAP_VIDEO_OVERLAY</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. The overlay I/O method specified
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below must be supported. Tuners and 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 overlay 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 overlay devices.</para>
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</section>
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<section>
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<title>Setup</title>
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<para>Before overlay can commence applications must program the
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driver with frame buffer parameters, namely the address and size of
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the frame buffer and the image format, for example RGB 5:6:5. The
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&VIDIOC-G-FBUF; and &VIDIOC-S-FBUF; ioctls are available to get
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and set these parameters, respectively. The
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<constant>VIDIOC_S_FBUF</constant> ioctl is privileged because it
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allows to set up DMA into physical memory, bypassing the memory
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protection mechanisms of the kernel. Only the superuser can change the
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frame buffer address and size. Users are not supposed to run TV
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applications as root or with SUID bit set. A small helper application
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with suitable privileges should query the graphics system and program
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the V4L2 driver at the appropriate time.</para>
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<para>Some devices add the video overlay to the output signal
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of the graphics card. In this case the frame buffer is not modified by
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the video device, and the frame buffer address and pixel format are
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not needed by the driver. The <constant>VIDIOC_S_FBUF</constant> ioctl
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is not privileged. An application can check for this type of device by
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calling the <constant>VIDIOC_G_FBUF</constant> ioctl.</para>
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<para>A driver may support any (or none) of five clipping/blending
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methods:<orderedlist>
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<listitem>
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<para>Chroma-keying displays the overlaid image only where
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pixels in the primary graphics surface assume a certain color.</para>
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</listitem>
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<listitem>
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<para>A bitmap can be specified where each bit corresponds
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to a pixel in the overlaid image. When the bit is set, the
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corresponding video pixel is displayed, otherwise a pixel of the
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graphics surface.</para>
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</listitem>
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<listitem>
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<para>A list of clipping rectangles can be specified. In
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these regions <emphasis>no</emphasis> video is displayed, so the
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graphics surface can be seen here.</para>
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</listitem>
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<listitem>
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<para>The framebuffer has an alpha channel that can be used
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to clip or blend the framebuffer with the video.</para>
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</listitem>
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<listitem>
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<para>A global alpha value can be specified to blend the
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framebuffer contents with video images.</para>
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</listitem>
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</orderedlist></para>
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<para>When simultaneous capturing and overlay is supported and
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the hardware prohibits different image and frame buffer formats, the
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format requested first takes precedence. The attempt to capture
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(&VIDIOC-S-FMT;) or overlay (&VIDIOC-S-FBUF;) may fail with an
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&EBUSY; or return accordingly modified parameters..</para>
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</section>
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<section>
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<title>Overlay Window</title>
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<para>The overlaid image is determined by cropping and overlay
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window parameters. The former select an area of the video picture to
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capture, the latter how images are overlaid and clipped. Cropping
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initialization at minimum requires to reset the parameters to
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defaults. An example is given in <xref linkend="crop" />.</para>
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<para>The overlay window is described by a &v4l2-window;. It
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defines the size of the image, its position over the graphics surface
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and the clipping to be applied. To get the current parameters
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applications set the <structfield>type</structfield> field of a
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&v4l2-format; to <constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant> and
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call the &VIDIOC-G-FMT; ioctl. The driver fills the
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<structname>v4l2_window</structname> substructure named
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<structfield>win</structfield>. It is not possible to retrieve a
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previously programmed clipping list or bitmap.</para>
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<para>To program the overlay window 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_OVERLAY</constant>, initialize the
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<structfield>win</structfield> substructure and call the
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&VIDIOC-S-FMT; ioctl. The driver adjusts the parameters against
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hardware limits and returns the actual parameters as
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<constant>VIDIOC_G_FMT</constant> does. Like
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<constant>VIDIOC_S_FMT</constant>, the &VIDIOC-TRY-FMT; ioctl can be
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used to learn about driver capabilities without actually changing
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driver state. Unlike <constant>VIDIOC_S_FMT</constant> this also works
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after the overlay has been enabled.</para>
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<para>The scaling factor of the overlaid image is implied by the
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width and height given in &v4l2-window; and the size of the cropping
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rectangle. For more information see <xref linkend="crop" />.</para>
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<para>When simultaneous capturing and overlay is supported and
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the hardware prohibits different image and window sizes, the size
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requested first takes precedence. The attempt to capture or overlay as
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well (&VIDIOC-S-FMT;) may fail with an &EBUSY; or return accordingly
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modified parameters.</para>
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<table pgwide="1" frame="none" id="v4l2-window">
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<title>struct <structname>v4l2_window</structname></title>
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<tgroup cols="3">
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&cs-str;
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<tbody valign="top">
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<row>
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<entry>&v4l2-rect;</entry>
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<entry><structfield>w</structfield></entry>
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<entry>Size and position of the window relative to the
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top, left corner of the frame buffer defined with &VIDIOC-S-FBUF;. The
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window can extend the frame buffer width and height, the
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<structfield>x</structfield> and <structfield>y</structfield>
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coordinates can be negative, and it can lie completely outside the
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frame buffer. The driver clips the window accordingly, or if that is
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not possible, modifies its size and/or position.</entry>
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</row>
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<row>
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<entry>&v4l2-field;</entry>
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<entry><structfield>field</structfield></entry>
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<entry>Applications set this field to determine which
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video field shall be overlaid, typically one of
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<constant>V4L2_FIELD_ANY</constant> (0),
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<constant>V4L2_FIELD_TOP</constant>,
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<constant>V4L2_FIELD_BOTTOM</constant> or
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<constant>V4L2_FIELD_INTERLACED</constant>. Drivers may have to choose
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a different field order and return the actual setting here.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>chromakey</structfield></entry>
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<entry>When chroma-keying has been negotiated with
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&VIDIOC-S-FBUF; applications set this field to the desired pixel value
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for the chroma key. The format is the same as the pixel format of the
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framebuffer (&v4l2-framebuffer;
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<structfield>fmt.pixelformat</structfield> field), with bytes in host
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order. E. g. for <link
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linkend="V4L2-PIX-FMT-BGR32"><constant>V4L2_PIX_FMT_BGR24</constant></link>
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the value should be 0xRRGGBB on a little endian, 0xBBGGRR on a big
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endian host.</entry>
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</row>
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<row>
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<entry>&v4l2-clip; *</entry>
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<entry><structfield>clips</structfield></entry>
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<entry>When chroma-keying has <emphasis>not</emphasis>
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been negotiated and &VIDIOC-G-FBUF; indicated this capability,
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applications can set this field to point to an array of
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clipping rectangles.</entry>
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</row>
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<row>
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<entry></entry>
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<entry></entry>
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<entry>Like the window coordinates
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<structfield>w</structfield>, clipping rectangles are defined relative
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to the top, left corner of the frame buffer. However clipping
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rectangles must not extend the frame buffer width and height, and they
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must not overlap. If possible applications should merge adjacent
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rectangles. Whether this must create x-y or y-x bands, or the order of
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rectangles, is not defined. When clip lists are not supported the
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driver ignores this field. Its contents after calling &VIDIOC-S-FMT;
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are undefined.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>clipcount</structfield></entry>
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<entry>When the application set the
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<structfield>clips</structfield> field, this field must contain the
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number of clipping rectangles in the list. When clip lists are not
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supported the driver ignores this field, its contents after calling
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<constant>VIDIOC_S_FMT</constant> are undefined. When clip lists are
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supported but no clipping is desired this field must be set to
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zero.</entry>
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</row>
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<row>
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<entry>void *</entry>
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<entry><structfield>bitmap</structfield></entry>
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<entry>When chroma-keying has
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<emphasis>not</emphasis> been negotiated and &VIDIOC-G-FBUF; indicated
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this capability, applications can set this field to point to a
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clipping bit mask.</entry>
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</row>
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<row>
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<entry spanname="hspan"><para>It must be of the same size
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as the window, <structfield>w.width</structfield> and
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<structfield>w.height</structfield>. Each bit corresponds to a pixel
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in the overlaid image, which is displayed only when the bit is
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<emphasis>set</emphasis>. Pixel coordinates translate to bits like:
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<programlisting>
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((__u8 *) <structfield>bitmap</structfield>)[<structfield>w.width</structfield> * y + x / 8] & (1 << (x & 7))</programlisting></para><para>where <structfield>0</structfield> ≤ x <
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<structfield>w.width</structfield> and <structfield>0</structfield> ≤
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y <<structfield>w.height</structfield>.<footnote>
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<para>Should we require
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<structfield>w.width</structfield> to be a multiple of
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eight?</para>
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</footnote></para><para>When a clipping
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bit mask is not supported the driver ignores this field, its contents
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after calling &VIDIOC-S-FMT; are undefined. When a bit mask is supported
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but no clipping is desired this field must be set to
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<constant>NULL</constant>.</para><para>Applications need not create a
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clip list or bit mask. When they pass both, or despite negotiating
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chroma-keying, the results are undefined. Regardless of the chosen
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method, the clipping abilities of the hardware may be limited in
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quantity or quality. The results when these limits are exceeded are
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undefined.<footnote>
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<para>When the image is written into frame buffer
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memory it will be undesirable if the driver clips out less pixels
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than expected, because the application and graphics system are not
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aware these regions need to be refreshed. The driver should clip out
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more pixels or not write the image at all.</para>
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</footnote></para></entry>
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</row>
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<row>
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<entry>__u8</entry>
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<entry><structfield>global_alpha</structfield></entry>
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<entry>The global alpha value used to blend the
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framebuffer with video images, if global alpha blending has been
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negotiated (<constant>V4L2_FBUF_FLAG_GLOBAL_ALPHA</constant>, see
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&VIDIOC-S-FBUF;, <xref linkend="framebuffer-flags" />).</entry>
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</row>
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<row>
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<entry></entry>
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<entry></entry>
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<entry>Note this field was added in Linux 2.6.23, extending the structure. However
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the <link linkend="vidioc-g-fmt">VIDIOC_G/S/TRY_FMT</link> ioctls,
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which take a pointer to a <link
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linkend="v4l2-format">v4l2_format</link> parent structure with padding
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bytes at the end, are not affected.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table pgwide="1" frame="none" id="v4l2-clip">
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<title>struct <structname>v4l2_clip</structname><footnote>
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<para>The X Window system defines "regions" which are
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vectors of struct BoxRec { short x1, y1, x2, y2; } with width = x2 -
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x1 and height = y2 - y1, so one cannot pass X11 clip lists
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directly.</para>
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</footnote></title>
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<tgroup cols="3">
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&cs-str;
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<tbody valign="top">
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<row>
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<entry>&v4l2-rect;</entry>
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<entry><structfield>c</structfield></entry>
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<entry>Coordinates of the clipping rectangle, relative to
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the top, left corner of the frame buffer. Only window pixels
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<emphasis>outside</emphasis> all clipping rectangles are
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displayed.</entry>
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</row>
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<row>
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<entry>&v4l2-clip; *</entry>
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<entry><structfield>next</structfield></entry>
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<entry>Pointer to the next clipping rectangle, NULL when
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this is the last rectangle. Drivers ignore this field, it cannot be
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used to pass a linked list of clipping rectangles.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<!-- NB for easier reading this table is duplicated
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in the vidioc-cropcap chapter.-->
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<table pgwide="1" frame="none" id="v4l2-rect">
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<title>struct <structname>v4l2_rect</structname></title>
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<tgroup cols="3">
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&cs-str;
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<tbody valign="top">
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<row>
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<entry>__s32</entry>
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<entry><structfield>left</structfield></entry>
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<entry>Horizontal offset of the top, left corner of the
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rectangle, in pixels.</entry>
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</row>
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<row>
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<entry>__s32</entry>
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<entry><structfield>top</structfield></entry>
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<entry>Vertical offset of the top, left corner of the
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rectangle, in pixels. Offsets increase to the right and down.</entry>
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</row>
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<row>
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<entry>__s32</entry>
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<entry><structfield>width</structfield></entry>
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<entry>Width of the rectangle, in pixels.</entry>
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</row>
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<row>
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<entry>__s32</entry>
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<entry><structfield>height</structfield></entry>
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<entry>Height of the rectangle, in pixels. Width and
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height cannot be negative, the fields are signed for hysterical
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reasons. <!-- video4linux-list@redhat.com on 22 Oct 2002 subject
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"Re:[V4L][patches!] Re:v4l2/kernel-2.5" --></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</section>
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<section>
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<title>Enabling Overlay</title>
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<para>To start or stop the frame buffer overlay applications call
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the &VIDIOC-OVERLAY; ioctl.</para>
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</section>
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<!--
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Local Variables:
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mode: sgml
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sgml-parent-document: "v4l2.sgml"
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indent-tabs-mode: nil
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End:
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-->
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