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f90580ca01
As discussed on the media summit 2013, there is no reason for the width and height to be signed. Therefore this patch is an attempt to convert those fields from __s32 to __u32. Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com> Acked-by: Sakari Ailus <sakari.ailus@iki.fi> (documentation and smiapp) Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com> Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com> Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
369 lines
16 KiB
XML
369 lines
16 KiB
XML
<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>__u32</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>__u32</entry>
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<entry><structfield>height</structfield></entry>
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<entry>Height of the rectangle, in pixels.</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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