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