path: root/Documentation/DocBook/media/v4l/dev-overlay.xml

  <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>

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