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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>Video Overlay Interface</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="LINUX MEDIA INFRASTRUCTURE API" /><link rel="up" href="ch04.html" title="Chapter 4. Interfaces" /><link rel="prev" href="ch04.html" title="Chapter 4. Interfaces" /><link rel="next" href="ch04s03.html" title="Video Output Interface" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Video Overlay Interface</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch04.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Interfaces</th><td width="20%" align="right"> <a accesskey="n" href="ch04s03.html">Next</a></td></tr></table><hr /></div><div class="section" title="Video Overlay Interface"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="overlay"></a>Video Overlay Interface</h2></div><div><h3 class="subtitle">Also known as Framebuffer Overlay or Previewing</h3></div></div></div><div class="toc"><dl><dt><span class="section"><a href="ch04s02.html#id2651328">Querying Capabilities</a></span></dt><dt><span class="section"><a href="ch04s02.html#id2651364">Supplemental Functions</a></span></dt><dt><span class="section"><a href="ch04s02.html#id2652514">Setup</a></span></dt><dt><span class="section"><a href="ch04s02.html#id2652661">Overlay Window</a></span></dt><dt><span class="section"><a href="ch04s02.html#id2653512">Enabling Overlay</a></span></dt></dl></div><p>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.</p><p>Video overlay devices are accessed through the same character
special files as <a class="link" href="ch04.html#capture" title="Video Capture Interface">video capture</a> devices.
Note the default function of a <code class="filename">/dev/video</code> device
is video capturing. The overlay function is only available after
calling the <a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_S_FMT</code></a> ioctl.</p><p>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.</p><p>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.<sup>[<a id="id2651289" href="#ftn.id2651289" class="footnote">20</a>]</sup></p><div class="section" title="Querying Capabilities"><div class="titlepage"><div><div><h3 class="title"><a id="id2651328"></a>Querying Capabilities</h3></div></div></div><p>Devices supporting the video overlay interface set the
<code class="constant">V4L2_CAP_VIDEO_OVERLAY</code> flag in the
<em class="structfield"><code>capabilities</code></em> field of struct <a class="link" href="re56.html#v4l2-capability" title="Table A.66. struct v4l2_capability">v4l2_capability</a>
returned by the <a class="link" href="re56.html" title="ioctl VIDIOC_QUERYCAP"><code class="constant">VIDIOC_QUERYCAP</code></a> ioctl. The overlay I/O method specified
below must be supported. Tuners and audio inputs are optional.</p></div><div class="section" title="Supplemental Functions"><div class="titlepage"><div><div><h3 class="title"><a id="id2651364"></a>Supplemental Functions</h3></div></div></div><p>Video overlay devices shall support <a class="link" href="ch01s05.html" title="Audio Inputs and Outputs">audio input</a>, <a class="link" href="ch01s06.html" title="Tuners and Modulators">tuner</a>, <a class="link" href="ch01s08.html" title="User Controls">controls</a>,
<a class="link" href="ch01s11.html" title="Image Cropping, Insertion and Scaling">cropping and scaling</a> and <a class="link" href="ch01s12.html" title="Streaming Parameters">streaming parameter</a> ioctls as needed.
The <a class="link" href="ch01s04.html" title="Video Inputs and Outputs">video input</a> and <a class="link" href="ch01s07.html" title="Video Standards">video standard</a> ioctls must be supported by
all video overlay devices.</p></div><div class="section" title="Setup"><div class="titlepage"><div><div><h3 class="title"><a id="id2652514"></a>Setup</h3></div></div></div><p>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
<a class="link" href="re40.html" title="ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF"><code class="constant">VIDIOC_G_FBUF</code></a> and <a class="link" href="re40.html" title="ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF"><code class="constant">VIDIOC_S_FBUF</code></a> ioctls are available to get
and set these parameters, respectively. The
<code class="constant">VIDIOC_S_FBUF</code> 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.</p><p>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 <code class="constant">VIDIOC_S_FBUF</code> ioctl
is not privileged. An application can check for this type of device by
calling the <code class="constant">VIDIOC_G_FBUF</code> ioctl.</p><p>A driver may support any (or none) of five clipping/blending
methods:</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>Chroma-keying displays the overlaid image only where
pixels in the primary graphics surface assume a certain color.</p></li><li class="listitem"><p>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.</p></li><li class="listitem"><p>A list of clipping rectangles can be specified. In
these regions <span class="emphasis"><em>no</em></span> video is displayed, so the
graphics surface can be seen here.</p></li><li class="listitem"><p>The framebuffer has an alpha channel that can be used
to clip or blend the framebuffer with the video.</p></li><li class="listitem"><p>A global alpha value can be specified to blend the
framebuffer contents with video images.</p></li></ol></div><p>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
(<a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_S_FMT</code></a>) or overlay (<a class="link" href="re40.html" title="ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF"><code class="constant">VIDIOC_S_FBUF</code></a>) may fail with an
<span class="errorcode">EBUSY</span> error code or return accordingly modified parameters..</p></div><div class="section" title="Overlay Window"><div class="titlepage"><div><div><h3 class="title"><a id="id2652661"></a>Overlay Window</h3></div></div></div><p>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 <a class="xref" href="ch01s11.html" title="Image Cropping, Insertion and Scaling">the section called “Image Cropping, Insertion and Scaling”</a>.</p><p>The overlay window is described by a struct <a class="link" href="ch04s02.html#v4l2-window" title="Table 4.1. struct v4l2_window">v4l2_window</a>. 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 <em class="structfield"><code>type</code></em> field of a
struct <a class="link" href="re41.html#v4l2-format" title="Table A.46. struct v4l2_format">v4l2_format</a> to <code class="constant">V4L2_BUF_TYPE_VIDEO_OVERLAY</code> and
call the <a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_G_FMT</code></a> ioctl. The driver fills the
<span class="structname">v4l2_window</span> substructure named
<em class="structfield"><code>win</code></em>. It is not possible to retrieve a
previously programmed clipping list or bitmap.</p><p>To program the overlay window applications set the
<em class="structfield"><code>type</code></em> field of a struct <a class="link" href="re41.html#v4l2-format" title="Table A.46. struct v4l2_format">v4l2_format</a> to
<code class="constant">V4L2_BUF_TYPE_VIDEO_OVERLAY</code>, initialize the
<em class="structfield"><code>win</code></em> substructure and call the
<a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_S_FMT</code></a> ioctl. The driver adjusts the parameters against
hardware limits and returns the actual parameters as
<code class="constant">VIDIOC_G_FMT</code> does. Like
<code class="constant">VIDIOC_S_FMT</code>, the <a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_TRY_FMT</code></a> ioctl can be
used to learn about driver capabilities without actually changing
driver state. Unlike <code class="constant">VIDIOC_S_FMT</code> this also works
after the overlay has been enabled.</p><p>The scaling factor of the overlaid image is implied by the
width and height given in struct <a class="link" href="ch04s02.html#v4l2-window" title="Table 4.1. struct v4l2_window">v4l2_window</a> and the size of the cropping
rectangle. For more information see <a class="xref" href="ch01s11.html" title="Image Cropping, Insertion and Scaling">the section called “Image Cropping, Insertion and Scaling”</a>.</p><p>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 (<a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_S_FMT</code></a>) may fail with an <span class="errorcode">EBUSY</span> error code or return accordingly
modified parameters.</p><div class="table"><a id="v4l2-window"></a><p class="title"><b>Table 4.1. struct <span class="structname">v4l2_window</span></b></p><div class="table-contents"><table summary="struct v4l2_window" width="100%" border="0"><colgroup><col /><col /><col /></colgroup><tbody valign="top"><tr><td valign="top">struct <a class="link" href="ch04s02.html#v4l2-rect" title="Table 4.3. struct v4l2_rect">v4l2_rect</a></td><td valign="top"><em class="structfield"><code>w</code></em></td><td valign="top">Size and position of the window relative to the
top, left corner of the frame buffer defined with <a class="link" href="re40.html" title="ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF"><code class="constant">VIDIOC_S_FBUF</code></a>. The
window can extend the frame buffer width and height, the
<em class="structfield"><code>x</code></em> and <em class="structfield"><code>y</code></em>
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.</td></tr><tr><td valign="top">enum <a class="link" href="ch03s06.html#v4l2-field" title="Table 3.8. enum v4l2_field">v4l2_field</a></td><td valign="top"><em class="structfield"><code>field</code></em></td><td valign="top">Applications set this field to determine which
video field shall be overlaid, typically one of
<code class="constant">V4L2_FIELD_ANY</code> (0),
<code class="constant">V4L2_FIELD_TOP</code>,
<code class="constant">V4L2_FIELD_BOTTOM</code> or
<code class="constant">V4L2_FIELD_INTERLACED</code>. Drivers may have to choose
a different field order and return the actual setting here.</td></tr><tr><td valign="top">__u32</td><td valign="top"><em class="structfield"><code>chromakey</code></em></td><td valign="top">When chroma-keying has been negotiated with
<a class="link" href="re40.html" title="ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF"><code class="constant">VIDIOC_S_FBUF</code></a> 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 (struct <a class="link" href="re40.html#v4l2-framebuffer" title="Table A.43. struct v4l2_framebuffer">v4l2_framebuffer</a>
<em class="structfield"><code>fmt.pixelformat</code></em> field), with bytes in host
order. E. g. for <a class="link" href="re01.html#V4L2-PIX-FMT-BGR32"><code class="constant">V4L2_PIX_FMT_BGR24</code></a>
the value should be 0xRRGGBB on a little endian, 0xBBGGRR on a big
endian host.</td></tr><tr><td valign="top">struct <a class="link" href="ch04s02.html#v4l2-clip" title="Table 4.2. struct v4l2_clip">v4l2_clip</a> *</td><td valign="top"><em class="structfield"><code>clips</code></em></td><td valign="top">When chroma-keying has <span class="emphasis"><em>not</em></span>
been negotiated and <a class="link" href="re40.html" title="ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF"><code class="constant">VIDIOC_G_FBUF</code></a> indicated this capability,
applications can set this field to point to an array of
clipping rectangles.</td></tr><tr><td valign="top"> </td><td valign="top"> </td><td valign="top">Like the window coordinates
<em class="structfield"><code>w</code></em>, 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 <a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_S_FMT</code></a>
are undefined.</td></tr><tr><td valign="top">__u32</td><td valign="top"><em class="structfield"><code>clipcount</code></em></td><td valign="top">When the application set the
<em class="structfield"><code>clips</code></em> 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
<code class="constant">VIDIOC_S_FMT</code> are undefined. When clip lists are
supported but no clipping is desired this field must be set to
zero.</td></tr><tr><td valign="top">void *</td><td valign="top"><em class="structfield"><code>bitmap</code></em></td><td valign="top">When chroma-keying has
<span class="emphasis"><em>not</em></span> been negotiated and <a class="link" href="re40.html" title="ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF"><code class="constant">VIDIOC_G_FBUF</code></a> indicated
this capability, applications can set this field to point to a
clipping bit mask.</td></tr><tr><td colspan="3" valign="top"><p>It must be of the same size
as the window, <em class="structfield"><code>w.width</code></em> and
<em class="structfield"><code>w.height</code></em>. Each bit corresponds to a pixel
in the overlaid image, which is displayed only when the bit is
<span class="emphasis"><em>set</em></span>. Pixel coordinates translate to bits like:
</p><pre class="programlisting">
((__u8 *) <em class="structfield"><code>bitmap</code></em>)[<em class="structfield"><code>w.width</code></em> * y + x / 8] & (1 << (x & 7))</pre><p>where <em class="structfield"><code>0</code></em> ≤ x <
<em class="structfield"><code>w.width</code></em> and <em class="structfield"><code>0</code></em> ≤
y <<em class="structfield"><code>w.height</code></em>.<sup>[<a id="id2653161" href="#ftn.id2653161" class="footnote">a</a>]</sup></p><p>When a clipping
bit mask is not supported the driver ignores this field, its contents
after calling <a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_S_FMT</code></a> are undefined. When a bit mask is supported
but no clipping is desired this field must be set to
<code class="constant">NULL</code>.</p><p>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.<sup>[<a id="id2653198" href="#ftn.id2653198" class="footnote">b</a>]</sup></p></td></tr><tr><td valign="top">__u8</td><td valign="top"><em class="structfield"><code>global_alpha</code></em></td><td valign="top">The global alpha value used to blend the
framebuffer with video images, if global alpha blending has been
negotiated (<code class="constant">V4L2_FBUF_FLAG_GLOBAL_ALPHA</code>, see
<a class="link" href="re40.html" title="ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF"><code class="constant">VIDIOC_S_FBUF</code></a>, <a class="xref" href="re40.html#framebuffer-flags" title="Table A.45. Frame Buffer Flags">Table A.45, “Frame Buffer Flags”</a>).</td></tr><tr><td valign="top"> </td><td valign="top"> </td><td valign="top">Note this field was added in Linux 2.6.23, extending the structure. However
the <a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT">VIDIOC_G/S/TRY_FMT</a> ioctls,
which take a pointer to a <a class="link" href="re41.html#v4l2-format" title="Table A.46. struct v4l2_format">v4l2_format</a> parent structure with padding
bytes at the end, are not affected.</td></tr></tbody><tbody class="footnotes"><tr><td colspan="3"><div class="footnote"><p><sup>[<a id="ftn.id2653161" href="#id2653161" class="para">a</a>] </sup>Should we require
<em class="structfield"><code>w.width</code></em> to be a multiple of
eight?</p></div><div class="footnote"><p><sup>[<a id="ftn.id2653198" href="#id2653198" class="para">b</a>] </sup>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.</p></div></td></tr></tbody></table></div></div><br class="table-break" /><div class="table"><a id="v4l2-clip"></a><p class="title"><b>Table 4.2. struct <span class="structname">v4l2_clip</span><sup>[<a id="id2653292" href="#ftn.id2653292" class="footnote">21</a>]</sup></b></p><div class="table-contents"><table summary="struct v4l2_clipThe 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." width="100%" border="0"><colgroup><col /><col /><col /></colgroup><tbody valign="top"><tr><td valign="top">struct <a class="link" href="ch04s02.html#v4l2-rect" title="Table 4.3. struct v4l2_rect">v4l2_rect</a></td><td valign="top"><em class="structfield"><code>c</code></em></td><td valign="top">Coordinates of the clipping rectangle, relative to
the top, left corner of the frame buffer. Only window pixels
<span class="emphasis"><em>outside</em></span> all clipping rectangles are
displayed.</td></tr><tr><td valign="top">struct <a class="link" href="ch04s02.html#v4l2-clip" title="Table 4.2. struct v4l2_clip">v4l2_clip</a> *</td><td valign="top"><em class="structfield"><code>next</code></em></td><td valign="top">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.</td></tr></tbody><tbody class="footnotes"><tr><td colspan="3"><div class="footnote"><p><sup>[<a id="ftn.id2653292" href="#id2653292" class="para">21</a>] </sup>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.</p></div></td></tr></tbody></table></div></div><br class="table-break" /><div class="table"><a id="v4l2-rect"></a><p class="title"><b>Table 4.3. struct <span class="structname">v4l2_rect</span></b></p><div class="table-contents"><table summary="struct v4l2_rect" width="100%" border="0"><colgroup><col /><col /><col /></colgroup><tbody valign="top"><tr><td valign="top">__s32</td><td valign="top"><em class="structfield"><code>left</code></em></td><td valign="top">Horizontal offset of the top, left corner of the
rectangle, in pixels.</td></tr><tr><td valign="top">__s32</td><td valign="top"><em class="structfield"><code>top</code></em></td><td valign="top">Vertical offset of the top, left corner of the
rectangle, in pixels. Offsets increase to the right and down.</td></tr><tr><td valign="top">__s32</td><td valign="top"><em class="structfield"><code>width</code></em></td><td valign="top">Width of the rectangle, in pixels.</td></tr><tr><td valign="top">__s32</td><td valign="top"><em class="structfield"><code>height</code></em></td><td valign="top">Height of the rectangle, in pixels. Width and
height cannot be negative, the fields are signed for hysterical
reasons. </td></tr></tbody></table></div></div><br class="table-break" /></div><div class="section" title="Enabling Overlay"><div class="titlepage"><div><div><h3 class="title"><a id="id2653512"></a>Enabling Overlay</h3></div></div></div><p>To start or stop the frame buffer overlay applications call
the <a class="link" href="re53.html" title="ioctl VIDIOC_OVERLAY"><code class="constant">VIDIOC_OVERLAY</code></a> ioctl.</p></div><div class="footnotes"><br /><hr width="100" align="left" /><div class="footnote"><p><sup>[<a id="ftn.id2651289" href="#id2651289" class="para">20</a>] </sup>A common application of two file descriptors is the
XFree86 <a class="link" href="ch07s03.html#xvideo" title="X Video Extension">Xv/V4L</a> interface driver and
a V4L2 application. While the X server controls video overlay, the
application can take advantage of memory mapping and DMA.</p><p>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
<span class="emphasis"><em>must</em></span> support two file descriptors and
<span class="emphasis"><em>may</em></span> support single fd operation.</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch04.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch04.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch04s03.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 4. Interfaces </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Video Output Interface</td></tr></table></div></body></html>
