mirror of
https://github.com/torvalds/linux.git
synced 2024-12-27 05:11:48 +00:00
8e080c2e6c
The V4L and DVB API's are there for a long time. however, up to now, no efforts were done to merge them to kernel DocBook. This patch adds the current versions of the specs as an unique compendium. Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
380 lines
16 KiB
XML
380 lines
16 KiB
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. 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] & (1 << (x & 7))</programlisting></para><para>where <structfield>0</structfield> ≤ x <
|
|
<structfield>w.width</structfield> and <structfield>0</structfield> ≤
|
|
y <<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>
|
|
|
|
<!--
|
|
Local Variables:
|
|
mode: sgml
|
|
sgml-parent-document: "v4l2.sgml"
|
|
indent-tabs-mode: nil
|
|
End:
|
|
-->
|