Provided by: libx11-protocol-other-perl_31-1build1_all 
NAME
X11::Protocol::Ext::DOUBLE_BUFFER - window off-screen double buffering
SYNOPSIS
use X11::Protocol;
$X = X11::Protocol->new;
$X->init_extension('DOUBLE-BUFFER')
or print "DOUBLE-BUFFER extension not available";
DESCRIPTION
The DOUBLE-BUFFER extension lets a program draw into an off-screen "back buffer" on a window and when
ready swap it to the user-visible "front". A back buffer is a drawable with the same size, depth,
visibility, etc as the window proper.
Drawing off-screen then swapping to visible is good for smooth frame by frame animations or if some
drawing is complex or poorly implemented and has clears or overwriting which would flash if done directly
to the window.
Off-screen drawing can also be implemented by a pixmap and copy into the window but the server might
implement a back buffer more efficiently. In particular the back buffer only needs to be visible
portions of a window so memory is not used for overlapped areas.
The server might support double buffering only on certain visuals. DbeGetVisualInfo() lists those which
are supported, or just try to create a back buffer for a window and watch for an error reply.
See examples/dbe-swap.pl for a simple program drawing with double buffering.
REQUESTS
The following requests are made available with an init_extension() per "EXTENSIONS" in X11::Protocol.
my $bool = $X->init_extension('DOUBLE-BUFFER');
"($server_major, $server_minor) = $X->DbeGetVersion ($client_major, $client_minor)"
Negotiate a protocol version with the server. $client_major and $client_minor is what the client
would like, the returned $server_major and $server_minor is what the server will do, which might be
less than requested (but not higher).
The code here supports 1.0 and automatically negotiates within init_extension() so direct use of
DbeGetVersion() is not necessary.
"$X->DbeAllocateBackBufferName ($window, $buffer, $action_hint)"
Create $buffer (a new XID) as the back buffer on $window. $buffer is a drawable and can be used with
all usual drawing operations.
my $buffer = $X->new_rsrc;
$X->DbeAllocateBackBufferName ($window, $buffer, 'Copied');
$action_hint is the most likely $action in later DbeSwapBuffers() requests (see below). But this is
just a hint and doesn't restrict what can be done.
If $window is already double buffered then $buffer becomes another reference to that back buffer.
If $window is destroyed (DestroyWindow()) then $buffer continues to exist and should still be
deallocated (below), but attempting to draw into it gives a "Resource" error reply.
"$X->DbeDellocateBackBufferName ($buffer)"
Deallocate $buffer and release that XID.
If multiple DbeAllocateBackBufferName() requests have been made on a window then all the other XIDs
continue to refer to the window back buffer. The underlying buffer remains until all buffer XIDs for
it are deallocated.
"$X->DbeSwapBuffers ($window1,$action1, $window2,$action2,...)"
Swap the front and back buffers on each given $window (XIDs). The back buffer becomes visible and
what was the front becomes the back.
$X->DbeSwapBuffers ($window1, 'Background',
$window2, 'Untouched');
Only the content is swapped, the XIDs are unchanged, so $window is still the visible window front and
any $buffer XIDs to it are still the back.
The contents of each back buffer after swapping are controlled by the corresponding $action for each
window (string type "DbeSwapAction"),
$action new back buffer contents
--------- --------------------------
"Undefined" undefined contents
"Background" cleared to the window background
"Untouched" left at current content (previous visible)
"Copied" content of the old back buffer (unchanged)
"Untouched" means the contents of the front buffer are swapped to the back buffer unchanged.
"Copied" is as if the back buffer content is copied to the front, making both now the same.
"$X->DbeBeginIdiom ()"
"$X->DbeEndIdiom ()"
Hint to the server that a sequence of swap and/or drawing operations between Begin and End might be
done as an atomic combination for higher performance. If the server doesn't recognise the sequence
then it runs it sequentially as normal.
If a DbeSwapBuffers() is in the idiom then it should be the first request, immediately following the
Begin.
# swap then clear back buffer to a GC stipple
# no guarantee any server would actually optimize this!
$X->DbeBeginIdiom;
$X->DbeSwapBuffers ($window, 'Undefined');
$X->PolyFillRectangle ($buffer, $gc, [0,0,$width,$height]);
$X->DbeEndIdiom;
There doesn't need to be a swap in an idiom. For example a CopyArea() of some parts of the back
buffer to the window might be in a Begin/End and might perhaps be optimized by the server.
$X->DbeBeginIdiom;
$X->CopyArea ($buffer, $window, # from buffer to window
$gc, $x,$y,$width,$height, $dstx,$dsty);
# more stuff ...
$X->DbeEndIdiom;
The idea of idiom groupings is to have a flexible way to express combination operations, including
things not yet imagined, rather than adding specific requests to the protocol. In principle the
server can always optimize consecutive requests but that depends on them arriving at the server
together. A DbeBeginIdiom() is like permission to the server to defer performing the requests and
wait, if it wishes, to see if what follows can be combined.
"@infos = $X->DbeGetVisualInfo ($drawable1, $drawable2, ...)"
"@infos = $X->DbeGetVisualInfo ()"
For each $drawable, return a list of the visual IDs on that screen which support double-buffering.
my ($info_aref_drawable1, $info_aref_drawable2)
= $X->DbeGetVisualInfo ($drawable1, $drawable2);
If no drawables are given then return information about each screen on the server.
my @list_of_info_aref = $X->DbeGetVisualInfo ();
Each returned value is an arrayref. Each arrayref contains a list of visual ID and visual data
pairs,
# each $info_aref is
[ $visual_id1, [ $depth, $perflevel ],
$visual_id2, [ $depth, $perflevel ],
...
]
$depth is the visual's depth the same as in the server info
"$X->{'visuals'}->{$visual_id}->{'depth'}".
$perflevel is an integer indicating how good the performance of double buffering is on this visual.
A higher value means higher performance, but the actual number has no meaning and in particular
cannot be compared between different servers.
If enquiring about a single drawable's screen then use a list context like the following. The result
in scalar context is unspecified as yet.
my ($info_aref) = $X->DbeGetVisualInfo ($X->root);
The visual+perf are pairs so they can be put into a hash to check support for double buffering on a
given visual,
my %hash = @$info_aref; # pairs $visualid => [$d,$p]
if ($hash{$my_visual_id}) {
print "double buffering is available on my_visual_id\n";
}
If you've got a choice of equally suitable visuals for application display then the performance level
might be compared to choose the best.
"List::Pairwise" has some grep and map functions for pair lists like the $info_aref.
See examples/dbe-info.pl for a simple program printing this info.
"$window = $X->DbeGetBackBufferAttributes ($buffer)"
Return the window (an integer XID) which $buffer is for. If its target window has been destroyed
(DestroyWindow()) then the return is "None".
ENUM TYPES
The following types are available for "$X->interp()" and "$X->num()", after init_extension().
DbeSwapAction
"Undefined" 0
"Background" 1
"Untouched" 2
"Copied" 3
For example,
my $num = $X->num("DbeSwapAction", "Background");
# sets $num to 2
BUGS
In some XFree86 3.x servers there was a bug in DbeGetVisualInfo() where the reply length was
miscalculated, being bytes instead of CARD32s, resulting in a length value bigger than the actual data
sent. The symptom is the client hangs waiting for data the length says should follow but which never
does.
This affects all client code, including the Xlib XdbeGetVisualInfo() as used for instance by the
"xdpyinfo" program.
Is there a good way to notice the problem? Probably not beyond looking at the server name and version
and either forbidding the request or doing something nasty to the way handle_input() reads as a
workaround.
SEE ALSO
X11::Protocol, X11::Protocol::Ext::Composite
/usr/share/doc/x11proto-xext-dev/dbe.txt.gz
HOME PAGE
<http://user42.tuxfamily.org/x11-protocol-other/index.html>
LICENSE
Copyright 2011, 2012, 2013, 2014, 2017 Kevin Ryde
X11-Protocol-Other is free software; you can redistribute it and/or modify it under the terms of the GNU
General Public License as published by the Free Software Foundation; either version 3, or (at your
option) any later version.
X11-Protocol-Other is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
Public License for more details.
You should have received a copy of the GNU General Public License along with X11-Protocol-Other. If not,
see <http://www.gnu.org/licenses/>.
perl v5.40.1 2025-11-24 X11::Protocol::Ext::DOUBLE_BUFFER(3pm)