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 in the X11-Protocol-Other sources 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 $info_aref.
See examples/dbe-info.pl in the X11-Protocol-Other sources for a
simple program printing this info.
- "$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 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/>.