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X11-Protocol-Other-11
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/ X11::Protocol::Ext::DOUBLE_BUFFER

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 client 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 and has 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 and redraws which would flash if done directly to the window.

Off-screen drawing can also be done to a pixmap then copy into the window, but a back buffer might be implemented more efficiently and in particular it only keeps 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, $swap_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');

$swap_action_hint is the most likely $swap_action in later DbeSwapBuffers requests (see below). This is just a hint and doesn't restrict what can be done.

If $window is already double buffered then $buffer becomes a further reference to the existing back buffer.

If $window is destroyed (DestroyWindow) before $buffer then $buffer continues to exist and should still be deallocated (per 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 are made on a single window then the other XIDs continue to refer to the window back buffer. The underlying buffer is released when all buffer XIDs to it are deallocated.

$X->DbeSwapBuffers ($window1,$action1, $window2,$action2,...)

Swap the front and back buffers on given $windows (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 the back buffer after swapping are controlled by $action (an enum string) for each window,

 $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" is untouched in the sense that the front visible buffer has now been swapped to be the back buffer and that content left 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 just runs it sequentially as normal.

If a DbeSwapBuffers is in the idiom then it should be the first request, immediately following the Begin.

# swap and clear buffer to a GC stipple
# no guarantee any server would actually optimize this!
$X->DbeBeginIdiom;
$X->DbeSwapBuffers ($window, 'Untouched');
$X->PolyFillRectangle ($buffer, $gc, [0,0,$width,$height]);
$X->DbeEndIdiom;

There need not be a swap in an idiom, for example a CopyArea of 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 optimize any consecutive requests, but that depends on them arriving at the server together. A DbeBeginIdiom is a kind of permission to the server not to perform requests immediately, but to wait (if it wishes) and 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. If no drawables are given then return information about each screen on the server.

my @infos = $X->DbeGetVisualInfo ($drawable1, $drawable2);
# $info[0] and $info[1] are arrayrefs

Each returned value is an arrayref containing a list of visual ID and visual data pairs,

# each info arrayref 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 the visual. A higher value means higher performance, but the actual number has no meaning and in particular cannot be compared between different servers.

So for example,

my ($info_aref_1, $info_aref_2)
  = $X->DbeGetVisualInfo ($drawable1, $drawable2);

Or if enquiring about a single drawable's screen then as follows. Note it must be called with array context, the result in scalar context is unspecified as yet.

my ($info_aref) = $X->DbeGetVisualInfo ($X->root);

The visual+perf pairs are arranged 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 could be used to choose the best among them.

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 XID) which $buffer is working on. If the target window has been destroyed (DestroyWindow) then return "None".

SEE ALSO

X11::Protocol, X11::Protocol::Ext::Composite

HOME PAGE

http://user42.tuxfamily.org/x11-protocol-other/index.html

LICENSE

Copyright 2011 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/>.