X11::Protocol::Ext::DAMAGE(3) drawing notifications


use X11::Protocol;
my $X = X11::Protocol->new;
or print "DAMAGE extension not available";
my $damage = $X->new_rsrc;
$X->DamageCreate ($damage, $drawable, 'NonEmpty');
sub my_event_handler {
my %h = @_;
if ($h{'name'} eq 'DamageNotify') {
my $drawable = $h{'drawable'};
$X->DamageSubtract ($damage, 'None', $parts_region);
# do something for $parts_region changed in $drawable


The DAMAGE extension lets a client listen for changes to drawables (windows, pixmaps, etc) due to drawing operations, including drawing into sub-windows which appears in the parent.

This can be used for various kinds of efficient copying or replicating of window contents, such as cloning to another screen, showing a magnified view, etc. The root window can be monitored to get changes on the whole screen.

Content changes due to drawing are conceived as ``damage''. A server-side damage object accumulates areas as rectangles to make a server-side ``region'' per the XFIXES 2.0 extension (see X11::Protocol::Ext::XFIXES)

A DamageNotify event is sent from a damage object. A reporting level controls the level of detail, ranging from just one event on becoming non-empty, up to an event for every drawing operation affecting the relevant drawable.

Fetching an accumulated damage region (or part of it) is reckoned as a ``repair''. It doesn't change any drawables in any way, just fetches the region from the damage object. This fetch is atomic, so nothing is lost if the listening client is a bit lagged etc.

See examples/damage-duplicate.pl in the "X11-Protocol-Other" sources for one way to use damage to duplicate a window in real-time.


The following requests are made available with an "init_extension()", as per ``EXTENSIONS'' in X11::Protocol.

    my $is_available = $X->init_extension('DAMAGE');


"($server_major, $server_minor) = $X->DamageQueryVersion ($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 more).

The current code supports up to 1.1. If asking for higher then be careful that it's upwardly compatible. The module code negotiates a version in "init_extension()" so an explicit "DamageQueryVersion()" is normally not needed.

"$X->DamageCreate ($damage, $drawable, $level)"
Create a new damage object in $damage (a new XID) which monitors changes to $drawable. If $drawable is a window then changes to its subwindows are included too.

    # listening to every change on the whole screen
    my $damage = $X->new_rsrc;
    $X->DamageCreate ($damage, $X->root, 'RawRectangles');

$level is an enum string controlling how often "DamageNotify" events are emitted (see ``EVENTS'' below).

    RawRectangles      every change
    DeltaRectangles    when damage region expands
    BoundingBox        when damage bounding box expands
    NonEmpty           when damage first becomes non-empty
"$X->DamageDestroy ($damage)"
Destroy $damage.
"$X->DamageSubtract ($damage, $repair_region, $parts_region)"
Move the accumulated region in $damage to $parts_region (a region XID), and clear it from $damage.

If $parts_region is ``None'' then $damage is cleared and the region discarded. This can be used if for example the entire $drawable will be copied or re-examined, so the exact parts are not needed.

$repair_region is what portion of $damage to consider. ``None'' means move and clear everything in $damage. Otherwise $repair_region is a region XID and the portion of the damage region within $repair_region is moved and cleared. Anything outside is left in $damage.

If anything is left in $damage then a new "DamageNotify" event is immediately sent. This can be good for instance if you picked out a $repair_region corresponding to what you thought was the window size (perhaps from the "geometry" field of a "DamageNotify" event), but it has grown in the interim.

Region objects here can be created with the XFIXES 2.0 extension (see X11::Protocol::Ext::XFIXES). It should be available whenever DAMAGE is available. If using ``None'' and ``None'' to clear and discard then region objects are not required and there's no need for an "init_extension('XFIXES')".


"$X->DamageAdd ($drawable, $region)"
Report to any interested damage objects that changes have occurred in $region (a region XID) of $drawable.

This is used by clients which modify a drawable in ways not seen by the normal protocol drawing operations. For example an MIT-SHM shared memory pixmap modified by writing to the memory (see X11::Protocol::Ext::MIT_SHM), or the various ``direct rendering'' to graphics hardware or GL etc.


"DamageNotify" events are sent to the client which created the damage object. These events are always generated, there's nothing to select or deselect them. The event has the usual fields

    name             "DamageNotify"
    synthetic        true if from a SendEvent
    code             integer opcode
    sequence_number  integer

and event-specific fields

    damage        XID, damage object
    drawable      XID, as from DamageCreate
    level         enum, as from DamageCreate
    more          boolean, if more DamageNotify on the way
    time          integer, server timestamp
    area          arrayref [$x,$y,$width,$height]
    geometry      arrayref [$rootx,$rooty,$width,$height]

"drawable" and "level" are as from the "DamageCreate()" which made the "damage" object.

"more" is true if there's further "DamageNotify" events on the way for this damage object. This can happen when the ``level'' means there's a set of "area" rectangles to report.

"area" is a rectangle within "drawable", as a 4-element arrayref,

    [ $x, $y, $width, $height ]

What it covers depends on the reporting level requested,

  • "RawRectangles" --- a rectangle around an arc, line, etc, drawing operation which changed "drawable".
  • "DeltaRectangles" --- an additional rectangle extending the damage region. Only new rectangles are reported, not any of the existing damage region. Reporting a region addition may require multiple "DamageNotify" events.
  • "BoundingBox" --- a bounding box around the damage region accumulated, bigger than previously reported.
  • "NonEmpty" --- umm, something, maybe the entire drawable.

"geometry" is the current size and position of the drawable as a 4-element arrayref in root window coordinates. For a pixmap $root_x and $root_y are 0.

    [ $root_x, $root_y, $width, $height ]


The reporting level above is type ``DamageReportLevel''. So for example (after a successful "$X->init_extension('DAMAGE')"),

    $number = $X->num('DamageReportLevel', 'RawRectangles');
    $string = $X->interp('DamageReportLevel', 3);

See ``SYMBOLIC CONSTANTS'' in X11::Protocol.


Error type ``Damage'' is a bad $damage resource XID in a request.


The server extension version number is queried in the "init_extension()", but not yet made available as such. The version determines whether "DamageAdd()" ought to work. Currently that request is always setup, but presumably generates an Opcode error if the server doesn't have it.


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.

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