EWMH proposal for _NET_WM_WINDOW_OPACITY

[Keith Packard]

Lars Knoll has asked that a feature from Qt available on Windows and OSX 
be available under X.  In those systems, applications may set an overall 
opacity value for their top level windows which the system uses to 
modulate the window content as it is composited to the desktop.  Please 
find a proposal for a simple addition to the wm-spec.xml file.  I've 
implemented this in xcompmgr and transset, but without window manager 
cooperation, applications won't be able to use this capability as it 
requires reflecting any application window property value up to the frame 
where the composting manager can see it.

-keith

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    <sect2><title>_NET_WM_WINDOW_OPACITY</title>
    <programlisting><![CDATA[
_NET_WM_WINDOW_OPACITY, CARDINAL/32
]]></programlisting>
        <para>
This property indicates the desired opacity for this window with 0 being
transparent and 0xffffffff being opaque.  Window managers acting as
compositing managers MAY take this into account when displaying a window.
Window managers MUST forward the value of this property to any enclosing
frame window.  This property MAY change while the window is mapped and the
window manager MUST respect changes while the window is mapped.
        </para>
        <para>
This opacity value SHOULD be multiplied with the original window alpha value
(which is 1 for visuals not including an alpha component) so that the window
content is modulated by the opacity value.
        </para>
        <para>
Rationale:  This property allows applications to provide values for window
opacity in place of any inferred value.
        </para>
    </sect2>