The new graphics state parameter compositing-mode controls the way that new drawings are combined with the existing value in the target of the drawing to generate the result.
Two values for compositing-mode are supported on Microsoft Windows, Cocoa and GTK+.
:overmeans draw over the existing values, blending alpha values if they exist.
:copymeans that the source is written to the destination ignoring the existing values. If the source has alpha and the target does not, that has the effect of converting semi-transparent source to solid.
:copyis especially useful for creating transparent and semi-transparent pixmap ports, which can be displayed directly or converted to images by
A number of other compositing-mode values are supported on later versions of Cocoa and GTK+.
is only supported in with
For further details and examples see the manual page for
LispWorks Release Notes and Installation Guide - 23 Dec 2011