When the port's
drawing-mode
is
:quality
the graphics state parameter
compositing-mode
determines how the colors are combined, and
operation
is ignored.
compositing-mode
:over
means draw over the existing values, blending alpha values if they exist.
compositing-mode
:copy
means 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.
:copy
is especially useful for creating transparent and semi-transparent pixmap ports, which can be displayed directly or converted to images by
make-image-from-port
.
Further compositing-mode values are supported on later versions of Cocoa and GTK+.
CAPI User Guide (Unix version) - 30 Aug 2011