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