cairo-mask-surface (cr surface x y)

cr -- a cairo-t context
surface -- a cairo-surface-t instance
x -- a double float x coordinate at which to place the origin of surface
y -- a double float y coordinate at which to place the origin of surface

A drawing operator that paints the current source using the alpha channel of surface as a mask. Opaque areas of surface are painted with the source, transparent areas are not painted.