cairo:mask-surface (cr surface x y)

cr -- a cairo:context-t instance
surface -- a cairo:surface-t instance
x -- a number for the x coordinate at which to place the origin of surface
y -- a number for the 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. The numbers for the arguments are coerced to double floats before being passed to the foreign C function.