Specifies a dither for use within a specified body of code.
A body of Lisp code.
The return value of the last form executed in
function specifies a dither for use within
argument can be a dither mask object from make-dither or a size, in which case a dither of that size is created.
dithers do not affect drawing or the anti-aliasing that occurs when drawing in Cocoa.
CAPI Reference Manual - 15 Dec 2011