1. A method of creating an image, said image being formed by rendering at least a plurality of graphical objects to be composited according to a compositing expression, each said object having a predetermined outline, said method comprising the steps of: dividing a space in which said outlines are defined into a plurality of mutually exclusive regions wherein each of said regions is defined by a region outline substantially following at least one of said predetermined outlines or parts thereof; examining each said region to determine those said objects which contribute to said region; modifying said compositing expression on the basis of the contribution of each of said objects within said region to form an optimized compositing expression for each said region; and compositing said image using each of said optimized compositing expressions.