OPTIMISING IMAGE COMPOSITING

A method and apparatus for creating an image is disclosed. The image is formed by rendering at least a plurality of graphical objects to be composited according to a compositing expression. Each object has a predetermined outline. The method comprises the following steps. Firstly, dividing a space in which the outlines are defined into a plurality of mutually exclusive regions wherein each of the regions is defined by a region outline substantially following at least one of the predetermined outlines or parts thereof. Secondly, examining each of the regions to determine those objects which contribute to the regions. Thirdly, modifying the compositing expression on the basis of the contribution of each of the objects within the region to form an optimized compositing expression for each region. Fourthly, compositing the image using each of the optimized compositing expressions.