A method of creating an image is disclosed. The image is formed by rendering at least a plurality of graphical objects to be composited according to a first hierarchical structure (eg. 4500) representing a compositing expression for the image. The first hierarchical structure (4500) includes a plurality of nodes each representing at least a component of the image or an operation for combining sub-expressions of the compositing expression. The method stores a second hierarchical structure (300) for at least one node of the first hierarchical structure (4500). The second hierarchical structure (300) indicates at least an unobscured region of an object associated with the node. A space in which the object is defined is partitioned into a plurality of regions. The second hierarchical structure (300) is then overlayed on the object such that the object is substantially encompassed within the second hierarchical structure (300) and the overlayed second hierarchical structure (300) is traversed to identify any of the plurality of regions which include at least a portion of the unobscured region. Finally, the image is created by rendering the identified regions.