1. A method for simplifying composition of an image of a scene with a computer illustration system, comprising the steps of: defining the scene with shapes that are layered in a list in a memory; specifying one of the shapes in the list to be a lens with a filtering function; the filtering function of the lens having a scope of operation that includes those shapes in the list of shapes layered below the lens; limiting the scope of operation of the lens to exclude at least one of the shapes layered below the lens from being modified by the filtering function of the lens by defining a cluster of shapes that includes the lens in the list of shapes and selected shapes in the list of shapes layered below the lens; and composing the image of the scene with the limited scope of operation of the lens defined by the cluster of shapes; wherein said composing step further comprises: computing an under-list of shapes for the lens: the under-list of shapes identifying shapes in the list of shapes to which the filtering function of the lens may apply; freezing the lens by replacing any shape specified as a lens in the under-list with shapes that are formed independently of other shapes: and applying the filtering function of the lens to each shape in the under-list of shapes.