Patent ID: 7292255

Claim:
A method of creating an image using a computer comprised of a processor and a memory, the image being formed by rendering a plurality of graphical objects to be composited according to an expression tree representing a compositing expression for the image, the expression tree including a plurality of nodes each representing an object of the image or an operation for combining sub-expressions of the compositing expression, said method comprising the steps of: determining an opacity region representation for each leaf node of the nodes of the expression tree, the opacity region representation comprising one or more of three predetermined values, each predetermined value distinctly identifying whether a corresponding region of an object represented by the leaf node is an opaque region, a transparent region or a partially transparent region such that the opacity region representation simultaneously represents each opaque region, transparent region and partially transparent region of the object represented by the leaf node, wherein a union of each opacity region representation for the expression tree includes at least one of each of the three predetermined values; determining an obscurance region representation for at least one leaf node based on an analysis of the opacity region representation associated with the at least one leaf node, such that, for the image, the at least one leaf node simultaneously comprises both the opacity region representation and the obscurance region representation, with the obscurance region representation being separate from the opacity region representation of the at least one leaf node, the obscurance region representation being assigned one or more of a plurality of further predetermined values, each further predetermined value of the obscurance region representation distinctly identifying whether a corresponding region of the object represented by the at least one leaf node is hidden by another object or is visible in the image; partitioning the object into a plurality of regions; overlaying the obscurance region representation on the partitioned object such that the partitioned object is substantially encompassed within the obscurance region representation; traversing the overlaid obscurance region representation to identify any of the plurality of regions of the partitioned object which include at least a portion of the visible region; rendering the identified regions of the object that are visible in the image, based on the traversal of the overlayed obscurance region representation to create the image, wherein the opacity region representation and the obscurance region representation are associated with the same leaf node; and outputting the formed image to an image output device.