1. Apparatus for processing image data, comprising image data storage means, memory means for storing instructions and a data structure including a plurality of image data processing nodes representing processing to be performed upon said image data, processing means for processing said instructions, wherein said instructions define operations to be performed in order to process said image data according to said data structure and are processed by said processing means to perform the steps of generating at least one of said data processing nodes as a three-dimensional object within a compositing space having at least one light source; defining a viewport in said space configured with a frustrum enclosing said object; generating a matte of said object in relation to said light source within said space; accumulating said matte in an accumulating shadow texture; and rendering said object including said accumulating shadow texture in an image frame defined by said frustrum.