1. An apparatus for rendering an object-based image one pixel at a time in scanline order, said apparatus comprising: an optimization module for generating a set of compositing messages corresponding to a first pixel and generating a reduced set of compositing messages from the set of compositing messages; and a pixel compositing module comprising a stack for generating a resultant color and opacity for a current pixel in accordance with the compositing messages, said pixel compositing module, in addition to compositing the first pixel using the set of compositing messages, stores a result or partial result of the compositing of the first pixel, wherein said pixel compositing module utilizes the reduced set of compositing messages and the stored result or partial result to composite at least one subsequent pixel, wherein the first pixel and the at least one subsequent pixel form part of a run of pixels along a scanline of the image between adjacent edges of objects in the object-based image, said optimization module sets first data in one or more of the compositing messages in the set to indicate that a compositing result of the one or more compositing messages is to be used in compositing the at least one subsequent pixel, and said optimization module sets the first data of a current compositing message if a predicted stack depth of the stack is of a predetermined depth and the current compositing message and all of the compositing messages having their associated first data set are associated with objects that have a constant color and opacity along the scanline.