1. An apparatus for rendering an object-based image one pixel at a time in scanline order, the apparatus comprising: an optimisation module for minimising a number of compositing messages for compositing the object-based image, the optimisation module comprising: a first unit for setting, for a first pixel in a run of pixels between adjacent edges of objects, first data in one or more compositing messages of a sequence of compositing messages to indicate a compositing result of the one or more compositing messages is to be stored for use in compositing of each pixel subsequent to the first pixel in the run of pixels; and a second unit for generating, for each pixel subsequent to the first pixel in the run of pixels, a reduced set of compositing messages from the sequence of compositing messages, the second unit comprising: a sub-unit for generating, for each pixel subsequent to the first pixel in the run of pixels, the reduced set of compositing messages; and a sub-unit for setting, for each pixel subsequent to the first pixel in the run of pixels, second data in one or more compositing messages of the reduced set to indicate the stored compositing result is to be retrieved for use in compositing of each pixel subsequent to the first pixel in the run of pixels; and a compositing module for generating a resultant color and opacity for a currently scanned pixel in accordance with the compositing messages, wherein the compositing module generates the color and opacity of the first pixel in the run of pixels in accordance with the sequence of compositing operations, and the compositing module generates the color and opacity of each pixel in the run subsequent to the first pixel in accordance with the reduced set and the retrieved compositing result.