Apparatus 20 is disclosed for rendering an object-based image one
pixel at a time in scanline order. The apparatus 20 comprises an optimisation
module 550 for generating a set of compositing messages during a first pixel
in a run of pixels between adjacent edges of one or two objects, and for generating
a reduced set of compositing messages from the set during subsequent pixels in
the run of pixels. The apparatus 20 further comprises a pixel compositing
module 700 which in addition to compositing the first pixel in the run of
pixels also stores a result or partial result of the compositing of the first pixel.
The pixel compositing module 700 utilises the reduced set of compositing
messages and the stored result or partial result to composite pixels in the run
subsequent to the first pixel.