PARALLELISING PER-PIXEL COMPOSITING

A method of compositing layers by grouping the layers into a foreground group and a background group; identifying independent instructions of compositing model for execution independently from the background group and dependent instructions requiring a compositing output of a background layer in order to composite foreground layers; executing the independent instructions on the foreground layers in parallel with compositing the background layers, a first independent instruction storing a corresponding result in a first buffer and a second independent instruction storing a corresponding result in a second buffer; executing a dependent instruction by updating the second buffer using the background compositing output; and determining a compositing output for the foreground group dependent upon contents of the first buffer and the updated second buffer.