1. A method of rendering a graphical image described by an expression tree, said expression tree comprising one or more parent nodes and one or more leaf nodes, each said parent node representing a graphical operator and having branches to respective descendent nodes, and each said leaf node representing a graphic object, said method comprising the steps of: (a) determining regions of one or more pixel locations in an output space of said image, each said region corresponding to a combination in said image of one or more said graphical objects; (b) determining, for each said region, an activity state of corresponding parent nodes using a predefined function for each corresponding said operator; (e) generating in parallel for each active one of said operators, compositing instructions and pixel generation instructions; (f) storing the compositing instructions in an operator queue and generating pixel colors using the pixel generation instructions, said pixel colors being buffered in an operand queue; and (e) compositing the generated pixel colors, for each pixel in said region, using the operators defined in the operator queue to output a composited pixel value for each pixel in said region.