1. A computer-implemented method of forming an image described by data represented as graphical objects, the computer comprising at least a processor configured for executing the method and an image store, the method comprising the steps of: (a) receiving a list of objects describing the image, in which at least one of the objects is to be composited and at least one of the objects is not to be composited, wherein the objects are ordered in priority order from a lowest-priority object to a highest-priority object; (b) considering objects in the list in order of increasing priority and, for each currently-considered object: (ba) rendering the currently-considered object to the image store using an object sequential rendering method if the currently-considered object is not to be composited, the respective objects being rendered by the object sequential rendering method to the image store in sequence independently; and (bb) if the currently-considered object is to be composited, performing the steps of: (bb)(i) determining objects in the list having lower priority than the currently-considered object and having a bounding box that overlaps a bounding box of the currently-considered object; and (bb)(ii) rendering each pixel of a portion of the currently-considered object and the determined objects in sequence of a raster scan order using a pixel sequential rendering method if the portion of an object contributes to a rendered output of a set of objects processed by the pixel sequential rendering method, wherein the steps are performed on a computer.