1. A method of performing a plurality of shading operations for a graphic object in a scene displayed on a device comprising a plurality of processing units, the method comprising: receiving a set of criteria for defining a set of parameters relating to the shading operations; determining an allocation of the shading operations to the processing units based on the received criteria; allocating the shading operations to the processing units based on said determined allocations; and rendering the graphic object based on a plurality of instructions that comprise the plurality of shading operations.