1. A method of generating shader program instructions to control a fragment shader within a programmable graphics processing pipeline, wherein the programmable graphics processing pipeline includes a GPU driver and a combiner instruction processing unit (IPU), the method comprising: receiving, at the GPU driver, a fixed function application programming interface (API) call to perform one or more combiner operations on pixel data; converting, at the GPU driver, the fixed function API call into a fixed function state that represents the one or more combiner operations; providing the fixed function state to the combiner instruction processing unit; translating, at the combiner instruction processing unit, the fixed function state into at least one shader program instruction for performing the one or more combiner operations, wherein the at least one shader program instruction is suitable for execution by the fragment shader within the programmable graphics processing pipeline for shading one or more pixels for display; and after translating the fixed function state into the at least one shader program instruction, providing the at least one shader program instruction to the fragment shader within the programmable graphics processing pipeline for execution.