In a data processing system, an output surface, such as frame to be displayed, is generated as a plurality of respective regions with each respective region of the output surface being generated from a respective region or regions of one or more input surfaces. When a new version of the output surface is to be generated 80, for each region of the output surface it is determined which region or regions of the input surface or surfaces contribute to the region of the output surface 84 and then checked whether the contributing region or regions of the input surface or surfaces have changed since the previous version of the output surface region was generated 85. If there has been a change in the contributing region or regions of the input surface or surfaces since the previous version of the region in the output surface was generated 86, the region of the output surface is regenerated 87.