A graphics processing system includes a graphics processor and a memory for storing data to be used by and generated by the graphics processor. In a first rendering pass, the graphics processor generates an array of graphics data and stores the generated array of graphics data in the memory. The array of graphics data generated in the first rendering pass is used in a subsequent rendering pass. In the first rendering pass, the graphics processor determines one or more regions of the array of graphics data that have a particular characteristic, and generates information indicative of the one or more regions. In the subsequent rendering pass, the graphics processor uses the information indicative of the one or more regions to control the reading of the array of graphics data when it is to be used in the subsequent rendering pass.