1. A tile-based graphics processing pipeline comprising: a plurality of processing stages, including at least a rasteriser that rasterises input primitives to generate graphics fragments to be processed, each graphics fragment having one or more sampling points associated with it, and a renderer that processes fragments generated by the rasteriser to generate rendered fragment data; a tile buffer configured to store rendered fragment data locally to the graphics processing pipeline prior to that data being written out to an external memory, the tile buffer storing data values for an array of sample positions, with respective sets of the sample positions corresponding to and being associated with respective pixels of an output data array that the rendered fragment data relates to; a write out stage configured to write data stored in the tile buffer to an external memory; and a programmable processing stage operable under the control of graphics program instructions to, for respective pixel positions that data stored in the tile buffer represents, read data stored in the tile buffer for one or more sampling positions that are not associated with the pixel position in question, perform a processing operation using the read sampling position data, and write the result of the processing operation into the tile buffer or to an external memory.