A technique is provided for performing rasterisation of input primitives to generate graphics fragments to be processed to generate output data. The technique comprises determining a bounding box for an input primitive, and performing a multi-level patch analysis, each patch having an array of grid points defining boundaries of a set of sub-patches within that patch. The technique further comprises, when performing patch analysis of a selected patch, performing a bounding box evaluation step to determine if a condition exists where the bounding box does not cover any of the grid points, or if a special grid point coverage condition exists, and in the presence of the condition, adopting an alternative operation for that selected patch instead of a default operation. The alternative operation is configured to determine whether the primitive at least partially covers any of the sub-patches of the selected patch.