An input set of indices, which contains primitive restarts, is input into splitter stage, where it is split into blocks of index positions. Each of these blocks is processed by scanner stages and scan combiner stage, to determine for each index position, the index position of the end of the sequence of primitives in the set of indices that the index position is part of. The determined index positions of the ends of the sequences of primitives in the set of indices for the index positions are then used to determine for each index position, whether a triangular primitive should be output or not, at a triangle output decision stage. A parallel scan stage then determines each triangle's position in the output set of indices, and the triangle's vertex indices are then written out at index write-out stage to an output set of indices, which contains no primitive restarts.