1. A method of assigning primitives to tiles in a graphics processing system which has a rendering space subdivided into a plurality of tiles, wherein each tile comprises one or more polygonal regions, the method comprising: in said graphics processing system: including, in a display list for a tile, indications of primitives which are present within a polygonal region of the tile, to thereby assign the primitives to the tile, receiving a set of one or more primitives, for each of the one or more polygonal regions of a tile: (i) determining whether the set of one or more primitives entirely covers the polygonal region of the tile, and (ii) if it is determined that the set of one or more primitives entirely covers the polygonal region of the tile, comparing a depth threshold for the polygonal region of the tile with a depth value for at least one of the received primitives of the set which is present within the polygonal region of the tile, to thereby determine whether the set of one or more primitives entirely obscures previous primitives in the region which are included in the display list for the tile; and if it is determined that the set of one or more primitives entirely obscures previous primitives which are included in the display list in all of the one or more regions of the tile, setting a display list start pointer to indicate that an indication of one of the primitives of the set is a starting point in the display list for the tile.