1. A method in a tile based graphics system having a rendering space subdivided into a plurality of tiles, comprising: reading a patch of surface data; tessellating the patch of surface data; determining whether at least a portion of the patch is hidden in a tile, prior to rasterization of the patch; culling the patch from the tile if said portion of the patch is determined to be hidden in the tile; and rendering said portion if it is determined not to be hidden in the tile.