When performing conservative rasterisation in a graphics processing pipeline, modified edge information that accounts for an error in the dimensions of a primitive is determined by a primitive set-up stage. That modified edge information is then used by a rasterisation stage to determine whether the primitive covers one or more sampling points associated with pixels to be displayed. The same modified edge information can also be used to determine if the pixels are fully covered by the primitive irrespective of any rounding effects (errors) in the position of the (vertices of the) primitive.