1. A method for clipping modifier volumes where they intersect a clipping plane in a 3 dimensional computer graphics generating system, the method comprising the steps of: representing the volumes in the image is as a set of planar polygons; providing an indication as to which polygons form modifier volumes; determining which polygons of a modifier volume intersect the clipping plane; generating further polygons on the clipping plane within the intersections of the polygons of the modifier volume with the clipping plane; inserting these polygons into the set of planar polygons; and discarding the portions of the polygons determined to lie in front of the clipping plane.