1. An apparatus for rendering a two dimensional polygon image from volume data defining a volume and plane data defining a two dimensional plane which intersects the volume, the apparatus comprising a graphics processing unit (GPU), wherein: (a) the apparatus is operable to generate triangle data comprising an indirect representation of each triangle of a set of candidate triangles, wherein for a given plane/volume interaction a subset of the candidate triangles are valid triangles which are within the volume and on the given plane and the remaining triangles are invalid triangles; (b) the GPU is operable to process the indirect representation of each triangle of the set of candidate triangles to output for a valid triangle for the defined plane a direct representation of the valid triangle and to output for an invalid triangle for the defined plane a direct representation of a degenerate triangle; (c) the GPU is operable to reject each direct representation of a degenerate triangle from further processing; and (d) the GPU is operable to process each direct representation of a valid triangle to form one or more triangle images which form the two dimensional polygon image.