1. A method of producing BREP data from electronic polygonal data, the method comprising: accessing the polygonal data with a computer, the polygonal data defining a mesh of polygonal data points, each having a valence corresponding with the number of polygons defined thereby, wherein the data points comprise: a plurality of valence four data points having a valence of four, and a plurality of non-valence four data points having a valence other than four; selecting one or more of the non-valence four data points, wherein the selected non-valence four data points are selected at least partly on the basis of having a valence other than four; generating a plurality of continuous curves, wherein each of the continuous curves terminates on one of the selected non-valence four data points and passes through one or more particular valence four data points; based at least in part on the continuous curves, generating a plurality of surface boundaries defining one or more continuous surfaces; generating the BREP data based at least in part on the continuous surfaces; and storing the BREP data in a computer readable data storage.