The recovery of straight picture edges from digitizations of scenes containing polyhedra ("line finding") is central to the functioning of scene analysis programs. While recognizing that recovery properly involves a computational mobilization of a great deal of knowledge-supported context, there remain some basic issues of representation which govern the way in which the primary data—grey levels—are addressed. The paper describes a parametric representation of straight picture edges and its procedural deployment in the recovery of edges from digitizations of scenes whose contents are essentially polyhedra with strong visible shadows.