As explained by Rolf, on each triangle LDD has information about normal at each vertex. If the normals are the same on matching vertices of two triangles, then a condline should be set. If the normals are different, then it's considered a sharp edge (independantly of facet angle) and a normal edge line should be set.
Quote:Creating the lines manually would be prohibitive.Even using Edger2 automatic generation, there's still a LOT of manual work to do to sort out things between lines and condlines.