Interpreting line drawings of objects with k-vertices

As part of the goal of automatic creation of B-rep models of engineering objects from freehand sketches, we seek to take a single line drawing (with hidden lines removed), and from it deduce an initial 3D geometric realisation of the visible part of the drawn object. Junction and line labels, and provisional depth coordinates, are key parts of this frontal geometry. Many methods for producing frontal geometry only work correctly for drawings of trihedral objects. However, nontrihedral k-vertices commonly occur in engineering objects. We analyse the performance of a line-labelling method applied to k-vertices, and show why methods ignoring geometric considerations are inadequate. We give a new approach which produces both junction labels and provisional depth coordinates without any prior knowledge. Our results show that even a naive implementation outperforms previous methods.