Line-drawing interpretation: bilateral symmetry

Results previously derived by the author are used to investigate the implication of bilateral symmetry in line drawings. It is shown that the line drawing of an orthographically protected surface of revolution exhibits bilateral symmetry about the projection of its axis of revolution irrespective of the viewing direction. Barring one exception, a bilaterally symmetric line drawing is necessarily the orthographic projection of a local surface of revolution whenever its symmetry axis continues to be the projection of a fixed line in space under perturbation of viewpoint; the axis of revolution is the invariant preimage of the symmetry axis. Various line-drawing causes are detailed which facilitate the deduction of invariant preimages of symmetry axes, and consequently of local surfaces of revolution