We study non-crossing frameworks in the plane for
which the classical reciprocal on the dual graph is also non-crossing.
We give a complete description of the self-stresses on non-crossing
frameworks G whose reciprocals are non-crossing, in terms of: the
types of faces (only
pseudo-triangles and pseudo-quadrangles are allowed); the sign
patterns in the stress on G; and a geometric condition on the stress
vectors at some of the vertices.
As in other recent papers where the interplay of non-crossingness
and rigidity of straight-line plane graphs is studied,
pseudo-triangulations show up as objects of special interest.
For example, it
is known that all planar Laman circuits can be embedded as a
pseudo-triangulation with one non-pointed vertex. We show that for
such pseudo-triangulation embeddings of planar
Laman circuits which are sufficiently generic,
the reciprocal is non-crossing and again a
pseudo-triangulation embedding of a planar Laman circuit. For a
singular (non-generic) pseudo-triangulation embedding of a planar Laman
circuit, the reciprocal is still non-crossing and a
pseudo-triangulation, but its underlying graph may not be a Laman
circuit. Moreover, all the pseudo-triangulations which admit a
non-crossing reciprocal arise as the reciprocals of such, possibly
singular, stresses on pseudo-triangulation Laman circuits.
All self-stresses on a planar graph correspond to liftings to
piecewise linear surfaces in 3-space. We prove
characteristic geometric properties of the lifts of such non-crossing
reciprocal pairs.