<_description>
A 3D immersion of the real projective plane that smoothly deforms
between the Roman surface and the Boy surface.
You can walk on the surface of the real projective plane or rotate it
in 3D. Furthermore, it is possible to smoothly deform the real
projective plane between the Roman surface and the Boy surface while
turning it or walking on it. Inspired by François Apéry's book
"Models of the Real Projective Plane", Vieweg, 1987.
https://en.wikipedia.org/wiki/Boy%27s_surface
https://en.wikipedia.org/wiki/Roman_surface
http://mathworld.wolfram.com/BoySurface.html
http://mathworld.wolfram.com/RomanSurface.html
Written by Carsten Steger; 2014.