UNSUPPORTED: twist  twist the edges of a polyhedron
Usage: twist [options] [input_file]
Read a file in OFF format and twist the edges.
If input_file is not given the program reads from standard input.
Options
h,help this help message (run 'off_util H help' for general help)
version version information
c <cent> centre to twist about, in form "x_val,y_val,z_val"
(default, "0,0,0")
t val twist edges an amount between polyhedron (0.0) and dual (1.0)
s output struts only
o <file> write output to file (default: write to standard output)
Twist the edges of an icosahedron
src_extra/twist t 0.5 ico  antiview x f v 0.03
The twisted model is like a zigzag tensegrity, but without the
zigzag; the strut and its string loop lie on a plane.
An edge of a polyhedron has two vertices on the ends and is flanked
by two faces. In the dual these faces are associated with vertices,
and so an edge can be associated with four coordinates: two polyhedron
vertices and two dual vertices.
The other important point is the centre. This is the centre
of reciprocation used to find the dual.
An edge of a polyhedron can be associated, by the vertices
above, with an edge in the dual. The planes through these two
edges and the centre intersect in a line through the centre.
The centre of a twist edge, the same size as the polyhedron
edge can be put on this line and the twist edge rotated about
the line by t*90 degrees. Finally, the twist edge is translated
by the proportion t between the centre of the polyhedron edge
and the centre of the dual edge.
Finally, the twist edge segment is extended to see where it
intersects the planes through its edge neighbours, then the
extended edge is scaled (using the model centre) back to the
original size.
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UNSUPPORTED: spidron  spidronise a polyhedron
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