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canonical - canonicalize a polyhedon

Usage    |    Examples    |    Notes

Usage



Usage: canonical [options] [input_file]

Read a polyhedron from a file in OFF format. Canonicalize or planarize it.
Uses algorithms by George W. Hart, http://www.georgehart.com/
http://www.georgehart.com/virtual-polyhedra/conway_notation.html
http://www.georgehart.com/virtual-polyhedra/canonical.html
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
  -H        documention on algorithm 
  -e <opt>  edge distribution (default : none)
               s - project vertices onto a sphere
  -s <opt>  shuffle model indexes
               v - vertices, e - edges, f - faces, a - all (default: none)
  -t <opt>  target model
               b - work on base only (default)
               p - work on dual for planarization only
               c - work on dual for planarization and canonicalization
  -p <opt>  planarization (done before canoncalization. default: none)
               p - poly_form planarize (poly_form -a p)
               m - mathematica planarize
               a - moving edge planarize
               b - base/dual (reciprocate on face centroids magnitude squared)
  -i <itrs> maximum planarize iterations. -1 for unlimited (default: -1)
            WARNING: unstable models may not finish unless -i is set
  -c <opt>  canonicalization
               c - circle packings (default)
               m - mathematica version
               a - moving edge version
               b - base/dual version (reciprocate on face normals)
               x - none (default, if -p is set)
  -n <itrs> maximum canonical iterations. -1 for unlimited (default: -1)
            WARNING: unstable models may not finish unless -n is set
  -y        maintain symmetry of the base model (-p p, -c c)
  -O <args> output b - base, d - dual, i - intersection points (default: b)
               edge nearpoints, n - base, m - dual; C - base/dual convex hull
               edge nearpoints centroid, p - base, q - dual; o - origin point
               tangent sphere, u - minimum, U - maximum
               incircles, s - base, t - dual; as rings, S - base, T - dual
  -q <dist> offset for incircles to avoid coplanarity e.g 0.0001 (default: 0)
  -g <opt>  roundness of tangent sphere, positive integer n (default: 8)
  -d <perc> radius test. percent difference between minimum and maximum radius
               checks if polyhedron is collapsing. 0 for no test 
               (default: 80 for canonicalizing, not used for planarizing)
  -l <lim>  minimum distance change to terminate, as negative exponent
               (default: 12 giving 1e-12)
            WARNING: high values can cause non-terminal behaviour. Use -n
  -z <nums> number of iterations between status reports (implies termination
            check) (0 for final report only, -1 for no report), optionally
            followed by a comma and the number of iterations between
            termination checks (0 for report checks only) (default: 1000,1)
  -o <file> write output to file (default: write to standard output)

Extra Options
  -E <perc> percentage to scale edge tangency (default: 50) (-c m)
  -P <perc> percentage to scale face planarity (default: 20) (-c m, -p m, -p p)
  -f <adj>  initial percent adjustment factor, optionally followed by a comma
            and a maximum percent adjustment (default: 1,50) (-c c)
  -C        continue processing a near-canonical model (the initial
            intermediate processing model will preserves the geometry
            of the base model rather than avoid scrambling) (-c c)

Coloring Options (run 'off_util -H color' for help on color formats)
  -I <col>  intersection points and/or origin color (default: yellow)
  -N <col>  base near points, centroid color (default: red)
  -M <col>  dual near points, centroid color (default: darkgreen)
  -S <col>  base incircles color. keyword: f take color of face (default)
  -R <col>  dual incircles color. keyword: f take color of face (default)
  -D <col>  dual face color. keyword: b take color from base vertices (default)
  -J <col>  base edge color (default: unchanged)
  -K <col>  dual edge color (default: unchanged)
  -U <col>  unit sphere and/or convex hull color (default: white)
  -T <tran> base/dual transparency. range from 0 (invisible) to 255 (opaque)

Examples

Make a cube, distort it, and canonicalize it back into a cube
off_util cube | off_trans -S 1,2,3 | canonical | antiview


Make a geodesic sphere, canonicalize it, and add it to it reciprocal
geodesic -c 2 ico | canonical -O bd | antiview -v 0.02


Make a propeller cube, canonicalize it, and cover it with the tranparent convex hull of the base and reciprocal
conway pC | canonical -O C -U 255,255,255,128 | antiview -v 0.01


Make a gyro icosahedron, canonicalize it, make it transparent, and show the edge tangent sphere and base edge nearpoints
conway gI | canonical -O bnU -T 128 | antiview -v 0.01


A nice conway notation model and the same model as base and dual incircle rings
conway ageC | canonical | antiview -v 0.01
conway ageC | canonical -O ST | antiview -v 0.01


Notes

The program will not always converge, and produce the canonical form. In this cases it may help to distort the polyhedron before running canonical. This could be done with off_util -S, repel, poly_form, off_trans or even editing the OFF file by hand.

George Hart has a page on canonicalization.

Uses algorithms by George W. Hart, http://www.georgehart.com/. The 'Mathematica' algorithms have been written to follow George Hart's Mathematica implementation

The following extended help for the program may be displayed with canonical -H



Calculating Canonical Polyhedra

A relaxation algorithm is presented to determine a canonical form for an
arbitrary convex polyhedron.

by George W. Hart

One important role of high-level mathematical software such as Mathematica is
that it easily allows for the testing of experimental algorithms. Here we
explore a method of finding a canonical form of a polyhedron. The canonical
form is a polyhedron topologically equivalent to an input polyhedron, but with
all edges tangent to a unit sphere and with the center of gravity of the
tangent points being the origin.  A variety of examples will illustrate this
procedure. One application is that the algorithm constructs a geometrically
self-dual polyhedron given one which is only combinatorially self-dual.

A theorem of Schramm [Schramm 1992] states that for any given convex polyhedron
or 3-connected planar graph, there is a topologically equivalent polyhedron
with the following properties:

 1) each edge is tangent to the unit sphere,

 2) the center of gravity of the points of tangency is the origin.

The solution is unique, up to rotations and reflections of the sphere and so
provides a kind of canonical representation. Although we will not pursue it
here, the theorem is actually more general in that it allows the sphere to be
replaced with an arbitrary smooth convex body, e.g., an egg-shaped
quasi-ellipsoid. The theorem is closely related to the Koebe-Andreev-Thurston
Circle Packing theorem for planar graphs. See Ziegler [Ziegler 1995, p. 118]
for discussion and other references. 

For example, if our input polyhedron is a geometrically distorted (but
topologically unchanged) form of any Platonic or Archimedean solid, and we
calculate its canonical form, our algorithm should output its undistorted form
centered at the origin, with a midradius of 1, and with an arbitrary rotation.
E.g., given any parallelepiped as input, the canonical form output is a cube of
edge Sqrt[2]. For an arbitrary polyhedron, the canonical form is a way of
illustrating its combinatorial or topological structure, which often lets one
immediately see and understand its structure and symmetry.  

As an illustration, we will use the algorithm to find three geometrically
self-dual polyhedra, given starting points which are only combinatorially
self-dual. It follows from the above that the dual polyhedron also shares
properties (1) and (2). Thus, for these three self-dual examples, we can make
a compound of the polyhedron with its dual, i.e., with itself, in which both
polyhedra have the same points of tangencies and at these points their edges
cross each other's at right angles. 

The proofs cited above are of existence and not constructive, so we are
interested in an algorithm for determining the canonical form of an input
polyhedron. The algorithm proposed here operates by relaxation to iteratively
move the vertices of the given polyhedron along a trajectory which converges
at the canonical form. Although we begin and end with a polyhedron, during this
relaxation, the object defined by the vertices is likely not to be a polyhedron
geometrically.  Sets of points which belong to a face (combinatorially) are
likely not to be coplanar. So we add a third condition for a solution, to the
two above:

 3) the faces are planar.


Algorithm

Our algorithm inputs a polyhedron and iteratively adjusts its vertices to
slightly improve its conformance with the three conditions above. Within a
couple of hundred iterations, it typically finds all the conditions are
satisfied within a small tolerance, and stops. Three simple operations are all
that is needed, corresponding to the three conditions:

 A) For each edge, the closest point to the origin is determined; call it p.
    If p lies at unit distance from the origin, condition (1) is satisfied for
    that edge. If not, a small fraction of p is added to the two vertices
    which define the edge, (in proportion to the sign and amount of the error)
    so that at the next iteration the edge will be closer to tangency with the
    unit sphere. 

 B) The center of gravity of all the points p is determined. If it is zero,
    condition (2) is satisfied. If not, it is subtracted from all the vertices. 

 C) For each face, if the vertices lie in some plane in space, condition (3) is
    satisfied. If not, a plane which approximates it is computed. Each vertex
    of the face is then moved along a normal towards the plane. 

Iterating, it sometimes takes many steps for a correction at one part of the
polyhedron to percolate its way around and equalize the conditions everywhere.
On the examples below, between 50 and 100 iterations were sufficient to have
all conditions satisfied within a tolerance of 10^-8. Although the individual
steps are quite simple, this takes a minute or so in these examples and would
take longer on more complex polyhedra. Refinement and optimization are left as
future work.


Additional Work by Adrian Rossiter:

Edge near-point / circle-packing canonicalisation algorithm
===========================================================

Approach
--------

A polyhedron with a midsphere corresponds to two circle packings on the
same sphere: the incircles of the base faces and the incircles of the dual
faces. The circles from the two packs intersect at the edge tangency points
of the base (or dual) polyhedron in two orthogonal tangent pairs. By moving
a set of points until they satisfy the conditions for being the intersection
points of these circles, a model corresponding to a polyhedron with a
midsphere can be made.

If the midsphere is the unit sphere, and the edge tangency points have
their centroid at the sphere centre then the polyhedron is canonical.


Processing model
----------------

The processing model has a vertex for each edge tangency point, and a face
for each circle of the two packs. Each vertex is therefore surrounded by four
faces, and these faces correspond, in opposing pairs, to each of the two
circle packs.

The canonical model is solved when the processing model satisfies
the following conditions:

*  the vertices are at distance 1 from the origin

*  the vertices have their centroid at the origin

*  the faces are planar, hence each set of vertices corresponding to a
   base/dual face lies on a circle of the base/dual circle pack

*  each vertex lies on the two planes through the origin containing the
   normals of opposing faces, hence each pair of circles meeting at the
   vertex are tangent (and this is sufficient to ensure that they are
   also orthogonal)


Algorithm
---------

Initialisation

1. Translate the base model to carry the vertex centroid to the origin.

2. Converted to an 'ambo' form. The vertices are truncated to a single
   point on each edge. These points are initially set to either the
   centroid of the edge vertices (better for a general input), or the
   point on the edge line nearest to the origin (better for a
   near-canonical input).

3. Make a list of the cycle of four faces around each vertex. Alternate
   faces will be opposing faces.

4. Choose a small termination value, that if the minimum vertex
   movement is less than this then the iteration terminates.

5. The amount of vertex movement is controlled by an adjustment factor,
   which can change each iteration. Choose a starting value (e.g. .1),
   and a maximum value (e.g. .5).


Iteration

An offset will be calculated for each vertex, and will be applied
near the end of the iteration loop.

For each vertex:

1. Initialize the offset to zero.

2. Adjust for the tangency point centroid:
   Add
      -vertices_centroid
   to the offset.

3. Adjust for coplanar / circular points:
   Calculate the projection the vertex onto its four surrounding planes,
   and then calculate the centroid of these four projection points. Add
      (projection_point_centroid - vertex) * factor
   to the offset.

4. Adjust for mutual tangency / orthogonality:
   For each pair of opposing faces:
      find a normal to their normals (a base or dual edge direction)
      calculate the projection of the vertex onto a plane through the origin
      with this normal
   Add
      (projection_point_centroid - vertex) * factor
   to the offset.

5. Adjust for unscrambling:
   If a vertex does not lie inside the cycle of its four neighbouring
   vertices, make the following adjustment. Add
      (neighbour_vertices_centroid - vertex) * 0.5 * factor
   to the offset.

6. Add the offset to the vertex

7. Scale the vertex to have a length of exactly 1

8. Adjust the adjustment factor:
   If the maximum offset length is less than that of the last iteration
   then scale the factor by 1.01 (if this will not exceed the maximum
   specified), but if it is greater then scale the factor by 0.995.

9. Terminate iteration if the maximum offset length is less than the
   termination value.


Final model

The processing model has faces that correspond to base faces and those
that correspond to dual faces, which also correspond to base vertices as
they were the faces produced by vertex truncation.

The final base model retains the faces of the original base model, but
each vertex is set to the polar reciprocal of the corresponding processing
model face plane.

For each vertex in the base model

1. Determine the corresponding processing model face.

2. Calculate the point nearest to the origin on the face plane, and its
   distance from the origin, and the final vertex position is
     position = point / distance^2


Symmetry optimisation
---------------------

The algorithm is suitable for use with a symmetry optimisation. This
also forces the original symmetry to be maintained, as repeated processing
of the vertices may otherwise cause them to wander from the original
symmetry.

Use the symmetry group of the base model. In the processing model just one
vertex from each orbit is processed. The faces surrounding the vertex have
their other vertex positions calculated, once per iteration, by a symmetry
transformation of a processed vertex.

To avoid calculating all the vertex positions for the centroid, it can be
calculated as: the centroid of the processed vertices, each projected onto
the subspace left invariant by the subgroup that fixes it, and weighted by
the number of vertices in its orbit. 


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Contact: adrian@antiprism.com      -      Modified 29.1.2021