#29065 closed enhancement (fixed)
Inverse function for `gale_transform`
Reported by:  ghkliem  Owned by:  

Priority:  major  Milestone:  sage9.1 
Component:  geometry  Keywords:  polytopes, gale transform 
Cc:  JeanPhilippe Labbé, Laith Rastanawi, Yuan Zhou  Merged in:  
Authors:  Jonathan Kliem  Reviewers:  JeanPhilippe Labbé 
Report Upstream:  N/A  Work issues:  
Branch:  b6898dd (Commits, GitHub, GitLab)  Commit:  b6898dd2c7afae73958f4d3daaab063b1e751337 
Dependencies:  Stopgaps: 
Description
One can obtain a gale diagram from a polyhedron. Some interesting examples of polyhedra are obtained by the reverse approach.
We implement a method that converts a gale diagram to a Polyhedron
.
This is a preparation for adding polytopes to the library obtained from the Gale diagram (e.g. #27728).
Change History (28)
comment:1 Changed 3 years ago by
Branch:  → public/29065 

Commit:  → baaa7350520c1e65a66a4ac7c60fb66ce5b3b147 
Status:  new → needs_review 
comment:2 Changed 3 years ago by
Commit:  baaa7350520c1e65a66a4ac7c60fb66ce5b3b147 → b32c0735232e300bbe02806ef255ef38702a8d31 

Branch pushed to git repo; I updated commit sha1. New commits:
b32c073  various improvements

comment:3 Changed 3 years ago by
Commit:  b32c0735232e300bbe02806ef255ef38702a8d31 → 8f9129930d5bade23a7f260d24138721aa36d238 

Branch pushed to git repo; I updated commit sha1. New commits:
8f91299  do not echolonize kernel matrix

comment:4 Changed 3 years ago by
Cc:  Yuan Zhou added 

comment:5 Changed 3 years ago by
Small remarks:
 Typos:
 The vectors are scalled automatically such that they add up to zero. + The vectors are scaled automatically such that they add up to zero.
 # The vectors of our gale transform shall add up to one. + # The vectors of our gale transform shall add up to zero.
 Maybe explain why you want the sum of all
vectors
to be zero. Something like "so that the right kernel ofvectors
has the one vector in it".
comment:6 Changed 3 years ago by
Commit:  8f9129930d5bade23a7f260d24138721aa36d238 → 251e9f51633e46c6b9d102a160243cca818111d1 

Branch pushed to git repo; I updated commit sha1. New commits:
251e9f5  better documentation

comment:7 followup: 8 Changed 3 years ago by
Status:  needs_review → needs_work 

Here are some comments:
 I would name the function
gale_transform_to_polytope
since there is no unbounded case here.
def gale_transform_to_polyhedron(vectors, base_ring=None, backend=None): r"""  Return the polytope from a (linear) gale transform. + Return the polytope associated to the list of vectors forming a Gale transform. This function is the inverse of :meth:`~sage.geometry.polyhedron.base.Polyhedron_base.gale_transform`  up to projective isomorphism. + up to projective transformation.
+ The vectors are scaled automatically such that they add up to zero.
This needs to be more clear. Is it: The _vertices_ of the polytope are scaled so that the sum of their coordinates is zero? I believe not, right? Then it should say that the centroid (which is what you mean, I'm pretty sure) should be the origin.
I think that the following sentence should be changed to reflect what the function does internally (if it is not the case, you mention in the comments in the code that you do it, this should be mentioned here too!).
+ The function is much faster and gives nicer representation if this
+ is already the case.
+ INPUT: +   ``vectors``  the vectors of the gale transform +  ``vectors``  the ordered vectors of the Gale transform
Please use the suggested format when using default values for optional parameters:
+  ``base_ring``  (optional) the base ring to be used for the construction + +  ``backend``  (optional) the backend to use to create the polytope +
gale transform > Gale transform (but not in names of functions...)
straight forward > straightforward
comment:8 followup: 10 Changed 3 years ago by
This comment is totally cryptic to me. Should I change the format? To what? There is tons of different styles for this all over the place.
The coding conventions suggest the following:
 ``base_ring``  string (default: `None`); the base ring to be used for the construction
Why is the order of the vectors of importance?
Replying to jipilab:
Please use the suggested format when using default values for optional parameters:
+  ``base_ring``  (optional) the base ring to be used for the construction + +  ``backend``  (optional) the backend to use to create the polytope +gale transform > Gale transform (but not in names of functions...)
straight forward > straightforward
comment:9 Changed 3 years ago by
Branch:  public/29065 → public/29065reb 

Commit:  251e9f51633e46c6b9d102a160243cca818111d1 → e54147184c209d81990fe7bfb7f4816ca2fc27a5 
Status:  needs_work → needs_review 
comment:10 Changed 3 years ago by
Replying to ghkliem:
This comment is totally cryptic to me. Should I change the format? To what? There is tons of different styles for this all over the place.
The coding conventions suggest the following:
 ``base_ring``  string (default: `None`); the base ring to be used for the construction
I like the above convention, yes.
Why is the order of the vectors of importance?
Essentially, to make things clear and smooth, it is practical to talk about a "labeled" set of vector, where the set of labels has a total order a.k.a. essentially numbered from 1 to k.
In practice, when one uses Gale duality, to go from primal to dual notions and viceversa, it is important to have a precise ground set in order to do the translations: complementation should be welldefined, and as soon as you get into the oriented matroid business, it is important to get the signs right... This is why it is a soothing measure to mention here in the first line of the documentation that we do care that this is an (ordered) list of vectors.
This will make our life easier when explaining/doing things.
comment:11 Changed 3 years ago by
About this, we should also add a reference to another book.
I suggest the Triangulations book of De Loera, Rambau, Santos. Definition 2.5.1 and Definition 4.1.35.
Essentially, this function is not necessarily to get a polytope, right?
Or does it really check for the polytopality property?
I see the possibility here to split into two functions...
comment:12 Changed 3 years ago by
Status:  needs_review → needs_work 

comment:13 Changed 3 years ago by
Commit:  e54147184c209d81990fe7bfb7f4816ca2fc27a5 → 3a909c1afa6b6d472301e00bfd4069e9abe090a2 

comment:14 Changed 3 years ago by
Status:  needs_work → needs_review 

comment:15 Changed 3 years ago by
Reviewers:  → JeanPhilippe Labbé 

Status:  needs_review → needs_work 
Tiny things:
+ .. SEEALSO:: +  :func`~sage.geometry.polyhedron.library.gale_transform_to_polyhedron`. + :func`~sage.geometry.polyhedron.library.gale_transform_to_polytope`.
 OUTPUT: An ordered point confuration as list of vectors. + OUTPUT: An ordered point configuration as list of vectors.
 One can also specify the backend to be used interally:: + One can also specify the backend to be used internally::
 The input vectors are checked for being totally cyclic:: + The input vectors should be totally cyclic::
 # of the polyhedron. So dehomogenization is straight forward. + # of the polyhedron. So dehomogenization is straightforward.
I find the "algorithm" part a bit hard to understand, but that might just be me. I will see if I can come up with an suggestion of improvement.
comment:16 Changed 3 years ago by
Commit:  3a909c1afa6b6d472301e00bfd4069e9abe090a2 → 55f62a8936590627672cd3f261e1abbc95abcbf2 

Branch pushed to git repo; I updated commit sha1. New commits:
55f62a8  improved documentation

comment:17 Changed 3 years ago by
Status:  needs_work → needs_review 

I hope this is better now. I completely rewrote the algorithm part.
comment:18 Changed 3 years ago by
Tiny things:
+ sage: gale_transform_to_polytope(
+ ....: [(1,1), (1,1), (1,0),
+ ....: (1,0), (1,1), (2,1)],
+ ....: backend='cdd', base_ring=RDF)
+ A 3dimensional polyhedron in RDF^3 defined as the convex hull of 6 vertices
I know that cdd is the only backend handling RDF, but perhaps it would be good to still check the backend like the example just before...
Question: is this test equivalent to the fact that it is not the correct polytope:
+ if not P.n_vertices() == len(vertices):
+ raise ValueError("the gale transform does not correspond to a polytope")
It would be good to give a reference for that line in the code in a comment. I want to make sure to catch this error if and only if the thing go wrong (so that other types of errors are not hidden by that failing/passing like it happened a lot in polyhedron stuff...).
In gale_transform_to_primal
, be careful in describing _all_ the details properly. We do not assume that the centroid of the vectors to be the origin, but rather
 We assume the centroid of the (input) vectors to be the origin.  We stack ``Matrix(vectors)`` by a row of ones.  The right kernel of this is the dual point configuration. + If the centroid of the (input) vectors is not the origin, we do an appropriate transformation to make it so. + We add a row of ones on top of ``Matrix(vectors)``. + The right kernel of this larger matrix is the dual configuration space, and a basis of this space provides the dual point configuration.
Then paragraph 3 and 4 of the algorithm are a bit hard to parse... Perhaps rework them a bit?
comment:19 Changed 3 years ago by
Status:  needs_review → needs_work 

comment:20 Changed 3 years ago by
Branch:  public/29065reb → public/29065reb2 

Commit:  55f62a8936590627672cd3f261e1abbc95abcbf2 → 948d9920d25e1ccc10c1471c402d8dbe02326f91 
Status:  needs_work → needs_review 
Last 10 new commits:
c7b0446  various improvements

2f7aa65  do not echolonize kernel matrix

c2e1d33  better documentation

49a2e23  fixed failing doctest

988f81f  improved function name and documentation

e0e975d  added another reference

7aaa768  split function in two functions

b9f3860  improved documentation

0691033  tiny improvement of doctests

948d992  error message for nonspanning gale transform; improved documentation

comment:21 Changed 3 years ago by
Status:  needs_review → needs_work 

You will hate me for this, but "centroid" is the wrong term.
What you mean is "center" (the centroid method is not implemented yet, see https://ask.sagemath.org/question/8092/computethecentroidofapolytope/ ... which is also to be implemented ...Sighhhhhh).
One small thing:
 convex if and only if every hyperplane contains at least two vectors of + convex and if and only if every hyperplane contains at least two vectors of
Otherwise, looks very good!
comment:22 Changed 3 years ago by
Commit:  948d9920d25e1ccc10c1471c402d8dbe02326f91 → b6898dd2c7afae73958f4d3daaab063b1e751337 

Branch pushed to git repo; I updated commit sha1. New commits:
b6898dd  centroid > center; small correction

comment:24 Changed 3 years ago by
I'm working on #27728 which depends on this ticket, and I might have some more feedback about the functions. Let's wait for that and the bots.
comment:25 Changed 3 years ago by
Milestone:  sage9.1 → sage9.2 

Batch modifying tickets that will likely not be ready for 9.1, based on a review of the ticket title, branch/review status, and last modification date.
comment:26 Changed 3 years ago by
Status:  needs_review → positive_review 

Ok, looks good. If there are more modifications on this function, they can be addressed in #27728.
comment:27 Changed 3 years ago by
Branch:  public/29065reb2 → b6898dd2c7afae73958f4d3daaab063b1e751337 

Resolution:  → fixed 
Status:  positive_review → closed 
comment:28 Changed 3 years ago by
Milestone:  sage9.2 → sage9.1 

New commits:
implement function to convert from gale transform to polytope