2008/9/20 mhampton <[EMAIL PROTECTED]>: > > I'm not sure I understand what you think is wrong; the output you are > describing looks ok to me. The polyhedron is a square, 2- > dimensional. It has four 1-dimensional faces, > and .facial_incidences() is listing the indices of those faces > followed by the indices of the vertices contained in each face.
That's what I thought was wrong. I though it should have one 2D face with the four vertices rather that four 1D faces. But if you tell me this is right, I trust you. > Are you writing triangularization code for arbitrary dimensions or > just for 3d? I realized that although I was mainly using it for > rendering, other people are interested in triangularizations for other > reasons and in other dimensions. The way I wrote it, it should work for any dimension. It doesn't actually look at the points themselves, only at their adjacency (with vertex_adjacencies()) > Cheers, > Marshall > > On Sep 20, 3:08 pm, "Arnaud Bergeron" <[EMAIL PROTECTED]> wrote: >> About this, I have encountered behaviour which I find strange (and >> also breaks the current code), but it may just be because I am not >> familiar enough with polytopes >> >> sage: p2 = Polyhedron([[0,0,0], [0,0,10], [10,0,10], [10,0,0]]) >> >> This should just be a square on the y plane right? >> >> sage: p2.facial_incidences() >> [[0, [0, 3]], [1, [0, 1]], [2, [1, 2]], [3, [2, 3]]] >> >> But this says to me it's actually four segments with the center empty. >> >> So I have two questions: >> >> 1. Is the polyheron above actually a square? >> 2. Is the facial incidence list describing a square or four edges >> along the side of one? >> >> If 1 is yes, I think I found a bug. >> >> If 2 is yes, my new code works! >> >> Arnaud >> >> >> >> > Cheers, >> > Marshall Hampton >> >> > On Sep 20, 11:35 am, "Arnaud Bergeron" <[EMAIL PROTECTED]> wrote: >> >> 2008/9/20 mhampton <[EMAIL PROTECTED]>: >> >> >> > Hi everybody, >> >> >> > This is mainly about making the lrs optional package standard to >> >> > improve some polytope-related calculations. >> >> >> > My original motivation for lrs (linear reverse search) is that is a >> >> > very different algorithm/implementation for computing exact convex >> >> > hulls in arbitrary dimensions. Currently the default for this in Sage >> >> > is cddlib, which uses the "double description" method. There are >> >> > classes of polytopes for which lrs is much faster than cddlib, >> >> > although in my experience cddlib does better most of the time. lrs is >> >> > also nice in that it doesn't use much memory. Polymake also uses >> >> > these two methods, plus a third, the "beneath and beyond" method. I >> >> > still need to modify the Polyhedron class to make use of lrs if it is >> >> > present. >> >> >> > I am now thinking more about nice renderings of polytopes, and for >> >> > that I need better triangulation code. I have a really bad algorithm >> >> > in polyhedra.py right now that needs to be improved or replaced. I >> >> > see at least three options for this: >> >> >> > 1) I could just improve the code I already have. I've actually >> >> > already done this, and I guess I will submit a ticket soon. But I am >> >> > not an expert in this area and I don't think even my improved versions >> >> > would be good enough for serious users. >> >> >> > 2) The option I like best in the short term is to make lrs standard >> >> > and use it for computing triangulations. The lrs algorithm computes a >> >> > triangulation anyway, and its probably one of the faster methods >> >> > available for doing so. lrs is small (spkg is 120kb) and compiles >> >> > quickly. Its mature code, its been maintained and improved for about >> >> > 10 years. >> >> >> > 3) Eventually it would be good to add TOPCOM. Apparently it is what >> >> > people use who do research that involves triangulations. But this is >> >> > a bigger task than I can take on right now. Its a much larger piece >> >> > of code than lrs, but it would add more functionality. >> >> >> > So I can do option (1) very soon, and option (2) in the next month or >> >> > so unless people have objections. >> >> >> I am beginning to work on the triangularization code just now, since >> >> it was on the list of requested items when I asked for thing to do in >> >> graphics. It should be ready in about a week if all goes well. >> >> >> Arnaud >> >> >> > To help, install the optional package and test it by downloading: >> >> >http://www.d.umn.edu/~mhampton/lrs_test.ext >> >> > and run $SAGE_ROOT/local/bin/lrs your_path_to_lrs_test/lrs_test.ext >> >> > where of course you replace "your_path_to_lrs_test" with the path to >> >> > the file. I've done this on a couple of intel macs (10.4 and 10.5), I >> >> > don't expect any problems on linux and I have no idea about Solaris. >> >> >> > Cheers, >> >> > Marshall Hampton >> >> >> -- >> >> La brigade SnW veut vous recruter -http://brigade.snw.googlepages.com >> >> -- >> La brigade SnW veut vous recruter -http://brigade.snw.googlepages.com > > > -- La brigade SnW veut vous recruter - http://brigade.snw.googlepages.com --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to sage-devel@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sage-devel URLs: http://www.sagemath.org -~----------~----~----~----~------~----~------~--~---
