CGAL users discussion list

[Joshua A Levine <>]

Monique; Sylvain,

Thanks for the replies.  I agree that dealing with hidden points is awkward, and treating them as vertices has its problem.  Although I'm not so sure that it wouldn't be easier to create a dummy vertex that stores the point and give each cell a list of vertex handles which are hidden instead of actual points.  It would seem to me that using the vertex structure to gain an extra level of indirection might be worth the sacrifice of having vertices that aren't really vertices. 

Sylvain's solution of storing additional information at the traits level 
instead of the vertex level is good.  It solves how to store the information, 
but you still run into problems on the insertion itself.  When you insert a 
point, the return type leaves much to be desired.  If the vertex is hidden, 
where is it hiding?  Harder: If the point I inserted hides current vertices, 
which ones?  So now for every insertion extra work external to the 
triangulation has to be done to verify special cases for insertion.  To not 
duplicate work, this means gathering all information before inserting and 
then do a fast insert where I pass the cell to insert within.

Thanks again, I'll see what kind of solution I can come up with here and let 
you know if I find something that might be helpful.

Josh


On 11/16/2007 3:58 AM, 

 wrote:
> Dear Joshua,
>
> You are right, hidden points are stored as points in the 3D package.
> In my opinion, storing hidden _vertices_ is ackward since when a point 
> is hidden, then it is by definition not a vertex of the triangulation. 
> Possible additional information are related more to the points 
> themselves than to the combinatorial vertices.
>
> Still, the need for manipulating (accessing, removing,...) hidden points 
> in an easier way is known. It is noted in our todo lists.
>
> Best
>     Monique Teillaud
>
> Joshua A Levine wrote:
> > Hi CGALers,
> >
> > I was wondering about the mechanism for storing hidden vertices in the 
> > Regular_Triangulation_3 class.  It seems that unlike in the 2D case, 
> > these vertices are only stored as a list of weighted points associated 
> > with whichever cell they are contained in.  There seems to be no 
> > global way to access all of the hidden points for the triangulation 
> > (outside of checking every cell).  Is this correct?
>
>
>
> > Moreover, because they are stored as a list of weighted points instead 
> > of as a list of actual vertices, it doesn't seem like there is a 
> > mechanism for storing additional information associated with these 
> > vertices.  By this I mean if I have a custom Vertex class with some 
> > additional members, and if I'd like to use that class to store 
> > additional information for a point I want to insert, I may not be able 
> > to if that point becomes hidden.  Is there a recommended way to store 
> > this sort of information without using some external bookkeeping?  Is 
> > there even a moderately easy way to do it _with_ external bookkeeping?
> >
> > To keep it simple for discussion purposes, consider a 
> > Regular_Triangulation_3 class where I want to store an associated id 
> > for each vertex I insert.  But the triangulation is changing with both 
> > insertions and deletions, so at any time a point I insert could itself 
> > be hidden, or could hide other vertices already present.  Similarly, 
> > on deletion I could unhide some points, but I'm pretty sure never hide 
> > anything.
> >
> > Thanks,
> > Josh