Re: [OctDev] convhulln options.

[Rafael L. <rla...@us...>]

* Paul Kienzle <pki...@us...> [2003-12-17 20:07]:

> 
> On Dec 17, 2003, at 7:00 PM, Rafael Laboissiere wrote:
> 
  > [...]
> > would be represented in Octave like this:
> >
> >    A = {[3 4 2 1],
> >         [7 4 3],
> >         [5 7 3 1],
> >         [4 6 2],
> >         [6 7 5],
> >         [7 6 4],
> >         [6 5 1 2]}
> >
> >
> >(Notice that vertex indices in Octave start at 1 instead of zero.)
> >
> >I do not know how backward incompatible would be such a change.
> 
> I think the most convenient would be to return a cell-array of matrices,
> one for each dimension of returned facet.

Sorry, I did not understand the above statement.  Could you give an example,
please?

> I think it would also be convenient if the default were to arbitrarily 
> split those facets that are non-simplicial so that those who just want 
> triangles don't have to deal with the occasional square.  I'm guessing that
> this is what matlab does --- convhulln returns this for the cube:
> 
> >> convhulln(cube)
> 
> ans =
> 
>      3     4     1
>      4     2     1
>      2     4     6
>      2     5     1
>      5     2     6
>      7     5     6
>      4     7     6
>      7     4     3
>      7     3     1
>      5     7     1

This is the case in Octave with the "QJ" option:

    octave:3> convhulln(cube,"QJ")
    qhull warning: joggle ('QJ') always produces simplicial output.
    Triangulated output ('Qt') does nothing.
    ans =

      7  8  6
      4  8  6
      5  7  1
      5  7  6
      2  4  1
      2  4  6
      2  5  1
      2  5  6
      3  7  8
      3  4  8
      3  7  1
      3  4  1
  
> So long as the default all-simplicial case is returned as a matrix, 
> backward compatibility doesn't matter.  The user will need special code to 
> handle the non-simplicial facets anyway.
> 
> Are these two cases different enough that we need different functions?

I do not think so, since the output format will be dependent on the data
and, in general, cannot be determined beforehand by the user.  

I have already an idea on how to implement this change (i.e., returning a
cell array instead of a matrix in the non-simplicial case.  Should I go
ahead?

> I think you are going to need an interface layer above the QHull options so
> that users can ask for them nicely:
> 
> [X,Y,Z] = convhulln(cube,'output',{'simplicial','ideal','geomview'})
> 
> This says that X will contain the simplicial output (as a full matrix),
> Y will contain a cell array of facet sizes, and Z will contain a string
> used to control geomview.  The default for output must be 'simplicial'
> for compatibility.
> 
> That means you will need other options for controlling the QHull 
> algorithm, such as 'qhull','QJ' to feed a raw option string, or if you are
> feeling creative, meaningful names for all the options.

That looks like a quite nice idea.  Two questions: 

    * Is the option passing format you described above standard in
      Matlab/Octave/octave-forge?

    * Can I consider your suggestion as a final design or is it just a
      sketch?  If it is the later, could you please propose a complete
      design (so that implementation can follows easily)?

-- 
Rafael