Re: [OctDev] griddata / delaunay

[David B. <Dav...@mo...>]

I'm not a user of delaunay and so you know better. I just proposed a 
generic fix that should be applied wherever foreign code is used with
octave to prevent just this type of problem. If the "QJ" to qhull is
such a generic solution, why not just make the default for OPT in
delaunay (and elsewhere?) be "QJ" with something like

 if (nargin == 2)
   opt = "QJ";
 endif

and be done with it. This combined with the previous fix, should be
sufficient..

D.


According to Jonathan Stickel <jjs...@sb...> (on 09/01/04):
> David Bateman wrote:
> >According to Jonathan Stickel <jjs...@sb...> (on 09/01/04):
> >
> >>I have encountered a problem with griddata when using semi-regular input 
> >>points.  The issue has to do with facet merging when running delaunay, 
> >>via qhull, with defualt options.  To avoid this, delaunay should be run 
> >>with the "QJ" option.  Perhaps this should be mentioned explicitly in 
> >>the delaunay help.  For the worst case of a rectangular grid, a 
> >>segmentation fault occcurs:
> >>
> >>octave:1> [x,y]=meshgrid(1:10,1:10);
> >>octave:2> delaunay(x(:),y(:),"QJ")
> >>(plot of triangles)
> >>octave:3> delaunay(x(:),y(:))
> >>panic: Segmentation fault -- stopping myself...
> >>attempting to save variables to `octave-core'...
> >>save to `octave-core' complete
> >>Segmentation fault
> >>
> >>Attached is a patch for griddata which calls delaunay with the "QJ" 
> >>option.
> >
> >
> >Not sure if this is the right approach. What is the real reason for the
> >seg fault. If it is in the underlying qhull code, it might be better to
> >surround the calls to the qhull stuff with
> >
> >      BEGIN_INTERRUPT_IMMEDIATELY_IN_FOREIGN_CODE_1;
> >
> >      error ("delaunay: Internal qhull error. Please try the 'QJ' 
> >      option.");
> >      octave_throw_interrupt_exception ();
> >
> >      BEGIN_INTERRUPT_IMMEDIATELY_IN_FOREIGN_CODE_2;
> >
> >      <QHULL CODE HERE>
> >
> >      END_INTERRUPT_IMMEDIATELY_IN_FOREIGN_CODE;
> >
> >and inform the author of qhull of the problem within their code. At least
> >this way octave won't crash for qhulls problems....
> >
> 
> I agree.  I do believe that qhull is the real cause of the example 
> segfault, and octave functions should catch the problem like you 
> suggest.  However, for other semi-regular grids, delaunay does not 
> always segfault but will "drop points" when used without the QJ option. 
>   I can provide an example if you like. I think this should be 
> mentioned in the help for qhull based functions.
> 
> Please don't overlook my specific solution for griddata.  Griddata does 
> not currently take arguments to pass to qhull.  This could be added, of 
> course, but I see no reason not to always use the QJ option for delaunay 
> within griddata.
> 
> Jonathan
> 
> 
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-- 
David Bateman                                Dav...@mo...
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