Re: [OctDev] griddata / delaunay

[Paul K. <pki...@us...>]

See comments at:

http://www.octave.org/octave-lists/archive/help-octave.2003/ 
msg02420.html

and links therein.

- Paul

On Sep 1, 2004, at 7:09 PM, Jonathan Stickel wrote:

> Seems like a good solution to me.
>
> I'm rather new to open source development...  How do changes generally  
> get committed to octave-forge?  Should I submit a patch for delaunay  
> to this list?  Will someone then review and commit it?
>
> BTW:  I just started working with delaunay recently because griddata  
> was giving me trouble.  I really don't know much about the other  
> functions that use qhull, so I'd rather leave them alone right now.
>
> Thanks,
> Jonathan
>
>
> David Bateman wrote:
>> 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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