Re: convex hull


Thanks for your response Bill. I think I got some ideas, but still have a few questions:

1)  It doesn't appear that I can simply create the surface:
Irregular3DSet(xyz, samples,null, null,null, new DelaunayClarkson(samples));

I was thinking if there are some methods within DelaunayClarkson where I can obtain the proper samples and tri and simply feed them to the DelaunayCustom. For example something like the following:

DelaunayClarkson test = new DelaunayClarkson(samples);
set = new Irregular3DSet(xyz, samples,null, null,null, new DelaunayCustom(test.getSamples?,test.getTri?));

2) If there aren't any methods as mentioned in (1), I would imagine that I will need to determine which data points form the convex hull based on the Walk[i][j] and then feed those data to DelaunayCustom. While I know the order of the data set (samples) that is going into DelaunayClarkson(samples), how do I know their orders within Walk[i][j]? In other words, which points within samples triangular face j is composed of?

Like always, thanks very much,


At 11:12 AM 8/26/2002 -0500, you wrote:

Date: Sun, 25 Aug 2002 17:03:37 -0500 (CDT)
From: Bill Hibbard <billh@xxxxxxxxxxxxx>
To: John Yao <johnyao@xxxxxxxx>
cc: visad-list@xxxxxxxxxxxxx, qingwei Fu <qfu@xxxxxxxx>
Subject: Re: convex hull

Hi John,

The Watson and Clarkson algorithms included in VisAD for
finding Delaunay triangulations were also used by their
authors to find convex hulls.

I have never tried it, but you could probably apply either
visad/ or visad/
to find a tetrahedralization of your 3-D samples, then
look at its 'int[][] Walk' array to find faces that have
no adjacent tetrahedron. These faces should form the
convex hull of your data. Specifically, if Walk[i][j] = -1,
then triangular face j of tetrahedron i is part of the
convex hull.

Note this is just a theory off the top of my head, and
one that I have never tried. If anyone can correct me,
please do.

Bill Hibbard, SSEC, 1225 W. Dayton St., Madison, WI  53706
hibbard@xxxxxxxxxxxxxxxxx  608-263-4427  fax: 608-263-6738

On Sun, 25 Aug 2002, John Yao wrote:

> Hello,
> I have generated some shapes using (DelaunayCustom(samples,tri)) from the
> Irregular3DSet. However, the resulting geometry (surface covering) did not
> turn out the way we want it to be.  We are investigating a different
> algorithm called "convex hull"
> ( to generate the
> surface based on the computed data points.
> I am just curious to know if VisAD happens to support this algorithm or
> method of creating the surface cover.  Thanks very much.
> john

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