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From: slamont@network.ucsd.edu (Steve Lamont)
Newsgroups: comp.graphics.visualization
Subject: Re: Marching Cubes
Keywords: marching cubes, isosurface
Message-ID: <5266@network.ucsd.edu>
Date: 27 Apr 91 22:22:37 GMT
References: <1991Apr24.211855.11705@rice.edu> <1991Apr25.005705.16988@jarvis.csri.toronto.edu> <2551@lee.SEAS.UCLA.EDU>
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Organization: Naval Postgraduate School, Monterey, CA
Lines: 31

In article <2551@lee.SEAS.UCLA.EDU> payne@hyperion.loni.ucla.edu (Bradley Payne) writes:
>
>
>I've also encountered the hole problem with marching cubes.  This is caused
>by an ambiguous case in the cube classification table.  One possible fix is
>to decompose each cube into tetrahedra (there are methods which give 5 or 6
>per cube), and then tile the tetrahedra, which don't have any ambiguous cases.
>This does tend to give lumpier surfaces than marching cubes.

The problem with tetrahedra is effectively the same as the problem with cubes,
they can both produce surfaces with holes in them.  If you play around with
the tetrahedral algorithm, you'll see what I mean.

This was a topic of some discussion at the San Diego Workshop on Volume
Visualization back in November 1990 (proceedings in Computer Graphics, V 24,
No. 5).  Jane Wilhelms and Allen Van Gelder suggested a fairly robust solution
which essentially involves a surface fit in the ambiguous case (Topological
considerations in isosurface generation [extended abstract]).

A reasonable 'cheat' which may not work in all cases but works in the cases
that I've encountered, is to use the averaging technique discussed in G.
Wyvill, C. Mc Pheeters, and B. Wyvill, Data structures for soft objects,
published in "The Visual Computer," 2(4):227-234, August 1986.

							spl (the p stands for
							polygons `r' me)
-- 
Steve Lamont, SciViGuy -- (408) 646-2752 -- a guest at network.ucsd.edu --
NPS Confuser Center / Code 51 / Naval Postgraduate School / Monterey, CA 93943
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