IsosurfaceLat2 normals

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2 total messages Started by Tim Scheitlin Thu, 10 Sep 1992 22:04

#71

Author: Tim Scheitlin
Date: Thu, 10 Sep 1992 22:04

22 lines
1358 bytes

Hi Folks,

I have a question about the Explorer IsosufaceLat2 module.
Some of the polygons in the isosurfaces I am generating with this module 
appear to have their normals pointing in the wrong direction.

This is evidenced by the fact that some of the polygons appear black
while the other polygons in the isosurface are colored correctly.
Flipping the normal produces just the opposite effect.

The man page for this module says that a "KNOWN PROBLEM" is that the
module can produce surfaces with holes because of a bug in the original
Marching Cubes algorithm.

My question is:  Is the problem I am having with incorrect normals 
related to the bug in the original Marching Cubes algorithm?
Is there a fix?

Thanks for any pointers.

Tim Scheitlin
scheitln@ncar.ucar.edu

Re: IsosurfaceLat2 normals

#74

Author: Roy Hashimoto
Date: Fri, 11 Sep 1992 16:54

28 lines
1563 bytes

In article <1992Sep10.220413.11698@ncar.ucar.edu> scheitln@ncar.ucar.edu (Tim Scheitlin) writes:
>I have a question about the Explorer IsosufaceLat2 module.
>Some of the polygons in the isosurfaces I am generating with this module 
>appear to have their normals pointing in the wrong direction.

That's rather peculiar, all the normals should be right or all should be
wrong.  What kind of graphics do you have, do you see the same behavior
on the original IsosurfaceLat, and is your input lattice curvilinear?

>My question is:  Is the problem I am having with incorrect normals 
>related to the bug in the original Marching Cubes algorithm?

No, that bug just causes polygons to be missing.  Normal generation is
entirely separate.

Here are my guesses:

1.  You have a curvilinear lattice that reflects on itself so that cells
can have two different "senses".

2.  The module has a bug in normal generation.  Quite possible, but it's
strange that we haven't seen it before.

3.  There is a bug in the rendering code or the GL.  Very unlikely, but
not out of the question.

Roy Hashimoto
hashimoto@sgi.com

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