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On Tue, 14 Jul 1998 23:04:15 +0200, F.VERBAAS <106120,444### [at] COMPUSERVE COM> wrote:
>But, so thinking further, what is the actual reason that a mesh can not be
>used for CSG? I assume this is because :
>1 - there is no guarantee that a mesh describes a closed surface, so no
>'real' inside or outside
True.
>2 - there is no guarantee that all triangles are consistently defined with
>their corner vertices arranged such that a normal vector defined by the
>right hand rule points outward. (or was it the left hand rule? with POV you
>never know).
I've never understood this rule. While it can make the job of determining
insideness easier, it's not really necessary. All you have to do is shoot
a ray in an arbitrary direction and count how many times it intersects the
mesh surface. If it intersects at a boundary between two or more triangles,
you'll need to shoot another ray, but I believe this condition is easy to
detect, as the intersection queue will contain a duplicated point.
>3 - there is no guarantee that the surface does not intersect itself,
>exposing the inside surface to the outside, and having the normals pointing
>inward. (hard to tell this in words, but no time to render an image)
>But, what if we would allow meshes that are closed, consistently defined,
>and not self-intersecting to engage in CSG?
If you don't care about the surface normals, self-intersecting but otherwise
closed meshes have insides. Yes, even a 3d representation of a Klein bottle
has a well-defined inside.
>A practical solution would be to add a keyword 'CSG' to the definition of
>the mesh, like:
An even more practical solution is to only use "good" meshes in CSG
operations. If you get strange results because you gave it a strange mesh,
that's your problem, just as it is now if you use an open cylinder or SOR
in a CSG. Perhaps a keyword to declare that your normals are all
well-behaved, as in your point 2, would be useful to speed up calculations
for such meshes.
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