Subject:
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Re: surface normals and vertex normals for OpenGL
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Newsgroups:
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lugnet.cad.dev
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Date:
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Thu, 11 Mar 2004 13:48:47 GMT
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Viewed:
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1708 times
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In lugnet.cad.dev, Wayne Gramlich wrote:
> There is an interesting property of closed surfaces that can allow you to
> determine the normals for all polygons (or triangles) given one correct
> normal.
>
> A "well behaved" closed surface can be decomposed into a set of
> triangles in 3D space. Each edge of the closed surface will be a
> member of exactly two triangles. Furthermore, the winding rule for
> each triangle will be such that one triangle will traverse the edge
> in one direction and the winding rule for the adjacent triangle will
> go in the opposite direction. Thus, given one triangle whose normal
> is known, it is possible to keep visiting adjacent triangles until the
> normals of all triangles are known. Yes, this is really true!
>
> A "well behaved" closed surface is 1) non-self intersecting 2) is
> completely closed (i.e. no gaps or holes) 3) does not have any
> edges that are in more than two triangles. Two cubes that
> share an edge would violate criteria 3). Two cubes that share
> a single point would probably work, but should be avoided as well.
> I would postulate that it is the goal of LDRAW part designers to
> produce "well behaved" closed surfaces.
I don't know about that goal. I think it's better to make it easy
to assemble the various primitives into parts. That way we get more
people volunteering to make the parts. I don't think the primitives
*can* be closed. I also think it's much easier to paste a stud
primitive onto a flat surface than to break up the flat surface into
something with a round hole to match the open edges of the stud.
But this violates your "well behaved" principle.
Don
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Message has 1 Reply:
 | | Re: surface normals and vertex normals for OpenGL
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| (...) All: It is dangerous to write technical arguments at 1:30 in the morning, and I made some serious mistakes in my post above. 1) You are correct that most LDRAW parts are not closed and probably never will be. Optimizing for ease of part (...) (20 years ago, 11-Mar-04, to lugnet.cad.dev)
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Message is in Reply To:
| | Re: surface normals and vertex normals for OpenGL
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| (...) [snip] (...) [snip] (...) It is neither impossible nor particularly hard. I will snip the rest of the post here and explain. There is an interesting property of closed surfaces that can allow you to determine the normals for all polygons (or (...) (20 years ago, 11-Mar-04, to lugnet.cad.dev)
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