| Re: Tessellate Question--fewest convex polys [message #29231] |
Fri, 08 February 2002 15:11  |
karl_schultz
Messages: 13
Registered: August 2001
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Junior Member |
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David Fanning <david@dfanning.com> wrote in message news:<MPG.16cdc43310dbf739897fb@news.frii.com>...
> Noname (parrhasius@altavista.com) writes:
>
>> If MESH_DECIMATE really
>> does what I need, I apologize for this thread and don't bother to answer,
>> unless there's some trick to using it... I thought I checked it out.
>
> I've never had the ...er, opportunity to use this
> yet. I'm afraid someone else will have to address
> this question.
>
> Cheers,
>
> David
MESH_DECIMATE is used to simplify polygonal meshes by collapsing mesh
vertex pairs, selecting the pairs carefully so that the simplified
mesh represents the original mesh as closely as possible.
It is typically used for things like 3D terrain maps and polyognal
models in order to produce a mesh that has fewer triangles, yet still
retains the original 3D shape as much as possible. Try the little
demo program called 'decimate' in the IDL 5.4 and 5.5 releases.
Using MESH_DECIMATE on a 2D (flat) mesh is a little out of the
ordinary, since the criteria used to select the vertex pair to
collapse would select any vertex pair in the mesh. That is, it does
not matter which pair it selects; you will always have a flat mesh
regardless of which and how many pairs are collapsed. This would make
the pair collapse look a little random, or related to the order of the
triangles in the mesh. But it would still reduce the number of
triangles in the mesh, depending on the percentage setting.
It is a little hard to say if this is the right tool for the data
described in the original post. MESH_DECIMATE starts by turning the
mesh into triangles, so it actually increases the number of polygons
in the mesh as it gets started. This is required, because the vertex
collapse criteria evaluation depends on the mesh only containing
triangles. Then, it will collapse vertices and the number of tris
will decrease and the size of the triangles will increase. You'll end
up with a bunch of largeish triangles that define your mesh.
If you (original poster) really needs to have large n-gons instead of
large triangles, then this won't work. I'd suggest googling around on
the net for this sort of thing. In a way, this is sort of a reverse
constrained tessellation - combining triangles into convex polygons.
I don't think that there is anything in IDL that will do this
directly. The main purpose of the tessellator object is to allow you
to convert possibly concave polygons into a set of triangles that can
be drawn using IDLgrPolygon.
Karl
RSI
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