Hi,
I am VERY interested in that. I've got a model with 1200x1200 elevation datas.
I need a algorithm to make more simple resolution models from the original. Let
me know about the issu of this, or give some pointers on good URL's.
Thanks,
david.
Julian Scheid wrote:
> hello scott,
>
> here are my thoughts on decomposing geometry:
>
> >> I have seen a few pieces of code out there that can take a Geometry Array
> and
> >> create the same Geometry Array only with fewer vertices.
>
> what you mean is commonly named "mesh reduction". but there is something
> more powerful: NURBS tesselation!
>
> >> So, if a NURB object had say, 5000 vertices.
> >> this package would make the same NURB object, but only use 3000 vertices.
> >> making the object simpler, getting rid of redundant vertices, things like
> that.
>
> let me correct you: the singular is "NURBS" (non-uniform rational b-spline),
> not "NURB".
> yes i agree, the trailing 'S' is rather misleading.
>
> another thing I have to correct: in its natural "parametric" form, a NURBS
> object is usually not comprised of 5000 vertices. I think an average,
> not-too-complex NURBS object has perhaps between 30 and 300 vertices (plus
> another few hundred control points). However, because "welded planes" are
> spanned between the corners, a 30 corner NURBS objects can still bear a lot
> of detail - highest detail if you wish. in order to approximate a polygon
> mesh to the NURBS object you would probably require a few thousand vertices
> for good resolution, that's true.
>
> NURBS is a very powerful kind of 3d model representation and ideally suited
> for dynamic LOD, because it makes "decomposing" unnecessary: you just need
> to retesselate parametric NURBS objects given different tesselation
> thresholds, which produces polygon meshes with corresponding resolutions
> (=number of vertices).
>
> when you have a NURBS object in parametric form, the fine thing is that you
> can tesselate it to your preferred resolution, without introducing artifacts
> like some mesh reduction algorithms do.
>
> tesselating a NURBS object to, say 5000 vertices and then mesh-reducing it
> later to 3000 vertices would be possible, but you get higher quality by
> instead retesselating the original parametric NURBS object, this time
> specifying a lower precision.
>
> "tesselation" refers to breaking up the welded plane spanned up by a number
> of bezier curves and is, as far as i know, a very simple, fast and
> straightforward process. basically, you only have to evaluate the NURBS
> function given some "x" and "y" parameter for obtaining the vertices.
> (correct me if i'm wrong.)
>
> what i think is really cool is to have your model files contain NURBS in
> parametric form and when loading them, the system decides how precisely to
> tesselate them into polygons based on the system speed. in this case you
> could also specify something like "GENERATE_LOD" when loading, and the
> system would create a number of LOD geometries simply by specifying
> different tesselation thresholds (again, depending on the system
> performance).
>
> this means, the super-power-user with his 999 THz 128x pipeline accelerator
> card can enjoy a perfectly rounded model, while a TNT1 owner still sees an
> adequate approximation with 200
> vertices or so - and all this would happen automagically for any NURBS model
> imported.
>
> all you need to implement this "vision" is a NURBS tesselator (which is
> available in source-code on the net, if not java then c source code), a 3d
> file format that supports parametric NURBS (also available), and a cool
> operator who can create good parametric NURBS models.
>
> i can find out the URL of the NURBS tesselator C source code if you need it.
>
> >> Has anyone gotten something like this working for the Java3d geometry
> classes?
>
> me not, but i suspect java is a little too slow for realtime mesh-reduction
> or realtime tesselation. static (pre-calculated) mesh-reduction should
> however be no problem, i already saw a number of (non-java, put adaptable)
> algorithms on the net. again, i may point to the excellent Graphics Gems,
> not sure but it should contain code on that matter. find the url in my
> previous postings.
>
> there are also ready-made products that enable you to load an object and
> reduce it to a given number of polygons, then save it in a number of
> formats - if this is what you need.
>
> i'm much of interested in your proceedings, please keep us up-to-date.
>
> -- julian
>
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