Thread: RE: [Algorithms] Heightfield to NURBS conversion
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From: Martin F. <mf...@ac...> - 2000-09-06 10:09:55
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What you're asking for is a easy solution to what is a very very complex problem. I would suggest instead of using NURBS looking at a curve equation that actually interpolates the control points. (but break the convex hull) Catmull Rom splines immediatly spring to mind. -----Original Message----- From: Christian R. M. Koerber [mailto:chr...@pu...] Sent: 06 September 2000 02:05 To: gda...@li... Subject: Re: [Algorithms] Heightfield to NURBS conversion "Christian R. M. Koerber" wrote: > > I have a question too: > > Could anyone of you point me to an algorithm on heightfield actually I meant 'easy, ready to use algorithm' > to NURBS conversion i.e an algorithm which takes as input > an heightfield and returns the control points of the NURBS- > surface which APPROXIMATES the heightfield to a user given > error? > I'd really like to avoid meddling with the later chapters > of Piegl & Tillers NURBS book! Maybe you even know a pro- > gram? _______________________________________________ GDAlgorithms-list mailing list GDA...@li... http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list |
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From: Martin F. <mf...@ac...> - 2000-09-06 10:16:22
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This really is a very very complex problem to attempt to solve, I don't know of any algorithms or programs that do this. What I would suggest instead is using a curve equation that interpolates all the control points. (But will break the convex hull) One such spline it the Catmull Rom spline, information about which can be found here. http://graphics.cs.ucdavis.edu/CAGDNotes/Catmull-Rom-Spline/Catmull-Rom-Spli ne.html Using Catmull Rom splines in certainly much easier (and quicker) than dealing with NURBS. Hope this helps, :) Cheers, Martin -----Original Message----- From: Christian R. M. Koerber [mailto:chr...@pu...] Sent: 06 September 2000 02:05 To: gda...@li... Subject: Re: [Algorithms] Heightfield to NURBS conversion "Christian R. M. Koerber" wrote: > > I have a question too: > > Could anyone of you point me to an algorithm on heightfield actually I meant 'easy, ready to use algorithm' > to NURBS conversion i.e an algorithm which takes as input > an heightfield and returns the control points of the NURBS- > surface which APPROXIMATES the heightfield to a user given > error? > I'd really like to avoid meddling with the later chapters > of Piegl & Tillers NURBS book! Maybe you even know a pro- > gram? _______________________________________________ GDAlgorithms-list mailing list GDA...@li... http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list |
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From: Christian R. M. K. <chr...@pu...> - 2000-09-06 10:59:43
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Martin Fuller wrote: > > This really is a very very complex problem to attempt to solve, I don't know > of any algorithms or programs that do this. > What I would suggest instead is using a curve equation that interpolates all > the control points. (But will break the convex hull) > One such spline it the Catmull Rom spline, information about which can be > found here. > > http://graphics.cs.ucdavis.edu/CAGDNotes/Catmull-Rom-Spline/Catmull-Rom-Spli > ne.html Though I can't make use of them, I'd like to have a look at those. Could it be that this link is broken? Christian |
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From: Garry C. M. <wb...@di...> - 2000-09-06 12:32:35
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Works for me, copy and paste the URL and add the two parts together, the mailer has split the long line... Garry. ----- Original Message ----- From: "Christian R. M. Koerber" <chr...@pu...> > > > > http://graphics.cs.ucdavis.edu/CAGDNotes/Catmull-Rom-Spline/Catmull-Rom-Spli > > ne.html > > Though I can't make use of them, I'd like to have a look at those. Could it be > that this link is broken? > > Christian > _______________________________________________ > GDAlgorithms-list mailing list > GDA...@li... > http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list |
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From: Martin F. <mf...@ac...> - 2000-09-06 10:25:18
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Sorry accidentally sent that prematurely. (Damm short cut keys) -----Original Message----- From: Martin Fuller [mailto:mf...@ac...] Sent: 06 September 2000 03:05 To: 'gda...@li...' Subject: RE: [Algorithms] Heightfield to NURBS conversion What you're asking for is a easy solution to what is a very very complex problem. I would suggest instead of using NURBS looking at a curve equation that actually interpolates the control points. (but break the convex hull) Catmull Rom splines immediatly spring to mind. -----Original Message----- From: Christian R. M. Koerber [mailto:chr...@pu...] Sent: 06 September 2000 02:05 To: gda...@li... Subject: Re: [Algorithms] Heightfield to NURBS conversion "Christian R. M. Koerber" wrote: > > I have a question too: > > Could anyone of you point me to an algorithm on heightfield actually I meant 'easy, ready to use algorithm' > to NURBS conversion i.e an algorithm which takes as input > an heightfield and returns the control points of the NURBS- > surface which APPROXIMATES the heightfield to a user given > error? > I'd really like to avoid meddling with the later chapters > of Piegl & Tillers NURBS book! Maybe you even know a pro- > gram? _______________________________________________ GDAlgorithms-list mailing list GDA...@li... http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list _______________________________________________ GDAlgorithms-list mailing list GDA...@li... http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list |
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From: Dave F. <df...@ra...> - 2000-09-06 16:29:30
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>Could anyone of you point me to an algorithm on heightfield >to NURBS conversion i.e an algorithm which takes as input >an heightfield and returns the control points of the NURBS- >surface which APPROXIMATES the heightfield to a user given >error? There are many techniques for this, ranging from the top-down approach that Dave suggests to the bottom-up approach that removes knots from a dense NURBS mesh. When you say it must be NURBS, do you mean it must have non-uniform knot spacing and non-zero wieghts for the control points, or would you be just as happy with a rational form of a standard B-spline? (ie. uniform knots + all wieghts at one). Do you want quadratic, cubic or quartic surfaces? Another way of putting this is to ask if you are happy with a smooth approximation or do you want a surface that has sharp edges in it? Also do you want just a single surface to do the approximation or are multiple surfaces ok? (If the latter then NURBs with triple end knots are equivalent to Bezier patches). Is the hieghtfield a grid or scattered data? What modelling system are you using? Most have some sort of "shrink-wrap" facility built-in (or with a plug-in) that you can wrangle into doing this for you (though probably quite slowly). Finally, how many times do you have to do this? If it is just once, then contract a company like Paraform to do the fitting for you. (or you can buy their software). Dave |
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From: Christian R. M. K. <chr...@pu...> - 2000-09-06 18:33:56
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Dave Forsey wrote: > > When you say it must be NURBS, do you mean it must have non-uniform > knot spacing and non-zero wieghts for the control points, or > would you be just as happy with a rational form of a standard > B-spline? (ie. uniform knots + all wieghts at one). It would to at first. > > Do you want quadratic, cubic or quartic surfaces? Cubic or quartic. > Another way of putting this is to ask if you are happy with a > smooth approximation or do you want a surface that has sharp > edges in it? I didn't get that right, I'm afraid. If the Heightfield implies sharp edges I'd need them. > Also do you want just a single surface to do the approximation > or are multiple surfaces ok? (If the latter then NURBs with triple > end knots are equivalent to Bezier patches). It needs to be a single one. > > Is the hieghtfield a grid or scattered data? It's distributed on a regular Grid. > What modelling system are you using? Most have some sort of "shrink-wrap" > facility built-in (or with a plug-in) that you can wrangle > into doing this for you (though probably quite slowly). We work together with geologist who model their data via single NURBS, with weights unequal zero and all that stuff. I'm working on a system for fast visualization of them. The Renderer works already. What I want to do, is to test if it works well for 'nurbsified' Heightfields too. And I want to take advantage of the fact that NURBS should be capable to model the same terrain as the heightfield with viewer controlpoints, since for flat areas few points are sufficient. That's why I need appro- ximation and not interpolation. I read over the approximation chapter in the NURBS Book and it's pretty complicated. I'm afraid it would take same time to implement this. So I thought maybe anyone here did this already. > Finally, how many times do you have to do this? If it is just once, > then contract a company like Paraform to do the fitting for you. > (or you can buy their software). That's impossible, unfortunately. Christian |
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From: Christian R. M. K. <chr...@pu...> - 2000-09-06 10:56:41
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Martin Fuller wrote: > > What you're asking for is a easy solution to what is a very very complex > problem. I would suggest instead of using NURBS looking at a curve > equation that actually interpolates the control points. (but break the > convex hull) Catmull Rom splines immediatly spring to mind. Unfortunately it MUST be NURBS. The project I participate in uses them and there is no way around that. And it MUST be approximating since I need fewer control points than there are samples in the heightfield. Christian |
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