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Technoflash

Nurbs curve exactly thru 3D locus points

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Here's a random 2D section thru that NURBS geodesic which shows the relationship of a few of the curves created at medium resolution for 2D & 3D. These images clearly illustrate the 'tolerance' issues discussed in this post:

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A closer zoomed in look at those curves:

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hi Biplab

I sent you the file (called "Boat") showing this problem last week and looking back my email (subject; NURBS Accuracy) wasn't really explicit about refering to this thread.

Have you had a chance to look at the file?

cheers,

N.

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Here's a thought that's been discussed before. Clearly there's a resolution setting for both 2D and 3D in VW. The resolution setting is a way for the display code to approximate the internal code representing the nurbs curve. So then, a nurbs curve can have high precision in memory, but be approximated on screen such that it draws on screen faster. I'm guessing Nemetschek has picked an upper limit of how much resolution it will display a nurbs curve on-screen...using the "very high" setting....and it appears that the resolution chosen by Nemetschek is not high enough for some. See the screenshot below. The first three views would indicate that the nurbs curve does indeed pass through all the loci..but when you zoom in, it clearly does not pass through the loci and in addition, the curve is segmented...these screenshots were taken with the all resolutions maxed out to very high. If you've ever found yourself trying to write code to draw a curve on-screen, you'll find yourself drawing a bunch of short line segments to approximate the curve...and if you've ever done a real good job of drawing a curve on screen by using so many line segments, then you'll find yourself trying to explain to end users why your curves redraw so slow. With regards to hard segments on your lofted hull, I can't explain that without seeing the original file...but then again, since the past post was almost a month ago, I could be standing in this room all by myself talking at the walls!

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[ 10-04-2005, 06:57 PM: Message edited by: tom kyler ]

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Hi Tom

No, I'm still listening LOL. Thanks for your thoughts. I've had to go back to my "day job" so this has gone on the back burner for a while.

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I was trying to design a boat with VW and gave up. Look at TouchCad. It does marine calculations. The control points on the mesh are alway on the mesh. It is 100% compatible with VW ... both ways. There are tutorials that show how easy it is to draw a hull on the TouchCad site.

Also, I can attest to the fairness of the hull lines.

www.touchcad.com

www.boatdesign.net forums has some posts on TouchCAD

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