Re: [Algorithms] Re: Heightfield to NURBS conversion

[Christian R. M. K. <chr...@pu...>]

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