Re: NURBS & Conic Sections

[jlim@xxxxxxxxxxxxxxxxxxxx (Jonathan Lim)]

#Inventor V2.1 ascii

Coordinate4 {
    point [ 1         0         0   1,
            0.707107  0.707107  0   0.707107,
            0         1         0   1,
           -0.707107  0.707107  0   0.707107,
           -1         0         0   1,
           -0.707107 -0.707107  0   0.707107,
            0        -1         0   1,
            0.707107 -0.707107  0   0.707107,
            1         0         0   1,        ]
}
NurbsCurve {
    numControlPoints    9
    knotVector  [ 0.00, 0.0, 0.0, 0.25,
                  0.25, 0.5, 0.5, 0.75,
                  0.75, 1.0, 1.0, 1.00 ]
}

              Jonathan Lim  _  Silicon Graphics  _  Mountain View
              GPS Graphics     Computer Systems        CA, USA

On Mon Jan 14 10:59:52 2002, srclark@xxxxxxxxxxxxxx wrote:
> 
> I know it should be easy to draw circles using Bezier curves in Open 
> Inventor. I have looked at some literature and it says for example that 
> a quadratic rational bezier curve can represent a circle - the middle 
> weighting should be cos(theta) where 2 * theta is the angular extension 
> of the arc. Yet I seem to get only parabolas. The closest one looking 
> like a circle has a middle point weighting of one. ??? I am lost.
> 
> Any help would be appreciated. Here are the values I am using:
> 
> float pts[3][4] = {
>                 {1.0,0.0,1.0,1.0},
>                 {1.0,1.0,1.0,0.707},
>                 {0.0,1.0,1.0,1.0}};
> 
> float knots [6] = {0,0,0,1,1,1};