Wrong BSpline representation after ToBeziers

[artemishenkov]

Hi, I am using Tinyspline library(C#) to break down the BSpline(from dxf ) to beziers and put them into svg(

) format. When I am trying to apply ToBeziers method on one particular spline which is supposed to be a circle Tinyspline breaks down it in a different shape.
My spline:
ControlPoints:
998.24, 702.13
997.58, 693.92
1005.01, 697.45
1012.44, 700.97
1005.67, 705.65
998.9, 710.33
998.24, 702.13
Knots:
0
0
0
2.0943951023932
2.0943951023932
4.188790204786389
4.188790204786389
6.283185307179588
6.283185307179588
6.283185307179588
Degree: 2
Dimension: 2
My code(C#):
BSpline spline = new((uint)ControlPoints.Count, 2, (uint)Degree) { ControlPoints = points, Knots = Knots };

        BSpline beziers = spline.ToBeziers();

        var offset = (int)(spline.Order * spline.Dimension);
        var controls = beziers.ControlPoints;
        var length = controls.Count / offset;

        if (spline.Order == 4)
            for (int n = 0; n < length; n++)
                Beziers.Add(new GCubicBezier()
                {
                    Start = new(controls[n * offset], controls[n * offset + 1]),
                    Second = new(controls[n * offset + 2], controls[n * offset + 3]),
                    Third = new(controls[n * offset + 4], controls[n * offset + 5]),
                    End = new(controls[n * offset + 6], controls[n * offset + 7])
                });
        else if (spline.Order == 3)
            for (int n = 0; n < length; n++)
                Beziers.Add(new GQuadraticBezier
                {
                    Start = new(controls[n * offset], controls[n * offset + 1]),
                    Middle = new(controls[n * offset + 2], controls[n * offset + 3]),
                    End = new(controls[n * offset + 4], controls[n * offset + 5])
                });
        else if (spline.Order == 2)
            for (int n = 0; n < length; n++)
                Beziers.Add(new GSporadicBezier
                {
                    Start = new(controls[n * offset], controls[n * offset + 1]),
                    End = new(controls[n * offset + 2], controls[n * offset + 3])
                });

I would appreciate any help. Thank you.

[msteinbeck]

It looks like you are mixing NURBS and B-Splines. I will take a closer look.

[msteinbeck]

In your screenshot you can see that there is a "Weight" component associated to your data points. The weight is probably not 1 for all control points (otherwise your B-Spline cannot form a circle; see: https://en.wikipedia.org/wiki/Non-uniform_rational_B-spline#Example:_a_circle). You have to encode your control points as homogeneous coordinates, pass them to TinySpline, and decode them back to the presentation supported by your rendering code.

[artemishenkov]

Thank you for your response @msteinbeck. You are totally right the Weights are different. I will try to follow your response.