Last Updated January 21, 2009

Catmull-Rom splines in XNA

The XNA Vector3 class provides a CatmullRom method to generate points on a Catmull-Rom curve. The function takes the 4 control points (Vector3) and a scalar between 0 & 1, indicating how far along the interval between the central control points we need.

e.g.

v=Vector3.CatmullRom(p1, p2, p3, p4,0.25);
//returns a point on the curve, at 25% of the distance between p2 & p3

The following code segment, adapted from the book Professional XNA Game Programming, shows how to convert an list of control points (unlimited control points permitted, minimum 4), into a list of points which can be used to build a Catmull-rom curve;

void GetTrackPoints()
        {
            for (int num = 0; num < inputpoints.Count; num++)
            {
                // Get the 4 required points for the catmull rom spline
                Vector3 p1 = inputpoints[num - 1 < 0 ? inputpoints.Count - 1 : num - 1];
                Vector3 p2 = inputpoints[num];
                Vector3 p3 = inputpoints[(num + 1) % inputpoints.Count];
                Vector3 p4 = inputpoints[(num + 2) % inputpoints.Count];

                // Calculate number of iterations we use here based
                // on the distance of the 2 points we generate new points from.
                float distance = Vector3.Distance(p2, p3);
                int numberOfIterations =
                    (int)(segmentsPer100Meters * (distance / 100.0f));
                if (numberOfIterations <= 0)
                    numberOfIterations = 1;

                for (int iter = 0; iter < numberOfIterations; iter++)
                {
                    Vector3 newVertex = Vector3.CatmullRom(p1, p2, p3, p4, iter / (float)numberOfIterations);
                    points.Add(newVertex);
                } // for (iter)
            } // for (num)
        }