public Arc(Vec startPt, Vec endPt, Vec ptOnArc)
{
Vec midPt1 = Vec.Multiply(Vec.Sum(endPt, ptOnArc), 0.5);
Vec midPt2 = Vec.Multiply(Vec.Sum(startPt, ptOnArc), 0.5);
Vec seg1 = Vec.Difference(ptOnArc, startPt);
Vec seg2 = Vec.Difference(endPt, ptOnArc);
Vec planeZ = Vec.Cross(seg1, seg2);
Vec rad1 = Vec.Cross(planeZ, seg1);
Vec rad2 = Vec.Cross(planeZ, seg2);
Vec center = Vec.IntersectLines(midPt1, rad1, midPt2, rad2);
Vec planeX = Vec.Difference(startPt, center);
Vec planeY = Vec.Cross(planeZ, planeX);
_plane = new Plane(center, planeX, planeY, planeZ);
_radius = Vec.Difference(center, startPt).Length;
_startAngle = 0;
_endAngle = 2 * Vec.AngleBetween(rad1, rad2);
}