/***************************************************/
public static Vector TangentAtParameter(this PolyCurve curve, double parameter, double tolerance = Tolerance.Distance)
{
double length = curve.Length();
double paramTol = tolerance / length;
if (parameter > 1 + paramTol || parameter < 0 - paramTol)
{
return(null);
}
double cLength = parameter * length;
foreach (ICurve c in curve.SubParts())
{
double l = c.ILength();
if (l >= cLength)
{
return(c.ITangentAtParameter(cLength / l));
}
cLength -= l;
}
return(curve.EndDir());
}