## Spline Curves

C3D Solver uses NURBS curves as a mathematical basis for spline curves.
NURBS presentation has de-facto become an industry standard used to
present free-form curves, and it is supported by almost all CAD systems.
The NURBS Book by Les Piegl and Wayne Tiller (Springer Publishing
Company) is a widely recognized canonical book in this subject domain.

C3D Solver offers two spline definition methods:

-   a spline constructed based on a set of characteristic points defined
    by their coordinate values;

-   a spline that passes through a set of interpolation points defined
    by descriptors of previously registered geometric objects.

A spline can be created from a NURBS curve based on

```cpp
geom_item GCE_AddSpline(GCE_system gSys, GCE_spline spl).
```

The function accepts GCE_spline data structure that contains data
defining the spline: the starting values of control points, the
interpolation point array (if required), weights, closedness indicator,
etc.

A special constraint that fixes the spline derivative vector is created
by

```cpp
constraint_item GCE_FixSplineDerivative(GCE_system gSys, geom_item spline
                                     , double par
                                     , uint derOrder
                                     , GCE_vec2d * fixVal = NULL )
```