## Equal Curve Properties

C3D Solver supports equality of four curve properties: lengths
GCE_AddEqualLength, radii GCE_AddEqualRadius, offsets of equidistant
curves GCE_AddEqualOffset, and curvatures at specified points
GCE_AddEqualCurvature. Let's explore each of these types.

The lengths of two curves can be equalized using the GCE_AddEqualLength
function.

```cpp
constraint_item GCE_AddEqualLength(GCE_system gSys, geom_item curve1, 
                                                    geom_item curve2);
```

It takes a constraint system **gSys** and two curves, **curve1** and
**curve2**, and returns a descriptor of a new constraint.

The Equal Length constraint can be applied to curves that either have
endpoints (i.e., queries for their GCE_FIRST_END and GCE_SECOND_END
coordinates return a result other than GCE_NULL_G) or are closed. This
constraint is supported for any combination of curves for which the
[GCE_AddLength](./Length_Fixing.md) constraint is available.

The radii of two curves can be equalized using the GCE_AddEqualRadius
function.

```cpp
constraint_item GCE_AddEqualRadius(GCE_system gSys, geom_item c1, 
                                                    geom_item c2);
```

It takes a constraint system **gSys** and two curves, **curve1** and
**curve2**, and returns a descriptor of a new constraint.

The Equal Radius constraint can be applied to curves for which querying
the GCE_RADIUS value is valid.

The offsets of two equidistant curves can be equalized using the
GCE_AddEqualOffset function.

```cpp
geom_item GCE_AddEqualOffset(GCE_system gSys, geom_item curve1, 
                                              geom_item curve2);
```

It takes a constraint system **gSys** and two equidistant curves,
**curve1** and **curve2**, and returns a descriptor of a new constraint.

The Equal Offset constraint can be applied to curves for which querying
GCE_OFFSET value is valid.

The curvatures at given points on two curves can be equalized using the
GCE_AddEqualCurvature function.

```cpp
constraint_item GCE_AddEqualCurvature(GCE_system gSys, geom_item curves[2], 
                                                       var_item tPars[2]);
```

It takes a constraint system **gSys**, two curves **curves**, and
parameters (if required), in which curvatures should be equal, and
returns a descriptor of a new constraint

If given a GCE_CIRCLE or a GCE_BOUNDED_CURVE, based on a circle, the
corresponding tPars\[i\] is expected to be set to GCE_NULL_V, since the
curvature of a circle is same at every point. For a GCE_LINE or a
GCE_LINE_SEGMENT the method is applied only in conjunction with
GCE_SPLINE.