PK_ERROR_code_t     PK_CURVE_eval_curvature
(
--- received arguments ---
PK_CURVE_t          curve,            --- curve to receive message
double              t,                --- parametric position on curve

--- returned arguments ---
PK_VECTOR1_t *const tangent,          --- tangent
PK_VECTOR1_t *const principal_normal, --- principal normal
PK_VECTOR1_t *const binormal,         --- binormal
double       *const curvature         --- curvature
)


This function calculates the tangent, principal normal, binormal and
curvature of a curve at the given parametric position.


Specific Errors:

    PK_ERROR_at_terminator         t at terminator of curve
    PK_ERROR_invalid_geometry      curve fails to pass checks
    PK_ERROR_bad_parameter         curve cannot be evaluated at t


The tangent, principal_normal and binormal are returned as unit vectors.
If the curvature is very small, then the normals are not significant.

The curve must be capable of passing the checks imposed by PK_GEOM_check.