HM_ExtAPI::CreateCurveNURBS()

Syntax

bool CreateCurveNURBS(
HM_EntityGeometryCurve& new_nurbs,
int degree,
int num_knots,
int num_poles,
const double* knots,
const HM_Point* poles,
const double* weights
);

Type

HyperMesh Ext API Function

Description

The function creates the curve object of the type HM_ExtAPI::NURBS_CURVE.

The NURBS curve is defined by its degree D, the set of poles (also known as control points) {Pα} with index α taking values in the range 0 … N-1 where N is the total number of poles, and the set of parametric "knot" values {τβ} with index β taking values in the range 0 … M-1. In case the curve is rational then also a set of "weights" {wα} corresponding to each pole is required for the NURBS curve definition.

Parametric equation for the NURBS curve is given by

C(t) = ∑wα Pα NαD(t) / ∑ wα NαD(t)

where summation sign ∑ indicates the sum over all values of index α in the range 0 … N-1, and NαD(t) are the B-spline functions of the degree D defined by the knots vector {τβ}.

In the case of non-rational curve (the parameter weights is NULL) all pole weights are assumed to be equal and the parametric equation simplifies to

C(t) = ∑ Pα NαD(t)

Parameters of the NURBS curve for the function CreateCurveNURBS satisfy "open end" boundary conditions. This means that first D +1 knots and last D + 1 knots in the knots array knots must have the same value:

τo = τ1 = … = τD

τM -1 = τM -2 = … = τM -1 - D

The total number of knots M and the total number of poles N must satisfy the relation

M = N + D + 1

The following table clarifies correspondence between parameters used in the equations above and parameter values used by the function CreateCurveNURBS.

If the function succeeds, the return value is true. If the function fails, the return value is false. To get extended value information, call HM_ExtAPI::GetLastErrorCode().

Requires including hm_extapi.h.

Inputs

new_nurbs

[out] - Handle to new curve object.

degree

[in] - NURBS degree.

num_knots

[in] - Number of knots.

num_poles

[in] - Number of poles.

knots

[in] - Points to the array of knot values. The number of values in the array must benum_knots.

poles

[in] - Point to the array of NURBS poles. The number of values in the array must benum_poles.

weight

[in] - If not NULL then points to the array of NURBS weights. The number of values in the array in this case must be num_poles. If the values is NULL then the value of 1.0 is assumed for all poles (non-rational curve).

Errors

None.