nurbscurve(3G) nurbscurve(3G)
NAME
nurbscurve - controls the shape of a NURBS curve
C SPECIFICATION
void nurbscurve(long knotcount, double knotlist, long
double *ctlarray, long order, long type)
PARAMETERS
knot_count number of knots in knot_list. knot_count equals the number
of control points plus the order.
knot_list array of knot_count non-decreasing knot values.
offset offset (in bytes) between successive curve control points.
ctlarray pointer to an array of control points. The coordinates must
agree with the type specified below.
order order of the NURBS curve. order equals degree + 1, hence, a
cubic curve has an order of 4. The maximum order is given
by getgdesc (GD_NURBS_ORDER).
type type of curve:
N_P2D for a polynomial (u,v) trimming curve
N_P2DR for a rational (wu,wv,w) trimming curve
N_V3D for a polynomial (x,y,z) curve
N_V3DR for a rational (x,y,z,w) curve
DESCRIPTION
Use nurbscurve to describe a NURBS curve.
The curve is either a curve in space, or a curve used to trim (define the
visible region of) a NURBS surface. Curve types N_V3D and N_V3DR specify
curves in space; types N_P2D and N_P2DR specify curves used as trimming
curves.
Use bgncurve and endcurve to delimit the NURBS curve, unless it is a
trimming curve, in which case it will appear within a bgntrim/endtrim
block. Specify the knot sequence, the control points and the order of
the NURBS curve.
You specify trimming regions within a NURBS surface definition (see
bgnsurface) to define the visible regions of the NURBS surface. Trimming
loops are closed, oriented curves that define the trimming regions. You
can describe a trimming loop by using a series of NURBS curves, piecewise
linear curves (see pwlcurve), or both.
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nurbscurve(3G) nurbscurve(3G)
Use bgntrim to mark the beginning of a trimming loop definition; use
endtrim to mark the end of a trimming loop definition.
The system displays the region of the NURBS surface that is to the left
of the trimming curve as the parameter increases. Thus, for a counter-
clockwise oriented trimming curve, the region inside the curve is
displayed. For a clockwise oriented trimming curve, the region outside
the curve is displayed.
The offset parameter is used in case the control points are part of an
array of larger structure elements. The nurbscurve routine searches for
the n-th control point pair or triple beginning at byte address
ctlarray + n * offset.
See the Graphics Library Programming Guide for a mathematical description
of a NURBS curve.
EXAMPLE
double ctlpts [4] [3] ={
{-.75, -.75, 0.0}, {-.5, .75, 0.0}, {.5, .75, 0.0} {.75, -.75, 0.0}
};
double knots[8] = {0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0};
bgncurve()
nurbscurve(8, knots, 3*sizeof(double), &ctlpts[0] [0], 4, N_V3D);
endcurve();
SEE ALSO
bgncurve, bgnsurface, bgntrim, endcurve, endsurface, endtrim,
getnurbsproperty, nurbssurface, pwlcurve, setnurbsproperty
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