NON-UNIFORM B-SPLINE CURVE 3

PBSC3

CALL SEQUENCE

CALL PBSC3(SORD, NKA, KNOTS, PARL, RTYPE, NCP, PXA, PYA, PZA, PWA)

INPUT PARAMETERS

INTEGER SORD

Spline order

INTEGER NKA

Number of knots

INTEGER KNOTS(NKA)

Knots

REAL PARL(2)

Parameter range limits

INTEGER RTYPE

Rationality(PRAT,PNRAT)

INTEGER NCP

Number of control points

REAL PXA(NCP)

REAL PYA(NCP)

REAL PZA(NCP),PWA(NCP)

Control points(MC)

PHIGS STATE

EFFECT

This function specifies the non-uniform B-spline curve primitive.

The NON-UNIFORM B-SPLINE CURVE 3 element is inserted in the open structure after element pointer or replaces the element indicated by element pointer, depending on the edit mode value. Element pointer is updated and points to the NON-UNIFORM B-SPLINE CURVE 3 structure element. The values specified in the function are associated with the element.

The spline order must be a positive integer. The knots must form a non-decreasing sequence of numbers.

The RTYPE parameter may have the value PRAT or PNRAT. When PRAT is specified the control points are specified as homogeneous modelling coordinates (PXA,PYA,PZA,PWA), with the restriction that the fourth coordinate be greater than zero. When PNRAT is specified the control points are non-homogeneous PHIGS modelling coordinates (PXA,PYA,PZA).

The number of control points must be at least as large as the order. The number of control points plus the spline order must equal the number of knots.

The parameter range limit values, PARL(1) and PARL(2), specify over what range the B-spline curve is generated. PARL(1) must be less than PARL(2). PARL(1)must be greater than or equal to the SORD-th knot value. PARL(2) must be less than or equal to the (NKA+ 1 - SORD) - th knot value .

When an element of this type is interpreted a non-uniform B-spline curve is generated for the parameter values between tmin an tmax. No curve is generated if the curve order is not supported. The current values of the non-uniform B-spline curve attributes specified in and stored in the traversal state list are bound to the primitive.

Attributes associated with Non-Uniform B-Spline Curve 3 :

LINETYPE

LINEWIDTH SCALE FACTOR

LINE COLOUR

CURVE APPROXIMATION CRITERIA

EXTENSION TO THE PHIGS STANDARD

If used in NRS mode, the effect of this function is different from the effect described in the standard.

In NRS mode, (see OPEN PIPE), the element is not stored in a structure. It is sent to the workstation on which the pipe is opened.

ERRORS

5

Ignoring function, function requires state (PHOP,*, STOP,*)

500

Ignoring function, the specified order is less than one

501

Ignoring function, not enough control points for the specified order

502

Ignoring function, the specified order is inconsistent with number of knots and control points

503

Ignoring function, the knot sequence is not non-decreasing

506

Ignoring function, the parameter range is inconsistent with the knots

507

Ignoring function, the fourth coordinate of a rational control point is less than or equal to zero

SEE ALSO

open structure, open pipe (ext), non-uniform b-spline curve 3 with colour, polyline, set curve approximation criteria, inquire current element content, inquire current element type and size