IFC 4.3.x development

8.9.3.7 IfcBSplineSurface

Change log

IFC4

added

8.9.3.7.1 Semantic definitions at the entity

The IfcBSplineSurface is a general form of rational or polynomial parametric surface.

NOTE  Definition according to ISO/CD 10303-42:1992
A b_spline_surface is a general form of rational or polynomial parametric surface which is represented by control points, basis functions, and possibly, weights. As with the corresponding curve entity it has some special subtypes where some of the data can be derived.

  1. The symbology used here is:
    K1 = upper_index_on_u_control_points
    K2 = upper_index_on_v_control_points
    Pij = control_points
    wij = weights
    d1 = u_degree
    d2 = v_degree

  2. The control points are ordered as
    P00, P01, P02, ......, PK1(K2-1), PK1K2
    The weights, in the case of the rational subtype, are ordered similarly.

  3. For each parameter, s = u or v, if k is the upper index on the control points and d is the degree for s, the knot array is an array of (k + d + 2) real numbers [s-d, ...., sk+1], such that for all indices j in [-d, k]; sjsj+1. This array is obtained from the appropriate u_knots or v_knots list by repeating each multiple knot according to the multiplicity.

    Nid, the ith normalised B-spline basis function of degree d, is defined on the subset [si-d, ...., si+1] of this array.

  4. Let L denote the number of distinct values amongst the knots in the knot list; L will be referred to as the ‘upper index on knots’. Let mj denote the multiplicity (i.e., number of repetitions) of the jth distinct knot value. Then:
    formula
    All knot multiplicities except the first and the last shall be in the range 1, ...., d; the first and last may have a maximum value of d+1. In evaluating the basis functions, a knot u of, e.g., multiplicity 3 is interpreted as a sequence u, u, u, in the knot array.

  5. The surface form is used to identify specific quadric surface types (which shall have degree two), ruled surfaces and surfaces of revolution. As with the b-spline curve, the surface form is informational only and the spline data takes precedence.

  6. The surface is to be interpreted as follows: In the polynomial case the surface is given by the equation:
    formula
    In the rational case the surface equation is:
    formula
  7. NOTE  Entity adapted from b_spline_surface defined in ISO10303-42.

    HISTORY  New entity in IFC4.

    8.9.3.7.1 Attributes

    # Attribute Type Description
    IfcRepresentationItem
    LayerAssignment SET [0:1] OF IfcPresentationLayerAssignment FOR AssignedItemsAssignment of the representation item to a single or multiple layer(s). The LayerAssignments can override a LayerAssignments of the IfcRepresentation it is used within the list of Items.

    > IFC2x3 CHANGE  The inverse attribute LayerAssignments has been added.

    > IFC4 CHANGE  The inverse attribute LayerAssignment has been restricted to max 1. Upward compatibility for file based exchange is guaranteed.



    StyledByItem SET [0:1] OF IfcStyledItem FOR Item Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve.



    > IFC2x3 CHANGE  The inverse attribute StyledByItem has been added.



    IfcGeometricRepresentationItem
    IfcSurface
    Dim INVALID The space dimensionality of IfcSurface. It is always a three-dimensional geometric representation item.

    > IFC4 CHANGE Derived attribute promoted from subtypes.

    IfcBoundedSurface
    IfcBSplineSurface
    1 UDegree IfcInteger Algebraic degree of basis functions in u.
    2 VDegree IfcInteger Algebraic degree of basis functions in v.
    3 ControlPointsListLIST [2:?] OF LIST [2:?] OF IfcCartesianPoint This is a list of lists of control points.
    4 SurfaceForm IfcBSplineSurfaceForm Indicator of special surface types.
    5 UClosed IfcLogical Indication of whether the surface is closed in the u direction; this is for information only.
    6 VClosed IfcLogical Indication of whether the surface is closed in the v direction; this is for information only.
    7 SelfIntersect IfcLogical Flag to indicate whether, or not, surface is self-intersecting; this is for information only.
    UUpper INVALID Upper index on control points in u direction.
    VUpper INVALID Upper index on control points in v direction.
    ControlPoints INVALID Array (two-dimensional) of control points defining surface geometry. This array is constructed from the control points list.

    8.9.3.7.2 Entity inheritance

    dot_inheritance IfcBSplineSurfaceWithKnots IfcBSplineSurfaceWithKnots IfcBSplineSurface IfcBSplineSurface IfcBSplineSurfaceWithKnots->IfcBSplineSurface IfcBoundedSurface IfcBoundedSurface IfcBSplineSurface->IfcBoundedSurface IfcBoundedSurface_children 3 more... IfcBoundedSurface_children->IfcBoundedSurface IfcSurface IfcSurface IfcBoundedSurface->IfcSurface IfcSurface_children 3 more... IfcSurface_children->IfcSurface IfcGeometricRepresentationItem IfcGeometricRepresentationItem IfcSurface->IfcGeometricRepresentationItem IfcGeometricRepresentationItem_children 24 more... IfcGeometricRepresentationItem_children->IfcGeometricRepresentationItem IfcRepresentationItem IfcRepresentationItem IfcGeometricRepresentationItem->IfcRepresentationItem IfcRepresentationItem_children 3 more... IfcRepresentationItem_children->IfcRepresentationItem

    8.9.3.7.3 Formal representations

    ENTITY IfcBSplineSurface
     ABSTRACT SUPERTYPE OF (ONEOF
    	(IfcBSplineSurfaceWithKnots))
     SUBTYPE OF (IfcBoundedSurface);
    	UDegree : IfcInteger;
    	VDegree : IfcInteger;
    	ControlPointsList : LIST [2:?] OF LIST [2:?] OF IfcCartesianPoint;
    	SurfaceForm : IfcBSplineSurfaceForm;
    	UClosed : IfcLogical;
    	VClosed : IfcLogical;
    	SelfIntersect : IfcLogical;
     DERIVE
    	 UUpper : IfcInteger := SIZEOF(ControlPointsList) - 1;
    	 VUpper : IfcInteger := SIZEOF(ControlPointsList[1]) - 1;
    	 ControlPoints : ARRAY [0:UUpper] OF ARRAY [0:VUpper] OF IfcCartesianPoint := IfcMakeArrayOfArray(ControlPointsList,
    0,UUpper,0,VUpper);
    END_ENTITY;