IfcFirstProjAxis

Definition from ISO/CD 10303-42:1992: This function produces a three dimensional direction which is, with fully defined input, the projection of arg onto the plane normal to the z-axis. With arg defaulted the result is the projection of (1.0,0.0,0.0) onto this plane except that if z-axis = (1.0,0.0,0.0) then (0.0,1.0,0.0) is used as initial value of arg. A violation occurs if arg is in the same direction as the input z-axis.

NOTE Corresponding STEP function: first_proj_axis. Please refer to ISO/IS 10303-42:1994, p.102 for the final definition of the formal standard.

HISTORY New function in IFC Release 1.5

EXPRESS specification:

FUNCTION IfcFirstProjAxis

  (ZAxis, Arg : IfcDirection) : IfcDirection;
LOCAL
  XAxis : IfcDirection;
  V     : IfcDirection;
  Z     : IfcDirection;
  XVec  : IfcVector;
END_LOCAL;

  IF (NOT EXISTS(ZAxis)) THEN
    RETURN (?) ;
  ELSE
    Z := IfcNormalise(ZAxis);
    IF NOT EXISTS(Arg) THEN
      IF (Z.DirectionRatios <> [1.0,0.0,0.0]) THEN
        V := IfcRepresentationItem() || IfcGeometricRepresentationItem () || IfcDirection([1.0,0.0,0.0]);
      ELSE
        V := IfcRepresentationItem() || IfcGeometricRepresentationItem () || IfcDirection([0.0,1.0,0.0]);
      END_IF;
    ELSE
      IF (Arg.Dim <> 3) THEN
        RETURN (?) ;
      END_IF;
      IF ((IfcCrossProduct(Arg,Z).Magnitude) = 0.0) THEN
        RETURN (?);
      ELSE
        V := IfcNormalise(Arg);
      END_IF;
    END_IF;
    XVec  := IfcScalarTimesVector(IfcDotProduct(V, Z), Z);
    XAxis := IfcVectorDifference(V, XVec).Orientation;
    XAxis := IfcNormalise(XAxis);
  END_IF;
  RETURN(XAxis);

END_FUNCTION;