IfcVectorDifference

注：定义符合ISO/CD 10303-42:1992

此函数将输入参数的差值返回为（Arg1-Arg2）。函数返回两个输入向量的向量差作为向量。输入参数应具有相同的维数，但可以是方向或向量。如果两个输入参数都是向量，则它们必须用相同的单位表示，如果两个输入参数都是方向，则生成无单位结果。零差向量产生零量级的向量。

注：根据ISO 10303-42中不同定义的向量_改编的函数。

IFC1.5中的新函数

EXPRESS Specification

FUNCTION IfcVectorDifference
  (Arg1, Arg2 : IfcVectorOrDirection)
    : IfcVector;
LOCAL
  Result : IfcVector;
  Res, Vec1, Vec2 : IfcDirection;
  Mag, Mag1, Mag2 : REAL;
  Ndim : INTEGER;
END_LOCAL;

  IF ((NOT EXISTS (Arg1)) OR (NOT EXISTS (Arg2))) OR (Arg1.Dim <> Arg2.Dim) THEN
    RETURN (?) ;
  ELSE
    BEGIN
      IF 'IFCGEOMETRYRESOURCE.IFCVECTOR' IN TYPEOF(Arg1) THEN
        Mag1 := Arg1IfcVector.Magnitude;
        Vec1 := Arg1IfcVector.Orientation;
      ELSE
        Mag1 := 1.0;
        Vec1 := Arg1;
      END_IF;
      IF 'IFCGEOMETRYRESOURCE.IFCVECTOR' IN TYPEOF(Arg2) THEN
        Mag2 := Arg2IfcVector.Magnitude;
        Vec2 := Arg2IfcVector.Orientation;
      ELSE
        Mag2 := 1.0;
        Vec2 := Arg2;
      END_IF;
      Vec1 := IfcNormalise (Vec1);
      Vec2 := IfcNormalise (Vec2);
      Ndim := SIZEOF(Vec1.DirectionRatios);
      Mag  := 0.0;
      Res  := IfcRepresentationItem() || IfcGeometricRepresentationItem () || IfcDirection([0.0:Ndim]);

      REPEAT i := 1 TO Ndim;
        Res.DirectionRatios[i] := Mag1*Vec1.DirectionRatios[i] - Mag2*Vec2.DirectionRatios[i];
        Mag := Mag + (Res.DirectionRatios[i]*Res.DirectionRatios[i]);
      END_REPEAT;

      IF (Mag > 0.0 ) THEN
        Result := IfcRepresentationItem() || IfcGeometricRepresentationItem () || IfcVector( Res, SQRT(Mag));
      ELSE
        Result := IfcRepresentationItem() || IfcGeometricRepresentationItem () || IfcVector( Vec1, 0.0);
      END_IF;
    END;
  END_IF;
  RETURN (Result);
END_FUNCTION;