IfcTriangulatedFaceSet

IfcTriangulatedFaceSet是一个镶嵌细分的面集，所有面都由三角形绑定。面由三个笛卡尔点定义的隐式多段线构造。每个点的坐标由一个基于1的索引提供到二维列表CoordIndex提供的笛卡尔点的有序列表中，其中

①二维列表的第一个维度处理三角形面的列表；

②二维列表的第二个维度正好为IfcCartesianPointList提供了三个索引，这些索引由在超类型IfcTessellatedFaceSet中定义的坐标引用。每个索引指向一个笛卡尔点作为三角形的顶点。

可选每个顶点处的法线可以由二维列表NormalIndex提供，其中

①二维列表的第一维处理相应的三角形面列表；

②二维列表的第二个维度为在超类型IfcTessellatedFaceSet中定义的二维列表法线提供了三个索引。与顶点索引相对应的每个索引都指向三角形顶点处的法线方向。

如果对应的点和法线列表中的点的顺序与法线中的法线的顺序相同，则应使用单个索引列表。坐标索引列表将同时应用于点和法线，法线索引列表应为空。换言之，如果not empty中的法线列表和NormalIndex列表为空，则CoordIndex列表将用于指向CoordList和Normals列表中的相应位置。

注意：使用相应的点和法线列表可以只使用一个索引列表，而这会牺牲点和顶点的非唯一集合。

图显示了不带注释的IfcTriangulatedFaceSet的使用。IfcTriangulatedFaceSet的图表示索引和索引指向的有序列表。索引从1开始（索引为1到N），如果CoordIndex中的最大索引为N，则IfcCartesianPointList应具有3:3坐标的N个列表。

图-三角面集

 

实例化IfcTriangulatedFaceSet有三种可能，除了通过Coordinates和CoordIndex强制属性CoordList之外，可选属性有以下组合：

①Normals和NormalIndex都提供了，然后NormalIndex指向法线

②提供法线而不提供NormalIndex，则CoordIndex也指向法线

③法线和法线索引都未提供，则必须重新计算法线。

 图-三角形面集几何图形

CoordIndex: ((1,6,5), (1,2,6), (6,2,7), (7,2,3), (7,8,6), (6,8,5), (5,8,1), (1,8,4), (4,2,1), (2,4,3), (4,8,7), (7,3,4))
IfcCartesianPointList3D: ((0.,0.,0.), (1.,0.,0.), (1.,1.,0.), (0.,1.,0.), (0.,0.,2.), (1.,0.,2.), (1.,1.,2.), (0.,1.,2.))

注：IfcTriangulatedFaceSet的定义基于ISO/IEC 19775-1中定义的indexedFaceSet和indexedTriangleSet

IFC4中增加的新实体。

#	Attribute	Type	Cardinality	Description	C
IfcRepresentationItem
	LayerAssignment	IfcPresentationLayerAssignment @AssignedItems	S[0:1]	Assignment 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.	X
	StyledByItem	IfcStyledItem @Item	S[0:1]	Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve.	X
IfcGeometricRepresentationItem
IfcTessellatedItem
IfcTessellatedFaceSet
1	Coordinates	IfcCartesianPointList3D	[1:1]	An ordered list of Cartesian points used by the coordinate index defined at the subtypes of IfcTessellatedFaceSet.	X
2	Normals	IfcParameterValue	L[1:?]L[3:3]	An ordered list of directions used by the normal index defined at the subtypes of IfcTessellatedFaceSet. It is a two-dimensional list of directions provided by three parameter values.	X
3	Closed	IfcBoolean	[0:1]	Indication whether the IfcTessellatedFaceSet is a closed shell or not. If omited no such information can be provided.	X
	HasColours	IfcIndexedColourMap @MappedTo	S[0:1]	Reference to the indexed colour map providing the corresponding colour RGB values to the faces of the subtypes of IfcTessellatedFaceSet.	X
	HasTextures	IfcIndexedTextureMap @MappedTo	S[0:?]	Reference to the indexed texture map providing the corresponding texture coordinates to the vertices bounding the faces of the subtypes of IfcTessellatedFaceSet.	X
IfcTriangulatedFaceSet
4	CoordIndex	IfcPositiveInteger	L[1:?]L[3:3]	Two-dimensional list, where the first dimension represents the triangles (from 1 to N) and the second dimension the indices to three points defining the vertices (from 1 to 3). NOTE  The coordinates of the vertices are provided by the indexed list of SELFIfcTessellatedFaceSet.Coordinates.CoordList.	X
5	NormalIndex	IfcPositiveInteger	L[1:?]L[3:3]	Two-dimensional list, where the first dimension represents the triangle (from 1 to N) and the second dimension the indices to three normals (from 1 to 3) corresponding to the vertices. The directions of the normals are provided by the indexed list of SELFIfcTessellatedFaceSet.Normals.	X
	NumberOfTriangles :=SIZEOF(CoordIndex)	IfcInteger	[1:1]	Derived number of triangles used for this triangulation.	X

 

EXPRESS Specification

ENTITY IfcTriangulatedFaceSet
 SUBTYPE OF (IfcTessellatedFaceSet);
  CoordIndex : LIST [1:?] OF LIST [3:3] OF IfcPositiveInteger;
  NormalIndex : OPTIONAL LIST [1:?] OF LIST [3:3] OF IfcPositiveInteger;
 DERIVE
  NumberOfTriangles : IfcInteger := SIZEOF(CoordIndex);
END_ENTITY;