VTKBoundaryEdges类可以提取四种边界，分别为boundary edge, feature edge, non-manifold edge, manifold edge。通过这四种情况，基本上可以提取在曲面上任意的边界信息。
These edges are either
这些边界包括几种情况：
1) boundary (used by one polygon) or a line cell;
边界，即只属于一个多边形或者一个线结构。
2) non-manifold (used by three or more polygons);
非流形边界，同时属于三个或更多的多边形结构。
3) feature edges (edges used by two triangles and whose dihedral angle > FeatureAngle);
特征边界：同时属于两个三角形，并且两个三角形之间的夹角大于特征角度（特征角度-feature angle是可以进行设置的）。
4) manifold edges (edges used by exactly two polygons).
流形边界：属于两个多边形结构。
These edges may be extracted in any combination. Edges may also be "colored" (i.e., scalar values assigned) based on edge type. The cell coloring is assigned to the cell data of the extracted edges.