[docu] Docu for the tool ExtractBoundary.

Author: TomFischer

web/content/docs/tools/meshing-submeshes/extract-boundary/ExtractBoundary_square_1x1_quad_border.png

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+date = "2020-01-17T5:56:57+01:00"
+title = "Extract Boundary"
+author = "Thomas Fischer"
+
+[menu]
+  [menu.tools]
+    parent = "meshing-submeshes"
++++
+
+## General
+
+The tool extracts either lines in case of a 2D bulk mesh as input or
+quads/triangles in case of a 3D bulk mesh as input. The input mesh can be given
+either in the vtu or msh format. Since the algorithm uses the element surface
+normals a correct node ordering of the element nodes is required.
+
+## Usage
+
+```
+ExtractBoundary -i [<file name of input mesh>] [-o <file name of output mesh>]
+    [--face-property-name <string>]
+    [--element-property-name <string>]
+    [--node-property-name <string>]
+```
+
+The data arrays added to the boundary mesh by using the options
+`--face-property-name` (default value 'bulk_face_ids'),
+`--element-property-name` (default value 'bulk_element_ids'),
+and `--node-property-name` (default value 'bulk_node_ids')
+are used in other tools (for instance in
+[ComputeNodeAreasFromSurfaceMesh]({{< ref "compute-node-areas-from-surface-mesh" >}}))
+and are required for flux calculations during a simulation run of OpenGeoSys.
+
+## Examples
+
+# Extract the boundary from a quad mesh
+
+`ExtractBoundary -i square_1x1_quad.vtu -o square_1x1_quad_border.vtu`
+
+![](ExtractBoundary_square_1x1_quad_border.png)
+
+In the figure above the square mesh consists of 16 cells/elements. The numbers
+in the cells are the cell IDs. The generated boundary grid consists of the
+somewhat thicker and colored line elements.
+
+# Extract the boundary from a tri mesh
+
+`ExtractBoundary -i square_1x1_tri.vtu -o square_1x1_tri_border.vtu`
+
+![](ExtractBoundary_square_1x1_tri_border.png)
+
+In the figure above the square mesh consists of 32 triangle shaped cells. The
+numbers in the tri are the cell IDs. The generated boundary grid consists of the
+somewhat thicker and colored line elements.