[web] Documenting geometryToGmshGeo.

web/content/docs/tools/meshing/gmsh-interface/index.md

++++
+date = "2021-05-31T11:00:13+01:00"
+title = "geometryToGmshGeo"
+author = "Thomas Fischer"
+aliases = [ "/docs/tools/meshing/gmsh-interface" ]
+
+[menu]
+  [menu.tools]
+    parent = "meshing"
++++
+
+## Introduction
+
+The tool `geometryToGmshGeo` takes OGS geometries (gml files) and creates a Gmsh
+geo file. The user can specify several command line arguments to influence the
+creation procedure. A list of the arguments as well as a description of each
+argument can be obtained with the `--help` option.
+
+## Usage
+
+```bash
+geometryToGmshGeo --input <input file name> --output <output file>
+```
+The `--input` argument is accepted multiple times.
+
+## Simple examples
+
+### First Example: Simple Geometry
+
+First, the Gmsh geometry file is create from the gml file [square_1x1.gml](square_1x1.gml):
+```bash
+geometryToGmshGeo -i square_1x1.gml -o /tmp/square_1x1.geo --mesh_density_scaling_at_points 0.05
+```
+
+Then, the Gmsh geometry can be meshed:
+```bash
+gmsh /tmp/square_1x1.geo -2 -algo meshadapt -format msh22 -o /tmp/square_1x1.msh
+```
+
+![geometry](square_1x1.gml.png#OneThird "1x1 square geometry")
+![coarse mesh](square_1x1_adaptive_point_density_0.5.png#OneThird "simple coarse mesh (density scaling 0.5)")
+![fine mesh](square_1x1_adaptive_point_density_0.05.png#OneThird "simple fine mesh (density scaling 0.05)")
+![even finer mesh](square_1x1_adaptive_point_density_0.005.png "even finer mesh (density scaling 0.005)")
+
+### Second Example: Simple Geometry with Additional Geometrical Information
+
+First, the Gmsh geometry file is create from the gml files
+ - [square_1x1.gml](square_1x1.gml)
+ - [square_0.15_0.25x0.15_0.25.gml](square_0.15_0.25x0.15_0.25.gml)
+ - [square_0.45_0.55x0.45_0.55.gml](square_0.45_0.55x0.45_0.55.gml)
+
+ ```bash
+geometryToGmshGeo -i square_1x1.gml
+                   -i square_0.15_0.25x0.15_0.25.gml
+                   -i square_0.45_0.55x0.45_0.55.gml
+                   -o /tmp/square_1x1.geo --mesh_density_scaling_at_points 0.5
+```
+
+Then, the Gmsh geometry can be meshed:
+```bash
+gmsh /tmp/square_1x1.geo -2 -algo meshadapt -format msh22 -o /tmp/square_1x1.msh
+```
+
+![geometry](square_1x1_plus_subgeometries.png#OneThird "1x1 square geometry and sub geometries")
+![coarse mesh](square_1x1_plus_subgeometries_adaptive_point_density_0.5.png#OneThird "coarse mesh (density scaling 0.5)")
+![fine mesh](square_1x1_plus_subgeometries_adaptive_point_density_0.05.png#OneThird "fine mesh (density scaling 0.05)")

web/content/docs/tools/meshing/gmsh-interface/square_0.15_0.25x0.15_0.25.gml

+<?xml version="1.0" encoding="ISO-8859-1"?>
+<?xml-stylesheet type="text/xsl" href="OpenGeoSysGLI.xsl"?>
+
+<OpenGeoSysGLI xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ogs="http://www.opengeosys.org">
+    <name>square_0.15_0.25x0.15_0.25_geometry</name>
+    <points>
+        <point id="0" x="0.15" y="0.15" z="0"/>
+        <point id="1" x="0.15" y="0.25" z="0"/>
+        <point id="2" x="0.25" y="0.15" z="0"/>
+        <point id="3" x="0.25" y="0.25" z="0"/>
+    </points>
+
+    <polylines>
+        <polyline id="4" name="boundary">
+            <pnt>0</pnt>
+            <pnt>1</pnt>
+            <pnt>3</pnt>
+            <pnt>2</pnt>
+            <pnt>0</pnt>
+        </polyline>
+    </polylines>
+</OpenGeoSysGLI>

web/content/docs/tools/meshing/gmsh-interface/square_0.45_0.55x0.45_0.55.gml

+<?xml version="1.0" encoding="ISO-8859-1"?>
+<?xml-stylesheet type="text/xsl" href="OpenGeoSysGLI.xsl"?>
+
+<OpenGeoSysGLI xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ogs="http://www.opengeosys.org">
+    <name>square_0.45_0.55x0.45_0.55_geometry</name>
+    <points>
+        <point id="0" x="0.45" y="0.45" z="0"/>
+        <point id="1" x="0.45" y="0.55" z="0"/>
+        <point id="2" x="0.55" y="0.45" z="0"/>
+        <point id="3" x="0.55" y="0.55" z="0"/>
+    </points>
+
+    <polylines>
+        <polyline id="4" name="boundary">
+            <pnt>0</pnt>
+            <pnt>1</pnt>
+            <pnt>3</pnt>
+            <pnt>2</pnt>
+            <pnt>0</pnt>
+        </polyline>
+    </polylines>
+</OpenGeoSysGLI>

web/content/docs/tools/meshing/gmsh-interface/square_1x1.gml

+<?xml version="1.0" encoding="ISO-8859-1"?>
+<?xml-stylesheet type="text/xsl" href="OpenGeoSysGLI.xsl"?>
+
+<OpenGeoSysGLI xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ogs="http://www.opengeosys.org">
+    <name>square_1x1_geometry</name>
+    <points>
+        <point id="0" x="0" y="0" z="0"/>
+        <point id="1" x="0" y="1" z="0"/>
+        <point id="2" x="1" y="0" z="0"/>
+        <point id="3" x="1" y="1" z="0"/>
+    </points>
+
+    <polylines>
+        <polyline id="0" name="boundary">
+            <pnt>0</pnt>
+            <pnt>1</pnt>
+            <pnt>3</pnt>
+            <pnt>2</pnt>
+            <pnt>0</pnt>
+        </polyline>
+    </polylines>
+</OpenGeoSysGLI>