/*! \page constructing_meshes_page Constructing meshes To create a mesh on geometry, at first you create a mesh object by choosing - a geometrical shape produced in the Geometry module (main shape); - meshing parameters, including - \ref basic_meshing_algos_page "meshing algorithms" and - \ref about_hypo_page "hypotheses" specifying constraints to be taken into account by chosen meshing algorithms. Then you already can launch mesh generation by invoking \ref compute_anchor "Compute" command. \note Sometimes \a hypotheses term is used to refer to both algorithms and hypotheses. Generation of the mesh on the geometry is performed in the bottom-up flow: nodes on vertices are created first, then edges are divided into segments using nodes on vertices; the segments of the edges is then used while meshing faces; then the mesh of the faces is used while meshing solids. This automatically assures the conformity of the mesh. You are to choose a meshing algorithm for every dimension of sub-shapes up to the highest dimension you desire to generate. Note that some algorithms generate elements of several dimensions while others, of only one. But it's not necessary to define meshing parameters for all dimensions at once; you can start from 1D meshing parameters only, compute the 1D mesh, then define 2D meshing parameters and compute the 2D mesh (note that 1D mesh won't be re-computed). An algorithm of a certain dimension chosen at mesh creation is applied to discretize every sub-shape of this dimension. But you can specify a different algorithm or hypothesis to be applied to one or a group of sub-shapes by creating a \ref constructing_submeshes_page "sub-mesh". You can specify no algorithms at all at mesh object creation and specify the meshing parameters on sub-meshes only; then only sub-shapes for which you defined an algorithm and a needed hypothesis (if any) will be discretized. \n Construction of a mesh on some geometry includes at least two (mesh creation and computing) of the following steps: \anchor create_mesh_anchor

Creation of a mesh object

To construct a mesh:
  1. Select a geometrical object for meshing.
  2. In the \b Mesh menu select Create Mesh or click "Create Mesh" button in the toolbar.
    \image html image32.png "Create Mesh" button
    The following dialog box will appear: \image html createmesh-inv.png
  3. To filter off irrelevant meshing algorithms, you can select Mesh Type in the corresponding list from Any, Hexahedral, Tetrahedral, Triangular and \b Quadrilateral (there can be less items for the geometry of lower dimensions). Selection of a mesh type hides all meshing algorithms that cannot generate elements of this type.
  4. Apply \subpage basic_meshing_algos_page "meshing algorithms" and \subpage about_hypo_page "hypotheses" which will be used to compute this mesh. "Create mesh" dialog box contains several tab pages titled \b 3D, \b 2D, \b 1D and \b 0D. The title of each page reflects the dimension of the sub-shapes the algorithms listed on this page affect and the maximal dimension of elements the algorithms generate. For example, \b 3D page lists the algorithms that affect 3D sub-shapes (solids) and generate 3D mesh elements (tetrahedra, hexahedra etc.) As soon as you have selected an algorithm, you can create (or select already created) a hypothesis. A set of accessible hypotheses includes only hypotheses the selected algorithm can take into account. \note - Some page(s) can be disabled if the geometrical object does not include shapes (sub-shapes) of the corresponding dimension(s). For example, if the input object is a geometrical face, \b 3D page is disabled. - Some algorithms affect the geometry of several dimensions, i.e. 1D+2D or 1D+2D+3D. If such an algorithm is selected, the dialog box pages related to the corresponding lower dimensions are disabled. - \b 0D page does not refer to the 0D elements, but to 0D geometry (vertices). Mesh module does not provide algorithms that produce 0D elements. Currently \b 0D page provides only one algorithm "Segments around vertex" that allows specifying the required size of mesh edges about the selected vertex (or vertices). For example, you need to mesh a 3D object. First, you can change a default name of your mesh in the \b Name box. Then check that a selected geometrical object, whose name is shown in \b Geometry field, is that you wish to mesh; if not, click the right object in the Object Browser. Click "Select" button near \b Geometry field if the name of the object has not yet appeared in \b Geometry field.
    \image html image120.png "Select" button
    Now you can define 3D Algorithm and 3D Hypotheses, which will be applied to discretize the solids of your geometrical object using 3D elements. Click the "Add Hypothesis" button to add a hypothesis.
    \image html image121.png "Add Hypothesis" button
    Click the "Plus" button to enable adding more additional hypotheses. Click the "Edit Hypothesis" button to change the values for the current hypothesis.
    \image html image122.png "Edit Hypothesis" button
    Most 2D and 3D algorithms can work without hypotheses using default meshing parameters. Some algorithms do not require any hypotheses. After selection of an algorithm "Hypothesis" field of the dialog can contain: After selection of an algorithm Add. Hypothesis field can contain: Proceed in the same way with 2D and 1D Algorithms and Hypotheses that will be used to mesh faces and edges of your geometry. (Note that any object has edges, even if their existence is not apparent, for example, a sphere has 4 edges). Note that the choice of hypotheses and lower dimension algorithms depends on the higher dimension algorithm. If you wish you can select other algorithms and/or hypotheses for meshing some sub-shapes of your CAD model by \ref constructing_submeshes_page. Some algorithms generate mesh of several dimensions, while others produce mesh of only one dimension. In the latter case there must be one Algorithm and zero or several Hypotheses for each dimension of your object, otherwise you will not get any mesh at all. Of course, if you wish to mesh a face, which is a 2D object, you do not need to define a 3D Algorithm and Hypotheses. In the Object Browser the structure of the new mesh will be displayed as follows:
    \image html image88.jpg
    It contains: There is an alternative way to assign Algorithms and Hypotheses by clicking Assign a set of hypotheses button and selecting among pre-defined sets of algorithms and hypotheses. In addition to the built-in sets of hypotheses, it is possible to create custom sets by editing CustomMeshers.xml file located in the home directory. CustomMeshers.xml file must describe sets of hypotheses in the same way as ${SMESH_ROOT_DIR}/share/salome/resources/smesh/StdMeshers.xml file does (sets of hypotheses are enclosed between tags).
    \image html hypo_sets.png List of sets of hypotheses. Tag [custom] is automatically added to the sets defined by the user.
    \note - \a "Automatic" in the names of predefined sets of hypotheses came from previous versions of SALOME where \ref automatic_length_anchor "Automatic Length" hypothesis was included in these sets, and not that these sets are suitable for meshing any geometry. - The list of sets of hypotheses can be shorter than in the above image depending on the geometry dimension.
Consider trying a sample script for construction of a mesh from our \ref tui_creating_meshes_page "TUI Scripts" section. \anchor evaluate_anchor

Evaluating mesh size

After the mesh object is created and all hypotheses are assigned and before \ref compute_anchor "Compute" operation, it is possible to calculate the eventual mesh size. For this, select the mesh in the Object Browser and from the \b Mesh menu select \b Evaluate. The result of evaluation will be displayed in the following information box: \image html mesh_evaluation_succeed.png \anchor preview_anchor

Previewing the mesh

Before \ref compute_anchor "the mesh computation", it is also possible to see the mesh preview. For this, select the mesh in the Object Browser. From the \b Mesh menu select \b Preview or click "Preview" button in the toolbar or activate "Preview" item from the pop-up menu.
\image html mesh_precompute.png "Preview" button
Select 1D mesh or 2D mesh preview mode in the Preview dialog. \image html preview_mesh_1D.png "1D mesh preview shows nodes computed on geometry edges"
\image html preview_mesh_2D.png "2D mesh preview shows edge mesh elements, computed on geometry faces" Compute button computes the whole mesh. When the Preview dialog is closed, the question about the storage of temporarily created mesh elements appears: \image html preview_tmp_data.png These elements can be kept in the mesh. \anchor submesh_order_anchor

Changing sub-mesh priority

If the mesh contains concurrent \ref constructing_submeshes_page "sub-meshes", it is possible to change the priority of their computation, i.e. to change the priority of applying algorithms to the shared sub-shapes of the Mesh shape. To change sub-mesh priority: Choose "Change sub-mesh priority" from the Mesh menu or a pop-up menu. The opened dialog shows a list of sub-meshes in the order of their priority. There is an example of sub-mesh order modifications taking a Mesh created on a Box shape. The main Mesh object: The first sub-mesh Submesh_1 created on Face_1 is: The second sub-mesh Submesh_2 created on Face_2 is: And the last sub-mesh Submesh_3 created on Face_3 is: The sub-meshes become concurrent if they share sub-shapes that can be meshed with different algorithms (or different hypotheses). In the example, we have three sub-meshes with concurrent algorithms, because they have different hypotheses. The first mesh computation is made with:
\image html mesh_order_123.png "Mesh order SubMesh_1, SubMesh_2, SubMesh_3"
\image html mesh_order_123_res.png "Result mesh with order SubMesh_1, SubMesh_2, SubMesh_3 "
The next mesh computation is made with:
\image html mesh_order_213.png "Mesh order SubMesh_2, SubMesh_1, SubMesh_3"
\image html mesh_order_213_res.png "Result mesh with order SubMesh_2, SubMesh_1, SubMesh_3 "
And the last mesh computation is made with:
\image html mesh_order_321.png "Mesh order SubMesh_3, SubMesh_2, SubMesh_1"
\image html mesh_order_321_res.png "Result mesh with order SubMesh_3, SubMesh_2, SubMesh_1 "
As we can see, each mesh computation has a different number of result elements and a different mesh discretization on the shared edges (the edges that are shared between Face_1, Face_2 and Face_3) Additionally, sub-mesh priority (the order of applied algorithms) can be modified not only in a separate dialog box, but also in the Preview. This helps to preview different mesh results, modifying the order of sub-meshes.
\image html mesh_order_preview.png "Preview with sub-mesh priority list box"
If there are no concurrent sub-meshes under the Mesh object, the user will see the following information.
\image html mesh_order_no_concurrent.png "No concurrent submeshes detected"
\anchor compute_anchor

Computing the mesh

It is equally possible to skip \ref evaluate_anchor "the Evaluation" and \ref preview_anchor "the Preview" and to \b Compute the mesh after the hypotheses are assigned. For this, select your mesh in the Object Browser. From the \b Mesh menu select \b Compute or click "Compute" button of the toolbar.
\image html image28.png "Compute" button
After the mesh computation finishes, the Mesh Computation information box appears. If you close this box and click "Compute" button again, without previously changing meshing parameters, the mesh is NOT re-computed and the Mesh Computation information box with the same contents is shown. (To fully re-compute the mesh, invoke \ref clear_mesh_anchor "Clear Mesh Data" command before). In case of a success, the box shows information on number of entities of different types in the mesh. \image html meshcomputationsucceed.png \anchor meshing_failed_anchor If the mesh computation failed, the information about the cause of the failure is provided in \b Errors table. \image html meshcomputationfail.png After you select an error, Show Sub-shape button allows visualizing in magenta the geometrical entity that causes the error. \image html failed_computation.png 3D algorithm failed to compute mesh on a box shown using Show Sub-shape button Publish Sub-shape button publishes the sub-shape, whose meshing has failed, in the Geometry component as a child of the main shape, which allows analyzing the problematic geometry and creating a sub-mesh on it in order to locally tune the hypotheses. If the failure is caused by an invalid input mesh and the algorithm has found which mesh entities are bad, Show bad Mesh button appears in the dialog. Clicked, it shows the bad mesh entities in the Viewer in magenta. Sometimes the shown mesh entities are too small or/and hidden by other mesh elements. They can be seen after switching the mesh to Wireframe visualization mode or switching off the visualization of faces and volumes (if any). Bad Mesh to Group button creates groups of bad mesh entities to facilitate their analysis. \image html show_bad_mesh.png Edges bounding a hole in the surface are shown in magenta using Show bad Mesh button \note Mesh Computation Information box does not appear if you set "Mesh computation/Show a computation result notification" preference to the "Never" value. This option gives the possibility to control mesh computation reporting. There are the following possibilities: always show the information box, show only if an error occurs or never. By default, the information box is always shown after mesh computation operation.

\anchor edit_anchor

Editing the mesh

It is possible to \ref modifying_meshes_page "edit the mesh" of lower dimension before generation of mesh of higher dimension. For example you can generate 2D mesh, modify it using e.g. \ref pattern_mapping_page, and then generate 3D mesh basing on the modified 2D mesh. The workflow is following: - Define 1D and 2D meshing algorithms. - Compute the mesh. 2D mesh is generated. - Apply \ref pattern_mapping_page. - Define 3D meshing algorithms without modifying 1D and 2D algorithms and hypotheses. - Compute the mesh. 3D mesh is generated. \note Nodes and elements added \ref adding_nodes_and_elements_page "manually" can't be used in this workflow because the manually created entities are not attached to any geometry and thus (usually) can't be found by a mesher paving some geometry. See Also a sample TUI Script demonstrates the possibility of \ref tui_editing_while_meshing "Intermediate edition while meshing" */