smesh/doc/salome/gui/SMESH/input/constructing_meshes.doc

/*!

\page constructing_meshes_page Constructing meshes

To create a mesh on geometry, it is necessary to create a mesh object by choosing
- a geometrical shape produced in the Geometry module (<em>main shape</em>);
- <em>meshing parameters</em>, including 
  - \ref basic_meshing_algos_page "meshing algorithms" and
  - \ref about_hypo_page "hypotheses" specifying constraints to be
    taken into account by the chosen meshing algorithms.

Then you 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.

Mesh generation 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 node of segments are then
used to mesh faces; then the nodes of faces are used to mesh
solids. This automatically assures the conformity of the mesh.

It is required to choose a meshing algorithm for every dimension of
sub-shapes up to the highest dimension to be generated. Note
that some algorithms generate elements of several dimensions, and
others of only one. It is 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 will not be
re-computed).

An algorithm of a certain dimension chosen at mesh creation is applied
to discretize every sub-shape of this dimension. It is possible to
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 the sub-shapes, for which an algorithm and a hypothesis (if any)
have been defined will be discretized.

\n Construction of a mesh on a geometry includes at least two (mesh
creation and computing) of the following steps:
<ul>
  <li> \ref create_mesh_anchor "Creation of a mesh object", where you
  can specify meshing parameters to apply to all sub-shapes of the
  main shape.</li>
  <li> \ref constructing_submeshes_page "Creation of sub-meshes",
  (optional) where you can specify meshing parameters to apply to the
  selected sub-shapes.</li>
  <li> \ref evaluate_anchor "Evaluating mesh size" (optional) can be
  used to know an approximate number of elements before their actual generation.</li>
  <li> \ref preview_anchor "Previewing the mesh" (optional) can be
  used to generate mesh of only lower dimension(s) in order to
  visually estimate it before full mesh generation, which can be much
  longer.</li>
  <li> \ref submesh_order_anchor "Changing sub-mesh priority"
  (optional) can be useful if there are concurrent sub-meshes
  defined.</li>
  <li> \ref compute_anchor "Computing the mesh" uses defined meshing
  parameters to generate mesh elements.</li>
  <li> \ref edit_anchor "Editing the mesh" (optional) can be used to
  \ref modifying_meshes_page "modify" the mesh of a lower dimension before
  \ref compute_anchor "computing" elements of an upper dimension.</li>
</ul>

\anchor create_mesh_anchor
<h2>Creation of a mesh object</h2>
<em>To construct a mesh:</em>
<ol>
  <li>Select a geometrical object for meshing.</li>
  <li>In the \b Mesh menu select <b>Create Mesh</b> or click <em>"Create
      Mesh"</em> button in the toolbar. 

    <center>
    \image html image32.png
    <em>"Create Mesh" button</em>
    </center>

    The following dialog box will appear: 

    \image html createmesh-inv.png
    <br>
  </li>
  <li> To filter off irrelevant meshing algorithms, you can
    select <b>Mesh Type</b> in the corresponding list from <b>Any,
      Hexahedral, Tetrahedral, Triangular </b> 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.</li>

  <li>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 a
    hypothesis (or select an already created one). A set of accessible
    hypotheses includes only the hypotheses that can be used by the
    selected algorithm.

    \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 pages related to the corresponding lower dimensions are
    disabled.
    - \b 0D page refers to 0D geometry (vertices) rather than
    to 0D elements. 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 the selected geometrical object indicated in
    \b Geometry field, is what you wish to mesh; if not, select
    the correct 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.
    <center>
    \image html image120.png
    <em>"Select" button</em>
    </center>

    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 <em>"Add Hypothesis"</em> button to create
    and add a hypothesis.
    <center>
    \image html image121.png
    <em>"Add Hypothesis" button</em>
    </center>
    Click the <em>"Plus"</em> button to enable adding more additional hypotheses.

    Click the <em>"Edit Hypothesis"</em> button to change the values for the
    current hypothesis.
    <center>
    \image html image122.png
    <em>"Edit Hypothesis" button</em>
    </center>

    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:
    <ul>
      <li> <em>\<Default\></em> if the algorithm can work using default
      parameters.</li>
      <li> <em>\<None\></em> if the algorithm requires a hypothesis defining
      its parameters.</li>
      <li> If the algorithm does not use hypotheses, this field is grayed.</li>
    </ul>
    After selection of an algorithm <b>Add. Hypothesis</b> field can contain:
    <ul>
      <li> <em>\<None\></em> if the algorithm can be tuned
      using an additional hypothesis.</li>
      <li> If the algorithm does not use additional hypotheses, this field is grayed.</li>
    </ul>

    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 <b>Object Browser</b> the structure of the new mesh is
    displayed as follows:
    \image html image88.jpg
    It contains: 
    <ul>
      <li>a mesh name (<em>Mesh_mechanic</em>);
      <li>a reference to the geometrical object on the basis of
        which the mesh has been constructed (\a mechanic);</li> 
      <li><b>Applied hypotheses</b> folder containing the references
        to the hypotheses chosen at the construction of the mesh;</li>
      <li><b>Applied algorithms</b> folder containing the references
        to the algorithms chosen at the construction of the mesh.</li> 
      <li><b>SubMeshes on Face</b> folder containing the sub-meshes
        defined on geometrical faces. There also can be folders for
        sub-meshes on vertices, edges, wires, shells, solids and
        compounds.</li>
      <li><b>Groups of Faces</b> folder containing the groups of mesh
        faces. There also can be folders for groups of nodes, edges,
        volumes 0D elements and balls.</li>
    </ul>

    There is an alternative way to assign Algorithms and Hypotheses by
    clicking <b>Assign a set of hypotheses</b> 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 \<hypotheses-set-group\>
      tags). For example:
~~~~~~{.xml}
    <?xml version='1.0' encoding='us-ascii'?>
    <!DOCTYPE meshers PUBLIC "" "desktop.dtd">
    <meshers>
    <hypotheses-set-group>
        <hypotheses-set name="My favorite hypotheses"
                        hypos="AutomaticLength"
                        algos="CompositeSegment_1D, Quadrangle_2D, GHS3D_3D"/>
    </hypotheses-set-group>
    </meshers>
~~~~~~
<br>
<center>
      \image html hypo_sets.png
      List of sets of hypotheses. Tag <em>[custom]</em> is
      automatically added to the sets defined by the user.
</center>
      \note 
      - \a "Automatic" in the names of predefined sets of hypotheses
      does not actually mean that they 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.
  </li>
</ol>

Consider trying a sample script for construction of a mesh from our 
\ref tui_creating_meshes_page "TUI Scripts" section.

\anchor evaluate_anchor
<h2>Evaluating mesh size</h2>

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 <b>Object Browser</b> 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
<h2>Previewing the mesh</h2>

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.

<center>
\image html mesh_precompute.png
<em>"Preview" button</em>
</center>

Select <b>1D mesh</b> or <b>2D mesh</b> preview mode in the Preview dialog. 

\image html preview_mesh_1D.png "1D mesh preview shows nodes computed on geometry edges"
<br>
\image html preview_mesh_2D.png "2D mesh preview shows edge mesh elements, computed on geometry faces"

<b>Compute</b> 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
<h2>Changing sub-mesh priority</h2>

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.

<em>To change sub-mesh priority:</em>

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:
<ul>
  <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=20</li>
  <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis<b>Max Element Area</b>
  </li>
</ul>
The first sub-mesh <b>Submesh_1</b> created on <b>Face_1</b> is:
<ul>
  <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=4</li>
  <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis <b>MaxElementArea</b>=1200</li>
</ul>
The second sub-mesh <b>Submesh_2</b> created on <b>Face_2</b> is:
<ul>
  <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=8</li>
  <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis <b>MaxElementArea</b>=1200</li>
</ul>

And the last sub-mesh <b>Submesh_3</b> created on <b>Face_3</b> is:
<ul>
  <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=12</li>
  <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis <b>MaxElementArea</b>=1200</li>
</ul>

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:
<center>
\image html mesh_order_123.png
<em>"Mesh order SubMesh_1, SubMesh_2, SubMesh_3"</em></center>
<center>
\image html mesh_order_123_res.png
<em>"Result mesh with order SubMesh_1, SubMesh_2, SubMesh_3 "</em></center>

The next mesh computation is made with:
<center>
\image html mesh_order_213.png
<em>"Mesh order SubMesh_2, SubMesh_1, SubMesh_3"</em></center>
<center>
\image html mesh_order_213_res.png
<em>"Result mesh with order SubMesh_2, SubMesh_1, SubMesh_3 "</em></center>

And the last mesh computation is made with:
<center>
\image html mesh_order_321.png
<em>"Mesh order SubMesh_3, SubMesh_2, SubMesh_1"</em></center>
<center>\image html mesh_order_321_res.png
<em>"Result mesh with order SubMesh_3, SubMesh_2, SubMesh_1 "</em></center>

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 <b>Face_1</b>, <b>Face_2</b> and <b>Face_3</b>)

Additionally, sub-mesh priority (the order of applied algorithms) can
be modified not only in a separate dialog box, but also in
the <b>Preview</b>. This helps to preview different mesh results,
modifying the order of sub-meshes. 
<center>
\image html mesh_order_preview.png
<em>"Preview with sub-mesh priority list box"</em></center>

If there are no concurrent sub-meshes under the Mesh object, the user
will see the following information.
<center>
\image html mesh_order_no_concurrent.png
<em>"No concurrent submeshes detected"</em></center>


\anchor compute_anchor
<h2>Computing the mesh</h2>

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 <b>Object Browser</b>. From the \b Mesh menu select \b Compute or
click "Compute" button of the toolbar.

<center>
\image html image28.png
<em>"Compute" button</em>
</center>

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 will NOT be
re-computed and the Mesh Computation information box will be shown
with the same contents. (To fully re-compute the mesh, invoke 
\ref clear_mesh_anchor "Clear Mesh Data" command before). 

If the mesh computation has been a success, the box shows information
on the number of entities of different types in the mesh.

\image html meshcomputationsucceed.png

\anchor meshing_failed_anchor
If the mesh computation has failed, the information about the cause of the
failure is provided in \b Errors table.

\image html meshcomputationfail.png

After you select an error, <b>Show Sub-shape</b> button allows
visualizing in magenta the geometrical entity that causes the error.

<center>
\image html failed_computation.png 
<em>3D algorithm failed to compute mesh on a box shown using <b>Show
    Sub-shape</b> button</em>
</center>

<b>Publish Sub-shape</b> 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, <b>Show bad Mesh</b> 
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).

<b>Bad Mesh to Group</b> button creates groups of bad mesh entities
to facilitate their analysis.

<center>
\image html show_bad_mesh.png
<em>Edges bounding a hole in the surface are shown in magenta using <b>Show
    bad Mesh</b> button</em>
</center>

\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.

<p><p>
\anchor edit_anchor
<h2>Editing the mesh</h2>

It is possible to \ref modifying_meshes_page "edit the mesh" of a 
lower dimension before generation of the mesh of a higher dimension.

For example you can generate a 2D mesh, modify it using e.g. 
\ref pattern_mapping_page, and then generate a 3D mesh basing on the
modified 2D mesh. The workflow is as follows:
- 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" cannot be used in this workflow because the manually created
entities are not attached to any geometry and thus (usually) cannot be
found by the mesher paving a geometry.

<b>See Also</b> a sample TUI Script demonstrates the possibility of 
\ref tui_editing_while_meshing "Intermediate edition while meshing"

*/