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

/*!

\page constructing_meshes_page Constructing meshes

\n Construction of a mesh consists of:
<ul>
<li>Selecting a geometrical object for meshing</li>
<li>Applying \subpage basic_meshing_algos_page "meshing algorithms" and
\subpage about_hypo_page "hypotheses" which will be used at computation of
this mesh.</li>
</ul>

<em>To construct a mesh:</em>
<ol>
<li>In the \b Mesh menu select <b>Create Mesh</b> or click <em>"Create
Mesh"</em> button in the toolbar. 

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

The following dialog box will appear: 

\image html createmesh-inv.png
</li>
<li>For example, you need to mesh a 3d object.
\n First, type the name for your mesh in the "Name" box, by default,
it is "Mesh_1". Then select the object you wish to mesh in the Object
Browser and click the "Add" button.

\image html image120.png
<center><em>"Add" button</em></center>

Now you can define 1d Algorithm and 1d Hypotheses, which will be
applied to the edges of your object. (Note that any object has edges,
even if their existence is not apparent, for example, a sphere has 4
edges). Click the <em>"Add Hypothesis"</em>  button to add a hypothesis.

\image html image121.png
<center><em>"Add Hypothesis" button</em></center>

Click the <em>"Edit Hypothesis"</em> button to define values for the
current hypothesis.

\image html image122.png
<center><em>"Edit Hypothesis" button</em></center>

The use of additional hypotheses is optional (i.e. you may leave
"None" in this box).

Proceed in the same way with 2d and 3d Algorithms and Hypotheses, note
that the choice of hypotheses depends on the algorithm. There must be
one Algorithm and one 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 don't need to define 3d
Algorithm and Hypotheses.
\n In the <b>Object Browser</b> the structure of the new mesh will be
displayed as follows:

\image html image88.jpg

It contains:
<ul>
<li>a reference to the geometrical object on the basis of which the mesh has been constructed;</li>
<li><b>Applied hypotheses</b> folder containing the references to the
hypotheses applied to the construction of the mesh;</li>
<li><b>Applied algorithms</b> folder containing the references to the
algorithms applied to the construction of the mesh.</li>
</ul>

There is an alternative way to create a mesh on an object simply by
clicking <b>Assign a set of hypotheses</b> button and selecting between
Automatic Tetrahedralization or Hexahedralization.  The program will
automatically generate a 3D mesh with the most appropriate
settings. In the same way you can apply this functionality for meshing
2D objects, in which case 3D algorithms are not applied.</li>

\anchor preview_mesh_anchor

<li> After the mesh object is created and all hypotheses are assigned and
before the mesh computation, it is 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
<center><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"

\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 appers:

\image html preview_tmp_data.png

These elenents can be reused in the next mesh computation.
</li>


<li>It is equally possible to skip the Preview and \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. 

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

The Mesh Computation information box appears.

\image html meshcomputationsucceed.png

If the mesh computation failed, the information about the cause of the
failure is provided.

\image html meshcomputationfail.png

After you select the error, <b>Show Subshape</b> button allows
visualizing the geometrical entity that causes it.

\image html failed_computation.png "Example of the invalid input mesh"

\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 information box, only if an error occurs or never. 
By default, the information box is always shown after mesh computation operation.

<b>Publish Subshape</b> button publishes the subshape, whose meshing
failed, in GEOM component as a child of the mesh geometry, which
allows analyzing the problem geometry and creating a submesh on it in
order to locally tune hypotheses.

<b>NOTE</b> It is possible to define a 1D or a 2D mesh in a
python script and then use such submeshes in the construction of a 3D
mesh. For this, there exist two algorithms: <b>Use existing edges</b> and <b>Use
existing faces</b>. They are not entirely usable from the GUI, so a
mesh created using these algorithms should be exported into a python
script, edited and then imported into the GUi. 

<li>It is possible to calculate the eventual mesh size 
before \b Compute operation. 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

</li>

</ol>

\anchor mesh_order_anchor
It is allowed to change submesh priority in mesh computation when
there are concurrent submeshes present. I.e. user can change priority of
applying algorithms on shared subshapes of Mesh shape.
<em>To change submesh priority:</em>
<ol>
<li>From the Mesh menu choose the "Change submesh priority" on
selected Mesh item, or invoke from popup menu.

There are example of submesh order modifications of Mesh created on a Box
shape. The main Mesh object:
<ul>
<li><i>3D</i> <b>Tetrahedron (Netgen)</b> with Hypothesis<b>Max Element Volume</b></li>
<li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis<b>Max Element Area</b>
</li>
<li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=20</li>
</ul>
The first submesh object <b>Submesh_1</b> created on <b>Face_1</b>
is:
<ul>
<li><i>2D</i> Netgen 1D-2D with Hypothesis <b>Netgen Simple parameters</b>
(Number of Segments = 4)</li>
</ul>
The second submesh object <b>Submesh_2</b> created on <b>Face_2</b>
is:
<ul>
<li><i>2D</i> Netgen 1D-2D with Hypothesis <b>Netgen Simple parameters</b>
(Number of Segments = 8)</li>
</ul>

And the last third submesh object <b>Submesh_3</b> created on <b>Face_3</b>
is:
<ul>
<li><i>2D</i> Netgen 1D-2D with Hypothesis <b>Netgen Simple parameters</b>
(Number of Segments = 12)</li>
</ul>
The submeshes can become concurrent if thir algorithms leads to mesh shared subshape
with different algorithms (or different algorithms parameters, i.e. hypothesises).
In fact, we have three submeshes with concurrent algorithms, becase
they has different hypothesises assigned to them. 

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

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

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

As we can see each mesh computation has different number of result
elements and different mesh descretisation on shared edges (edges,
that shared between <b>Face_1</b>, <b>Face_2</b> and <b>Face_3</b>)

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

If no concurrent submeshes under Mesh object user will see the
following information dialog box
\image html mesh_order_no_concurrent.png
<center><em>"No concurrent submeshes detected"</em></center>
and no mesh order list box appear in Preview dialog box.

</ol>

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

*/