/*!

\page about_meshes_page About meshes

\n \b MESH represents a discrete approximation of a subset of the
three-dimensional space by \ref mesh_entities "elementary geometrical
elements".

A SALOME study can contain multiple meshes, but they do not
implicitly compose one super-mesh, and finally each of them
can be used (e.g. exported) only individually.

Mesh module provides several ways to create the mesh:
<ul>
  <li>The main way is to \subpage constructing_meshes_page "construct the mesh" 
    on the basis of the geometrical shape produced in the Geometry
    module. This way implies selection of 
    - a geometrical object (<em>main shape</em>) and
    - <em>meshing parameters</em> (\ref
      basic_meshing_algos_page "meshing algorithms" and
    characteristics (e.g. element size) of a
    required mesh encapsulated in \ref about_hypo_page "hypothesis"
    objects).

    Construction of \subpage constructing_submeshes_page "sub-meshes"
    allows to discretize some sub-shapes of the main shape, for example a face,
    using the meshing parameters that differ from those used for other sub-shapes.<br>
    Meshing parameters of meshes and sub-meshes can be 
    \subpage editing_meshes_page "edited". (Upon edition only mesh entities
    generated using changed meshing parameters are removed and will be
    re-computed).<br>
    \note Algorithms and hypotheses used at mesh level are referred to as
    \a global ones and those used at sub-mesh level are referred to as \a
    local ones.
  </li>
  <li>Bottom-up way, using \ref modifying_meshes_page "mesh modification"
    operations, especially \ref extrusion_page "extrusion" and \ref
    revolution_page "revolution". To create an empty mesh not based on a
    geometry, use the same dialog as to \ref constructing_meshes_page
    "construct the mesh on geometry" but do not specify a geometry
    or a meshing algorithm.
  </li>
  <li>The mesh can be \subpage importing_exporting_meshes_page "imported" from
    (and exported to) the file in MED, UNV, STL, CGNS, DAT, GMF and
    SAUVE formats.
  </li>
  <li>The 3D mesh can be generated from the 2D mesh, which was \ref
    importing_exporting_meshes_page "imported" or manually created. To
    setup the meshing parameters of a mesh not based on a geometry, just
    invoke \ref editing_meshes_page "Edit mesh / sub-mesh" command on
    your 2D mesh.
  </li>
  <li>Several meshes can be \subpage building_compounds_page "combined"
    into a new mesh.
  </li>
  <li>The whole mesh or its part (sub-mesh or group) can be 
    \subpage copy_mesh_page "copied" into a new mesh.
  </li>
  <li>A new mesh can be created from a transformed, e.g. \ref
    translation_page "translated", part of the mesh.</li>
</ul>

Meshes can be edited using the MESH functions destined for 
\ref modifying_meshes_page "modification" of meshes.

Attractive meshing capabilities include:
- 3D and 2D \ref viscous_layers_anchor "Viscous Layers" (boundary
  layers of highly stretched elements beneficial for high quality
  viscous computations);
- automatic conformal transition between tetrahedral and hexahedral
  sub-meshes.

The \b structure of a SALOME mesh is described by nodes and elements based on
these nodes. The geometry of an element is defined by the sequence of
nodes constituting it and
the <a href="http://www.code-aster.org/outils/med/html/connectivites.html">
  connectivity convention </a> (adopted from MED library). Definition of
the element basing on the elements of a lower dimension is NOT supported.

\anchor mesh_entities
The mesh can include the following entities:
<ul>
<li>\b Node &mdash; a mesh entity defining a position in 3D
  space with coordinates (x, y, z).</li>
<li>\b Edge (or segment) &mdash; 1D mesh element linking two nodes.</li>
<li>\b Face &mdash; 2D mesh element representing a part of
  surface bound by links between face nodes. A face can be a
  triangle, quadrangle or polygon.</li>
<li>\b Volume &mdash; 3D mesh element representing a part of 3D
  space bound by volume facets. Nodes of a volume describing each
  facet are defined by
  the <a href="http://www.code-aster.org/outils/med/html/connectivites.html">
  MED connectivity convention.</a> A volume can be a tetrahedron, hexahedron,
  pentahedron, pyramid, hexagonal prism or polyhedron.</li>
<li>\b 0D element &mdash; mesh element defined by one node.</li>
<li>\b Ball element &mdash; discrete mesh element defined by a
  node and a diameter.</li>
</ul>

Every mesh entity has an attribute associating it to a sub-shape it is
generated on (if any). The node generated on the geometrical edge or
surface in addition stores its position in parametric space of the
associated geometrical entity. This attribute is set up by meshing
algorithms generating elements and nodes.

Mesh entities are identified by integer IDs starting from 1.
Nodes and elements are counted separately, i.e. there can be a node
and element with the same ID.

SALOME supports elements of second order, without a central node
(quadratic triangle, quadrangle, polygon, tetrahedron, hexahedron,
pentahedron and pyramid) and with central nodes (bi-quadratic triangle
and quadrangle and tri-quadratic hexahedron).<br>
Quadratic mesh can be obtained in three ways:
- Using a global \ref quadratic_mesh_anchor "Quadratic Mesh"
hypothesis. (Elements with the central node are not generated in this way).
- Using \ref convert_to_from_quadratic_mesh_page operation.
- Using an appropriate option of some meshing algorithms, which
generate elements of several dimensions starting from mesh segments.
*/