/*!

\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".

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 mesh some sub-shapes of the main shape, for example a face,
    using different meshing parameters than 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 as
    \a global ones and those used at sub-mesh level are referred 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
    geometry, use the same dialog as to \ref constructing_meshes_page
    "construct the mesh on geometry" but do not specify any geometry
    nor meshing algorithm.
  </li>
  <li>The mesh can be \ref 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, \ref
    importing_exporting_meshes_page "imported" or manually created. To
    setup the meshing parameters of a mesh not based on geometry, just
    invoke \ref editing_meshes_page "Edit mesh / sub-mesh" command on
    your 3D 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>
</ul>

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


The \b structure of a SALOME mesh is described by nodes and elements based on
these nodes. Geometry of the 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 elements of lower dimension is NOT supported.

\anchor mesh_entities
The mesh can include the following entities (also referred as \a elements):
<ul>
<li>\b Node &mdash; an entity of a mesh defining a position in 3D
  space with coordinates (x, y, z).</li>
<li>\b Edge (or segment) &mdash; 1D element of a mesh linking two nodes.</li>
<li>\b Face &mdash; 2D element of a mesh 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 element of a mesh 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; element of a mesh defined by one node.</li>
<li>\b Ball element &mdash; discrete element of a mesh 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.

SALOME supports elements of second order, without central node
(quadratic triangle, quadrangle, 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 two 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.

*/