/*!

\page prism_3d_algo_page 3D extrusion meshing algorithm

3D extrusion algorithm can be used for meshing prisms, i.e. 3D shapes
defined by two opposing faces having the same number of vertices and
edges. These two faces should be connected by quadrangle "side" faces.

The prism is allowed to have sides composed of several faces. (A prism
side is a row of faces (or one face) connecting corresponding edges of
the top and base faces). But there is a limitation that a prism 
side is allowed to be split only vertically as indicated in the
picture below. 

\image html prism_ok_ko.png
In this picture, the left prism is suitable for meshing with 3D
extrusion algorithm; it has six sides two of which are split
vertically. And the right prism can't be meshed with this
algorithm because one of the prism sides is split horizontally (a
splitting edge is highlighted).

The algorithm can propagate 2D mesh not only between horizontal
(i.e. base and top) faces of one prism but also between faces of prisms
organized in a stack and between stacks sharing prism sides.

\image html prism_stack.png
In this picture, four neighboring prism stacks, each comprising two prisms,
are shown. The shown sub-mesh is used by the algorithm to mesh
all the eight prisms in the stacks.

To use <em>3D extrusion</em> algorithm you need to assign algorithms
and hypotheses of lower dimension as follows.
(A sample picture below shows algorithms and hypotheses used to
mesh a cylinder with prismatic volumes).

\image html prism_needs_hyps.png

\b Global algorithms and hypotheses to be chosen at 
\ref create_mesh_anchor "Creation of a mesh object" are:
<ul>
<li> 1D algorithm and hypothesis that will be applied for meshing
  (logically) vertical edges of the prism (these edges connect the top and
  base faces of prism). In the sample picture above these are
  "Regular_1D" algorithm and "Nb. Segments_1" hypothesis.</li>
</ul>

\b Local algorithms and hypotheses to be chosen at 
\ref constructing_submeshes_page "Constructing sub-meshes" are:
<ul>
  <li> 1D and 2D algorithms and hypotheses that will be applied for
    meshing the top and base prism faces. These faces can be meshed
    with any type of 2D elements: quadrangles, triangles, polygons or
    their mix. It's enough to define a sub-mesh on either top or base
    face. In the sample picture above, "BLSURF" algorithm meshes
    "Face_1" base surface with triangles. (1D algorithm is not
    assigned as "BLSURF" does not require divided edges to create 2D mesh.)
  </li>
  <li> Optionally you can define an 1D sub-mesh on some vertical edges
    of stacked prisms, which will override the global 1D hypothesis mentioned
    above. In the picture above the picture of Object Browser, the
    vertical division is not equidistant on all the length because of
    a "Number Of Segments" hypothesis with Scale Factor=3 assigned to
    the highlighted edge. 
</li></ul>

\image html image157.gif 

Prism with 3D extrusion meshing. "Vertical" division is different on
neighbor edges due to local 1D hypotheses assigned.

\sa a sample TUI Script of
\ref tui_prism_3d_algo "Use 3D extrusion meshing algorithm".

*/