smesh/doc/salome/gui/SMESH/files/about_meshes.htm

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<h1>About meshes</h1>

<p><img src="../i_blue.jpg" x-maintain-ratio="TRUE" width="30px" height="30px" border="0" class="img_whs1"> <span style="font-weight: bold;"><B>MESH</B></span> represents
 a discretization of a geometrical CAD model into a set of entities with
 a simple topology. In MESH there are two options of creation of meshes,
 you can:</p>

<p>&nbsp;</p>

<ul type="disc" class="whs2">

	<li class=kadov-p><p>generate meshes on the basis of geometrical shapes
 produced in the GEOM module, </p></li>

	<li class=kadov-p><p>create your own meshes using the MESH functions
 destined for modification of generated meshes. </p></li>
</ul>

<p>&nbsp;</p>

<p>The topology of a mesh is described by the relationships between its
 entities including:</p>

<p class="whs3">&nbsp;</p>

<ul type="disc" class="whs2">

	<li class=kadov-p><p class="whs3"><span style="font-weight: bold;"><B>Node</B></span>
 <20> 0D element of a mesh presented by a point with coordinates (x, y, z).</p></li>

	<li class=kadov-p><p class="whs3"><span style="font-weight: bold;"><B>Edge</B></span>
 <20> 1D element of a mesh defined by two nodes.</p></li>

	<li class=kadov-p><p class="whs3"><span style="font-weight: bold;"><B>Face</B></span>
 <20> 2D element of a mesh defined by three or four edges (closed contour).</p></li>

	<li class=kadov-p><p class="whs3"><span style="font-weight: bold;"><B>Volume</B></span>
 <20> 3D element of a mesh defined by several faces.</p></li>
</ul>

<p class="whs3">&nbsp;</p>

<p class="whs4">These entities are considered as topological
 entities and they don't imply any geometric representation. Only <span
 style="font-weight: bold;"><B>Nodes</B></span> reference &nbsp;geometric
 representations of points with definite coordinates. <span style="text-indent: 8px;">The
 node entity will contain &nbsp;additional
 information about its position in the space and its relations with the
 meshed CAD model. Its position could be described in the following way:</span></p>

<p class="whs4">&nbsp;</p>

<ul type="disc" class="whs2">

	<li class=kadov-p><p class="whs3"><span style="font-weight: bold;"><B>2D
 position</B></span>. It is a free position defined by only two coordinates
 x,y.</p></li>

	<li class=kadov-p><p class="whs3"><span style="font-weight: bold;"><B>3D
 position</B></span>. It is a free position defined by three coordinates x,y
 and z.</p></li>

	<li class=kadov-p><p class="whs3"><span style="font-weight: bold;"><B>Surface
 position</B></span>. It characterizes the position of a node on a geometric
 surface and is defined by the u,v position in the parametric space of
 the corresponding surface.</p></li>

	<li class=kadov-p><p class="whs3"><span style="font-weight: bold;"><B>Line
 position</B></span>. It characterizes the position of a node on a geometric
 curve and is defined by the u parameter and the corresponding curve.</p></li>

	<li class=kadov-p><p class="whs3"><span style="font-weight: bold;"><B>Vertex
 position</B></span>. It characterizes the position of a node on a geometric
 point of the meshed CAD model and is defined by the x,y,z coordinates
 of the corresponding vertex.</p></li>
</ul>

<p class="whs3">&nbsp;</p>

<p class="whs3">&nbsp;</p>

<h4 class="whs5">Connections</h4>

<p class="whs6">Each mesh entity bounds 0 or more mesh entities
 of higher dimension. In the same way each mesh entity is bounded by 0
 or more mesh entities of lower dimension:</p>

<p class="whs6">&nbsp;</p>

<ul type="disc" class="whs2">

	<li class=kadov-p><p class="whs6">A node bounds edges,
 faces and volumes</p></li>

	<li class=kadov-p><p class="whs6">An edge bounds faces,
 and volumes</p></li>

	<li class=kadov-p><p class="whs6">A face bounds volumes</p></li>

	<li class=kadov-p><p class="whs6">A volume is bounded
 by faces, edges and nodes</p></li>

	<li class=kadov-p><p class="whs6">A face is bounded by
 edges, and nodes</p></li>

	<li class=kadov-p><p class="whs6">An edge is bounded by
 nodes</p></li>
</ul>

<p class="whs6">&nbsp;</p>

<p class="whs6">You can notice that there are two types of
 connections: <span style="font-weight: bold;"><B>inverse</B></span> and <span
 style="font-weight: bold;"><B>direct</B></span> connections.</p>

<p class="whs6">&nbsp;</p>

<h4 class="whs5">Inverse connections</h4>

<p class="whs6">&nbsp;</p>

<p class="whs6">This relationship has a particularity that
 the order of bounded entities has not a direct meaning. Also the number
 of bounded entities is not fixed.</p>

<p class="whs6">&nbsp;</p>

<p class="whs6"><img src="../pics/exemple.gif" x-maintain-ratio="TRUE" width="36px" height="38px" border="0" class="img_whs7"> The edges surrounding a node.
 The 3rd edge has no more sense that the 5th one.</p>

<p class="whs6">&nbsp;</p>

<p class="whs8">Direct connections</p>

<p class="whs6">&nbsp;</p>

<p class="whs6">This relationship has a particularity that
 the order of bounding entities is meaningful. The number of bounding entities
 is fixed and depends on the type of the entity (hexahedron, tetrahedron,<2C>).</p>

<p class="whs6">&nbsp;</p>

<p class="whs6"><img src="../pics/exemple.gif" x-maintain-ratio="TRUE" width="36px" height="38px" border="0" class="img_whs7"> An edge is composed of two
 nodes. A face is composed of 3 or 4 edges depending if we are dealing
 with triangles or quadrangles.</p>

<p class="whs6">&nbsp;</p>

<p class="whs6">The connections are not only restricted to
 entities of one dimension higher or lower. For example some algorithms
 may be interested to retrieve all the faces surrounding a node.</p>

<p class="whs6">&nbsp;</p>

<p class="whs6">&nbsp;</p>

<p class="whs6">&nbsp;</p>

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