/*!

\page merging_nodes_page Merging nodes

This functionality allows user to detect groups of coincident nodes
with specified tolerance; each group of the coincident nodes can be
then converted to the single node.

\image html mergenodes_ico.png "Merge nodes menu button"

<em>To merge nodes of your mesh:</em>
<ol>
<li>Choose \b Modification -> \b Transformation -> <b>Merge nodes</b>
  menu item. The following dialog box shall appear:</li>
<br>
\image html mergenodes_auto.png
<br>
<ul>
<li>\b Name is the name of the mesh whose nodes will be merged.</li>
<li>\b Automatic or \b Manual mode allows choosing how the nodes are
  processed. In \b Manual mode you can adjust groups of coincident nodes
  detected by the program and/or select any nodes to be merged.</li>
<li>\b Tolerance is a maximum distance between nodes sufficient for
merging.</li>
<li>Activation of <b>No merge of corner and medium nodes of quadratic
    cells</b> check-box prevents merging medium nodes of quadratic
    elements with corner nodes. This check-box is enabled provided
    that the selected mesh includes quadratic elements.</li>
<li>Activation of <b>Avoid making holes</b> check-box prevents merging
    nodes that make elements invalid (but not degenerated) and hence
    removed. Thus, no holes in place of removed elements appear. </li>
<li><b>Exclude groups from detection</b> group allows to ignore the
  nodes which belong to the specified mesh groups. This control is
  active provided that the mesh includes groups.</li>
<li><b>Nodes to keep during the merge</b> group allows to specify
  nodes to keep in the mesh. (By default a node being the first in a
  group of coincident nodes is kept.) It is possible to either select
  nodes in the Viewer or select groups of any element type whose nodes
  will be kept.
  <ul>
    <li>\a Selection button activates selection of nodes to keep.</li>
    <li><b>Nodes</b> button activates selection of nodes in the
      Viewer.</li>
    <li><b>Groups and sub-meshes</b> button activates selection of
      groups and sub-meshes.</li>
    <li>\b Add button adds selected nodes or groups to the list.</li>
    <li> Nodes or groups selected in the list can be removed using \b
      Remove button.</li>
  </ul>
</li>
</ul>

<li><b>Automatic mode:</b>
<br>
<ul>
<li>In the \b Automatic Mode all nodes within the indicated tolerance
will be merged. The nodes which belong to the groups specified in
<b>Exclude groups from detection</b> will NOT be taken into account.</li>
</ul>
</li><br>
<li> The \b Manual mode gives you full control of what the operation will do.
In this mode additional controls are available:
<ul>
  <li>\b Detect button generates the list of coincident nodes for the given
    \b Tolerance.</li>
  <li><b>Coincident nodes</b> is a list of groups of nodes for
    merging. Upon \b Apply all nodes of each group will
    be united into one node. The first node of a group is kept and
    the others are removed. By default the first node has a lowest ID
    within the group.
<ul>
<li>\b Remove button deletes the selected group from the list.</li>
<li>\b Add button adds to the list a group of nodes selected in the
viewer.</li>
<li><b>Select all</b> check-box selects all groups.</li>
<li><b>Show double nodes IDs</b> check-box shows/hides identifiers of
nodes of selected groups in the 3D viewer.</li>
</ul>

<br>
\image html mergenodes.png
<br>
</li>
  <li><b>Edit selected group of coincident nodes</b> list allows
  editing the selected group:
    <br><br>
    \image html add.png
    <center>adds to the group the nodes selected in the viewer.</center>
    <br>
    \image html remove.png
    <center>removes from the group the selected nodes.</center>
    <br>
    \image html sort.png
    <center>moves the selected node to the first position in the
      group in order to keep it in the mesh.</center><br>
  </li>
</ul>
</li>
<li>To confirm your choice click \b Apply or <b>Apply and Close</b> button.</li>
</ol>

\image html merging_nodes1.png 
<center> The initial object. Nodes 25, 26 and 5 are added to <b>Nodes
    to keep during the merge</b> group.
</center>
<br>
\image html merging_nodes2.png
<center> The object has been merged
</center>

<br><b>See Also</b> a sample TUI Script of a 
\ref tui_merging_nodes "Merge Nodes" operation.  

*/