/*!

\page cutting_quadrangles_page Cutting quadrangles

\n This operation allows to cut one or several quadrangle elements by
addition of a supplementary edge which will connect two opposite
corners.

<em>To cut quadrangles:</em>
<ol>
<li>Display a mesh or a sub-mesh in the 3D viewer.</li>
<li>In the \b Modification menu select the <b>Cutting of quadrangles</b> item or
click <em>"Cutting of quadrangles"</em> button in the toolbar.

\image html image82.png
<center><em>"Cutting of quadrangles" button</em></center>

The following dialog box will appear:

\image html a-cuttingofquadrangles.png

\par
<ul>
<li>The main list contains the list of quadrangles. You can click on
an quadrangle in the 3D viewer and it will be highlighted (lock Shift
keyboard button to select several quadrangles). Click \b Add button and
the ID of this quadrangle will be added to the list. To remove a
selected element or elements from the list click \b Remove button. <b>Sort
list</b> button allows to sort the list of IDs. \b Filter button allows to
apply a definite filter to the selection of quadrangles.</li>
<li><b>Apply to all</b> check box allows to modify the orientation of all
quadrangles of the currently displayed mesh or sub-mesh.</li>
<li>\b Preview - provides a preview of cutting in the viewer.</li>
</ul>

<ul>
<li>\b Criterion
<ul>
<li><b>Use diagonal 1-3</b> and <b>Use diagonal 2-4</b> allows to
specify the opposite corners which will be connected by the cutting
edge.</li>
<li><b>Use numeric functor</b> - allows to choose a quality criterion
 which will be optimized at the outcome of the operation.
<ul>
<li><b>Minimum diagonal</b> - cuts by the shortest edge.</li>
<li><b>Aspect Ratio</b> - cuts by the edge splitting the quadrangle 
into triangles with optimal \ref aspect_ratio_page "Aspect Ratio". The reference value is 1.</li>
<li><b>Minimum Angle</b> -  cuts by the edge splitting the quadrangle 
into triangles with optimal  \ref minimum_angle_page "Minimum Angle". The reference value is 60 degrees.</li>
<li><b>Skew</b> -   cuts by the edge splitting the quadrangle 
into triangles with optimal \ref skew_page "Skew". The reference value 
is 0.0 degrees.</li>
</ul>
</li>
</ul>
</li>
<li><b>Select from</b> - allows to choose a sub-mesh or an existing
group whose quadrangle elements will be automatically added to the
list.</li>
</ul>

</li>
<li>Click the \b Apply or <b>Apply and Close</b> button to confirm the operation.</li>
</ol>

\image html image52.jpg "The chosen quadrangular element"

\image html image51.jpg "Two resulting triangular elements"

<br><b>See Also</b> a sample TUI Script of a 
\ref tui_cutting_quadrangles "Cutting Quadrangles" operation.  

*/