+
+\image html reduce_three_to_one.png "The fastest transition pattern: 3 to 1"
+
+<b>Base vertex</b> tab allows using Quadrangle (Mapping)
+algorithm for meshing of trilateral faces. In this case it is
+necessary to select the vertex, which will be used as the forth
+degenerated side of quadrangle.
+
+\image html hypo_quad_params_dialog_vert.png "Quadrangle parameters: Base Vertex"
+
+\image html hypo_quad_params_1.png "A face built from 3 edges"
+
+\image html hypo_quad_params_res.png "The resulting mesh"
+
+This parameter can be also used to mesh a segment of a circular face.
+Please, consider that there is a limitation on the selection of the
+vertex for the faces built with the angle > 180 degrees (see the picture).
+
+\image html hypo_quad_params_2.png "3/4 of a circular face"
+
+In this case, selection of a wrong vertex for the <b>Base vertex</b>
+parameter will generate a wrong mesh. The picture below
+shows the good (left) and the bad (right) results of meshing.
+
+\image html hypo_quad_params_res_2.png "The resulting meshes"
+
+\image html hypo_quad_params_dialog_enf.png "Quadrangle parameters: Enforced nodes"
+
+<b>Enforced nodes</b> tab allows defining points, where the
+algorithm should create nodes. There are two ways to define positions
+of the enforced nodes.
+<ul>
+ <li>\b Vertices group allows to set up shapes whose vertices will
+ define positions of the enforced nodes. Only vertices successfully
+ projected to the meshed face and located close enough to the
+ meshed face will be used to create the enforced nodes.</li>
+ <li> \b Points group allows to explicitly define coordinates of
+ points used to create the enforced nodes. Only points successfully
+ projected to the meshed face and located close enough to the
+ meshed face will be used to create the enforced nodes.</li>
+</ul>
+\note <b>Enforced nodes</b> cannot be created at \b Reduced transition type.
+
+Let us see how the algorithm works:
+<ul>
+ <li> Initially positions of nodes are computed without taking into
+ account the enforced vertex (yellow point).</li>
+\image html hypo_quad_params_enfnodes_algo1.png "Initial mesh"
+
+ <li> Then the node closest to the enforced vertex is
+ detected. Extreme nodes of the row and column of the detected node
+ are used to create virtual edges (yellow lines) ending at the
+ enforced vertex. </li>
+\image html hypo_quad_params_enfnodes_algo2.png "Creation of virtual edges"
+
+ <li> Consequently, the meshed face is divided by the virtual
+ edges into four quadrilateral sub-domains each of which is meshed
+ as usually: the nodes of the row and column of the detected node are
+ moved to the virtual edges and the quadrilateral elements are
+ constructed.
+
+\image html hypo_quad_params_enfnodes_algo3.png "Final mesh"
+
+</ul>
+If there are several enforced vertices, the algorithm is applied
+recursively to the formed sub-domains.