.. _cutting_quadrangles_page:

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Cutting quadrangles
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This operation allows cutting one or several quadrangle elements into two or four triangles.

*To cut quadrangles:*

.. |img| image:: ../images/image82.png

#. Select a mesh (and display it in the 3D Viewer if you are going to pick elements by mouse).
#. In the **Modification** menu select the **Cutting of quadrangles** item or click *"Cutting of quadrangles"* button |img| in the toolbar.

   The following dialog box will appear:

   .. image:: ../images/a-cuttingofquadrangles.png
      :align: center


   * The main list contains the list of quadrangles selected for cutting. You can click on a quadrangle in the 3D viewer and it will be highlighted (lock Shift keyboard button to select several quadrangles): 
	* Click **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 **Remove** button. 
	* **Sort list** button allows sorting the list of IDs. 
	* **Filter** button allows applying a definite :ref:`filter <filtering_elements>` to the selection of quadrangles.
   * **Apply to all** check box allows cutting all quadrangles of the selected mesh.
   * **Preview** provides a preview of cutting in the viewer. It is disabled for **Cut into 4 triangles** as this cutting way implies no ambiguity.
   * **Criterion** defines the way of cutting:
	* **Cut into 4 triangles** allows cutting a quadrangle into four triangles by inserting a new node at the center of the quadrangle. The other options allow cutting a quadrangle into two triangles by connecting the nodes of a diagonal.
	* **Use diagonal 1-3** and **Use diagonal 2-4** allow specifying the opposite corners, which will be connected to form two new triangles.
	* **Use numeric functor** allows selecting in the field below a quality metric, which will be optimized when choosing a diagonal for cutting a quadrangle: 
		* **Minimum diagonal** cuts by the shortest diagonal.
		* **Aspect Ratio** cuts by the diagonal splitting the quadrangle into triangles with :ref:`Aspect Ratio <aspect_ratio_page>` closer to 1
		* **Minimum Angle** cuts by the diagonal splitting the quadrangle into triangles with :ref:`Minimum Angle <minimum_angle_page>` closer to 60 degrees.
		* **Skew** cuts by the diagonal splitting the quadrangle into triangles with :ref:`Skew <skew_page>` closer to 0.0 degrees.
   * **Select from** allows choosing a sub-mesh or an existing group, whose quadrangle elements then can be added to the main list.

#. Click the **Apply** or **Apply and Close** button to confirm the operation.

.. image:: ../images/image52.jpg 
	:align: center

.. centered::
	The chosen quadrangular element

|

.. image:: ../images/image51.jpg 
	:align: center

.. centered::
	Two resulting triangular elements

**See Also** a sample TUI Script of a :ref:`tui_cutting_quadrangles` operation.