.. role:: blue
   :class: blue

Constructing Polygons for an Object
===================================
.. _Constructing_Polygons_for_an_Object_:

.. kernel version: 

The virtual method of three-dimensional geometric objects

.. code-block:: cpp

   void MbSpaceItem::CalculateWire( double sag,
                                    MbMesh & mesh );

fills the supplied polygonal object *mesh* with a set of polygons that approximate the geometric object.

The input parameter of the method is *sag* – the maximum allowable deviation of a polygonal element from the original by sag.

The output parameter of the method is the polygonal object *mesh*.

The method is declared in the *space_item.h* file and in the header files of :blue:`MbSpaceItem` inheritors.

The polygonal object is described in the subsection :ref:`MbMesh Polygonal Object <MbMesh_Polygonal_Object_>`. The *sag* parameter defines the maximum allowable distance between the object and the polyline passing through the polygon points. This method fills only the set of polygon pointers *mesh.wires* (polylines).

The method approximates a curve with a single polygon. It approximates a contour with several polygons, each passing along the corresponding contour segment. Fig. 1 shows a curve and its polygonal object consisting of a single polygon.

.. figure:: ../../pictures/R.1.4.1.png
   :width: 9cm
   :align: center

   Fig. 1

The method approximates a surface with a set of polygons passing along *u*-lines, *v*-lines and along the surface boundary. Fig. 2 shows a polygonal object of a surface consisting of several polylines.

.. figure:: ../../pictures/R.1.4.2.png
   :width: 9cm
   :align: center

   Fig. 2

The method approximates a solid with a set of polylines passing inside each face along the *u*-lines and *v*-lines of the face surfaces and with a set of polylines approximating the curves on which the solid edges are based. Fig. 3 (right) shows a polygonal object of a solid:

.. figure:: ../../pictures/R.1.4.3.png
   :width: 15cm
   :align: center

   Fig. 3

For geometric model objects, this method works similarly to the :blue:`CalculateMesh`\() method when *stepData.stepType* == *ist_SpaceStep*, *stepData.sag* == *sag*, *note.wire* == true, *note.seam* == true.