.. role:: blue
   :class: blue

Constructing a Polygonal Object
===============================
.. _Constructing_a_Polygonal_Object_:

.. kernel version: 

The virtual method of geometric model objects

.. code-block:: cpp

   MbItem * MbItem::CalculateMesh( const MbStepData &      stepData,
                                   const MbFormNote &      note,
                                         MbRegDuplicate *  iReg ) const;

constructs a polygonal object that approximates the given geometric model object.

The input parameters of the method are:

- *stepData* – data for calculating the approximation step;
- *note* – method of constructing the polygonal object;
- *iReg* – registrar of copied objects.

If the method succeeds, it returns a pointer to the newly constructed geometric model object *MbItem*; otherwise it returns null.

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

This method approximates model objects by creating polygonal copies with a similar structure. For a solid, a wireframe and a point frame, the method creates a polygonal object :ref:`MbMesh <MbMesh_Polygonal_Object_>` that approximates the original object and returns a pointer to the created object. Each triangulation *grids*\[*i*] of the :ref:`MbMesh <MbMesh_Polygonal_Object_>` object approximates the *i*-th face, each polygon *wires*\[*i*] of the :ref:`MbMesh <MbMesh_Polygonal_Object_>` object approximates the *i*-th edge, each apex *peaks*\[*i*] of the :ref:`MbMesh <MbMesh_Polygonal_Object_>` object approximates the *i*-th vertex. For a polygonal object *MbMesh* this method creates a polygonal copy. For an insertion this method creates an insertion with the object that this method creates for the insertion content. For an assembly unit this method creates an assembly unit with the objects that this method creates for the assembly unit objects.

The *stepData* parameter controls the density of the polygonal object mesh and contains data for calculating the step when moving along curves and surfaces of the model object. The *note* parameter defines the way to construct the polygonal object. The *stepData* and *note* parameters are described in the :ref:`Triangulation_Calculation_Control <Triangulation_Calculation_Control_>` paragraph. If *note.wire* == true, the method fills the set of polygon pointers *mesh.wires*; if *note.grid* == true, the method fills the set of triangulation pointers *mesh.grids* (for faces and surfaces), the set of polygon pointers *mesh.wires* (for edges), and the set of apex pointers *mesh.peakes* (for vertices).

The *iReg* parameter can be null. It is used to pass information about already processed objects to nested methods.

Fig. 1 shows a polygonal object of a solid constructed with *note.wire* == false, *note.grid* == true, containing triangulations, polygons and apexes. Fig. 2 shows a polygonal object of a solid constructed with *note.wire* == true, *note.grid* == false, containing only polygons.

.. figure:: ../../pictures/R.1.2.1.png
   :width: 9cm
   :align: center

   Fig. 1

.. figure:: ../../pictures/R.1.2.2.png
   :width: 9cm
   :align: center

   Fig. 2

Fig. 3 shows a polygonal object of an assembly unit constructed with *note.wire* == false, *note.grid* == true, consisting of an assembly unit of polygonal objects approximating the parts:

.. _R.1.2.3_figure:

.. figure:: ../../pictures/R.1.2.3.png
   :width: 9cm
   :align: center

   Fig. 3

This method is used to visualize geometric model objects. Polygonal objects are easy to transform, and intersections with a straight line can be found quickly for them. The screen displays exactly the polygonal copy of the geometric model object, while the object in its original representation remains behind the scenes.