# Nature of points, lines and faces

## Problems

There is a certain number of situations where the user may want to modify the consideration of the entities (points, lines, faces) during the automatic construction of the faces and/or volumes.

Two examples are given hereafter to illustrate this type of situation.

## Example 1

The first example reconsider the problem of intersection between faces (blocking for building the volumes). This example has already been presented.

In this example (figure to the right) the constitutive faces of the internal parallelepiped intersect the internal circular face of the torus. The volume of the bar inside the torus cannot be built.

To avoid this difficulty, it is necessary to ignore the two circular faces during the automatic construction of volumes.

## Example 2

The second example consists in the numerical modeling of a ship (“La Fayette” frigate of the French Marine represented in the figure below).

The ship structure is only made up of bars, which are represented and modeled via the lines (line regions). For this type of structure that is relatively complex, the use of the algorithm of faces and volumes construction is expensive, it often takes a long time and it generates many useless faces and volumes .

To avoid this difficulty, we should place the lines within an air volume without building the group of faces and volumes that will not be used.

## Solution: the nature attribute

To allow the user to modify the consideration of entities when building the faces and volumes, a specific feature (called nature) is attached to the points, lines and faces.

## Attribute nature

The nature attribute allows us to set the following functions:

Nature The entity is taken into account for:
the geometry the mesh
Standard (STANDARD) yes yes
in air (IN_AIR) no yes
ignored (NO_EXIST) no no

To ignore the annoying circular faces, the user should modify the nature of these faces (“ignored”) and restart the automatic building of volumes. These faces are then ignored at the geometry level (and also at the mesh level).

Note: These faces are not destroyed. They still exist and are visible on the screen (in the visibility conditions that allow their visualization).