## Face definition

- @FACE_DEFINITION {
- @FACE_NAME {
*FaceName*} { - @CONNECTED_TO_EDGES {
*Edge0Name, Edge1Name, Edge2Name, Edge3Name*} - @SURFACE_NAME {
*SurfaceName*} - @COMMENTS {
*CommentText*} - }
- }

### Introduction

Figure 1. Configuration of a face.

An **face** is a topological entity, see fig. 1, which points to four edges, ** Edge0Name**,

**,**

*Edge1Name***and**

*Edge2Name***. Figure 1 shows the configuration of a face, with its four associated edges. Note that two adjacent edges must share a common vertex; hence, the face is also associated with four distinct vertices.**

*Edge3Name*The order in which the vertices of each edge are defined is unimportant. Two adjacent edges, however, must share a common vertex. For instance, in fig. 1, edges ** EdgeAB** and

**share a common vertex,**

*EdgeAC***. Clearly, the face is then associated with four vertices**

*VertexA***,**

*v0 = VertexC***,**

*v1 = VertexA***and**

*v2 = VertexB***. In it not necessary to define the vertices since they are inherited from the definition of the edges. By definition, vertex**

*v3 = VertexD**v0*is the common vertex of edges

*and*

**e3***,*

**e0***that of edges*

**v1***and*

**e0***,*

**e1***that of edges*

**v2***and*

**e1***, and*

**e2***that of edges*

**v3***and*

**e2***. Note that the edges must be defined in a cyclic order while rotating around the face. It then becomes possible to assign a normal to the face: based on the right hand rule, the normal to the face in fig. 1 points out of the plane of the sceen.*

**e3**