# [Gmsh] question about the direction of normals on the surface

**Toru Takahashi**
ttaka at atlas.riken.go.jp

*Mon May 20 03:44:01 CEST 2002*

Hi.
I'm a beginner of 'gmsh' and interested in structuring surface data rather than
volume data or FE analysis.
In my situation, it is required that all the normals of triangle elements on the surface,
generated by the function of 2D-meshing, must be directed outward to the surface.
Here, the surface is simply modeled with the some commands (Point, Line, Line Loop,
Plane Surface or Extrude). And the method of constructing of normals using the gmsh's
mesh data is determined uniquely i.e. if 72 th triangle element is defined as
"72 2 1 1 3 45 48 50" in the mesh data under consideration, the direction of the normal
is determined as same as that of the vector calculated by
(position of 48th element - 45th ) x (50th - 45th),
where 'x' means the vector product of two vectors in 3D.
How should I satisfy the above requirement ?
thanks