[Gmsh] How are set up U and V vectors for elements ?

[bilou at netc.it]

Hello. I'm trying to parse a mesh.msh generated by gmsh.

I work with 9-quad elements. I see in documentation that inside of the elements, nodes are ordered like this :

           v
           ^
           |    
3-------6-------2
|          |         |
|          |         |
7        8-------5 --> u 
|                    |
|                    |
0-------4-------1

I made a 2D mesh (all surfaces normals are oriented in the same direction)

When i look at my 2D mesh (in plan XY) the element 265 :
265 2 44 1047 533 49 1102 1103 544 1104

is oriented like this related to the golbal X and Y axis

ie, when looking at the screen i see:
  44-------49-------2     
     |           |         |     Y
     |           |         |     ^
1102   1104    544    |
     |           |         |     |
     |           |         |     |
1047--1103----533    x------> X

it seems that u anv v vectors are oriented like this :
        1-----4-----0
        |        |       |
        |        |       |
u <---5-----8      7
        |        |       |
        |        |       |
        2-----6-----3
                 |
                V
                v
                   

How are V and U vectors defined for each element ? Is there a way to orient them along the natural X and Y axis ?

Thanks