Assign connectivity ID to 2D triangular mesh

This function assigns an ID based on the connectivity for each cell of a 2D triangular mesh.

Description

This function takes an unrestricted triangulation (only the list of triangles, not the node coordinates) and assigns an ID to each group of triangles. If some cells are connected together, each cell of that group will have the same ID. It works like the connectivity-filter of the graphical library VTK.

function groupID=connectivityTri(TRI)
Input :
         "TRI" is mx3 matrix which is the standard list of index of vertices
Output :
          "groupID" is mx1 matrix which is the list of ID-group of each cell

Simple example :

X=[2 1 3 2 5 5 8 6 7 5 8 9 10 12 10 12 13 15 1 0 2 1];
Y=[2 4 4 6 6 8 8 4 2 2 4 6 8 7 5 5 3 2 7 8 8 9];
Z=zeros(size(X));
TRI=[1 2 3;2 4 3;4 3 5;5 6 7;10 8 9;8 9 11;12 13 14;15 16 17;17 16 18;4 6 5;20 19 21;22 21 20];
groupID=connectivityTri(TRI);

trisurf(TRI,X,Y,Z,groupID) (the screenshot of the soumission)

Acknowledgements

The author wishes to acknowledge the following in the creation of this submission:
Toolbox Wavelets on Meshes
This submission has inspired the following:
Remove Unconnected Triangle

• MATLAB release: MATLAB 7.8 (R2009a)

[Óscar J. Rubio Martín]

Nice work, fast and accurate. It will let me save a lot of time.