# Determine boundary faces from tetrahedral mesh

## weblog/

Here's a matlab function that takes a list of tetrahedron indices (4 indices to a row) and finds the triangles that are on the surface of the volume. It does this simply by finding all of the faces that only occur once.
``````function F = boundary_faces(T)
% BOUNDARY_FACES
% F = boundary_faces(T)
% Determine boundary faces of tetrahedra stored in T
%
% Input:
%  T  tetrahedron index list, m by 4, where m is the number of tetrahedra
%
% Output:
%  F  list of boundary faces, n by 3, where n is the number of boundary faces
%

% get all faces
allF = [ ...
T(:,1) T(:,2) T(:,3); ...
T(:,1) T(:,3) T(:,4); ...
T(:,1) T(:,4) T(:,2); ...
T(:,2) T(:,4) T(:,3)];
% sort rows so that faces are reorder in ascending order of indices
sortedF = sort(allF,2);
% determine uniqueness of faces
[u,m,n] = unique(sortedF,'rows');
% determine counts for each unique face
counts = accumarray(n(:), 1);
% extract faces that only occurred once
sorted_exteriorF = u(counts == 1,:);
% find in original faces so that ordering of indices is correct
F = allF(ismember(sortedF,sorted_exteriorF,'rows'),:);
end
``````
With this you can easily determine the vertices of a tetmesh that are on the boundary:
``````% V are your vertex positions, T are your tet indices
% get boundary faces
F = boundary_faces(T);
% get boundary vertices
b = unique(F(:));
subplot(1,2,1);
% plot boundary positions
plot3(V(b,1),V(b,2),V(b,3),'.');
subplot(1,2,2);
% plot just  boundary faces
trisurf(F,V(:,1),V(:,2),V(:,3),'FaceAlpha',0.3)
``````