Find outline of triangle mesh and plot in MATLAB

Alec Jacobson

December 06, 2010

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Here's a little snippet to determine the edges along the boundary of a triangle mesh in matlab. Given that your vertex values are in V and your triangle indices are in F, this will fill O with a list of edges make up the outline of your mesh:

% Find all edges in mesh, note internal edges are repeated
E = sort([F(:,1) F(:,2); F(:,2) F(:,3); F(:,3) F(:,1)]')';
% determine uniqueness of edges
[u,m,n] = unique(E,'rows');
% determine counts for each unique edge
counts = accumarray(n(:), 1);
% extract edges that only occurred once
O = u(counts==1,:);

Then you can view the outline with, in 2D:

plot([V(O(:,1),1) V(O(:,2),1)]',[V(O(:,1),2) V(O(:,2),2)]','-')

and in 3D:

plot3([V(O(:,1),1) V(O(:,2),1)]',[V(O(:,1),2) V(O(:,2),2)]',[V(O(:,1),3) V(O(:,2),3)]','-')