triangulation_boundary, a MATLAB code which starts with a triangulation of a simple shape, and determines the triangle edges that form the boundary, and a sequence of nodes that trace the boundary.
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triangulation_boundary is available in a MATLAB version and an Octave version.
mesh_boundary, a MATLAB code which is given a mesh of a 2D region, formed by polygonal elements, and determines a sequence of pairs of polygonal vertices that define the boundary of the region.