Sources from http://www.mathworks.com/matlabcentral/fileexchange/6110-toolbox-fast-marching

This is an implementation of geodesic (shortest path) algorithm for triangular mesh (triangulated surface) first described by Mitchell, Mount and Papadimitriou in 1987[1] with some minor improvements, extensions and simplifications. The algorithm has O(n^2 log n) worst-case time complexity, but in practice can work with million-node meshes in reasonable time. For the quick overview, see [2].

The basic idea of the algorithm is very similar to Dijkstra's algorithm for finding shortest path on a weighted graph. It has two steps:

• propagation of the distance field from sources over the surface of the mesh (slow)
• tracing back the shortest path from target point to the closest source (fast)

For debugging and comparison purposes I also implemented two approximate algorithms

• Dijkstra shortest path on the graph created by the vertices and edges of the mesh
• Subdivision (put N additional vertices on every edge of the mesh, directly connect all vertices belonging to the same face, run Dijkstra on the resulting graph) The nice property of the subdivision algorithm is that it becomes Dijkstra when N=0 and computes exact distances when N->infinity.

The input mesh is represented as two arrays: vertices (each vertex has tree coordinates) and faces (each face is represented as indices of its vertices). Most of the communication with the algorithms is done through SurfacePoints (points on the surface of the mesh; they have three coordinates and a pointer to a mesh element they belong).

The algorithms are available as C++ code (downloadable at http://code.google.com/p/geodesic/) and Matlab toolbox (downloadable at MathWorks File Exchange).

C++ NOTES The base class of for all algorithms is GeodesicAlgorithmBase (defined in geodesic_algorithm_base.h). The most important functions defined in this class are

• propagate(...). It is possible to stop propagation after it reaches certain distance or covers certain points on the surface of the mesh; O(n^2 log n).
• trace_back(…). It traces the shortest path from target to the nearest source; O(n) .
• best_source(…). For a given point on the surface of the mesh, this function reports the closest source and the distance to this source; O(1). Read example0.cpp and example1.cpp – they are self-explanatory.

MATLAB NOTES

• you do not have to compile anything; the algorithm is already available as C library (it has _debug and _release versions)
• just in case, the header of the library is geodesic_matlab_api.h; all the functions are instantiated in geodesic_matlab_api.c (this is the only file you have to compile for the library).
• only Windows version of the library is provided. If you need them on other operating systems, you are on your own – download C++ code and compile geodesic_matlab_api.c into a library Play with example1.m – example5.m; they are self-explanatory.

Limitations and known issues.

The mesh should be edge-connected. I.e. for every two faces of the mesh there should exist a connecting path on the surface of the mesh that does not go through any vertex. This property implies the simple connectivity of the mesh. In particular, there should be no vertices that are not used by any of the triangles.

There is a list of possible features and improvements that could be done to the algorithm.

• Memory is currently a bottleneck for large meshes. In theory, it is possible to overcome it if all destinations are known BEFORE the propagation step.
• For large flat parts of the mesh (vertices whose total adjacent angles sum up to exactly 2*pi) the current version of the algorithm uses more time and memory than necessary.

ACKNOWLEDGEMENTS I am very grateful to Steven Gortler without whom this project would never exist. Hugues Hoppe helped to develop the early version of the algorithm by providing his mesh processing code as well as many helpful advices. Tatiana and Vitaly Surazhsky did a great job analyzing the complexity of the algorithm for the “regular” meshes.

REPORTING BUGS AND SENDING LOVE/HATE MAIL If you have a bug, first check your mesh and run the algorithm in debug mode. Most likely, the mesh is numbered incorrectly or disconnected or of somehow degenerate. Usually, I am not very good communicating with people, but you can try reaching me at exact.geodesic@gmail.com

Best, Danil Kirsanov, 01/2008

[1] J.S.B. Mitchell, D.~M. Mount, and C.~H. Papadimitriou. SIAM J. Comput., 16:647--668, 1987. [2] J. O'Rourke, Computational Geometry Column 35, SIGACT News, 30(2) Issue #111 (1999) 31-32, 1993 (could be found at citeseer.ist.psu.edu)

CHANGES ON 03/03/08

• fixed libpointer conflict with matlab 2007b
• fixed cellfun error with earlier versions of matlab