skbl doc

git-svn-id: svn+ssh://scm.gforge.inria.fr/svnroot/gudhi/trunk@363 636b058d-ea47-450e-bf9e-a15bfbe3eedb

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diff --git a/src/Contraction/include/gudhi/Edge_contraction.h b/src/Contraction/include/gudhi/Edge_contraction.h
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--- a/src/Contraction/include/gudhi/Edge_contraction.h
+++ b/src/Contraction/include/gudhi/Edge_contraction.h
@@ -95,8 +95,19 @@ package.
 
 \section Performance 
 
-
-TODO show curve CGAL
+The next figure shows the computation time to reduce a random 2-sphere to a single tetrahedron with 
+this package and with the CGAL equivalent package (to is specific to 2-manifold simplicial complexes).
+Despite this package is able to deal with \a arbitrary simplicial complexes (any dimensions, manifold or non manifold),
+it is still \a 65% times faster than CGAL package which is focused on 2-manifold. 
+The main reason is that few blockers appears during the simplification and hence,
+the algorithm only have to deal with the graph and not higher-dimensional simplices
+(in this case triangles). However, we recall that higher-dimensional simplices are \a implicitely 
+stored in the \ref skbl data-structure and hence, if one has to store nodes
+to simplices in an external map if storing information on simplices such 
+as orientation is needed.
+
+
+\image html "sphere_contraction.png" "Time to simplify random 2-spheres to a tetrahedron" width=10cm
 
 \section Example