small modif to avoid assert on compute_PD()

git-svn-id: svn+ssh://scm.gforge.inria.fr/svnroot/gudhi/branches/Nerve_GIC@3459 636b058d-ea47-450e-bf9e-a15bfbe3eedb

Former-commit-id: b0e73b4e1e77743419b06d142467ebfceacf83cf

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+---
+layout: page
+title: "Cover complex"
+meta_title: "Cover complex"
+teaser: ""
+permalink: /covercomplex/
+---
+{::comment}
+Leave the lines above as it is required by the web site generator 'Jekyll'
+{:/comment}
+
+
+## Nerve ##
+This program builds the Nerve of a point cloud sampled on an OFF file.
+The cover C comes from the preimages of intervals covering a coordinate function,
+which are then refined into their connected components using the triangulation of the .OFF file.
+
+The program also writes a file SC.txt.
+The first three lines in this file are the location of the input point cloud and the function used to compute the cover.
+The fourth line contains the number of vertices nv and edges ne of the Nerve. The next nv lines represent the vertices.
+Each line contains the vertex ID, the number of data points it contains, and their average color function value.
+Finally, the next ne lines represent the edges, characterized by the ID of their vertices.
+
+**Usage**
+
+`Nerve <OFF input file> coordinate resolution gain [-v]`
+
+where
+
+* `coordinate` is the coordinate function to cover
+* `resolution` is the number of the intervals
+* `gain` is the gain for each interval
+* `-v` is optional, it activates verbose mode.
+
+**Example**
+
+`Nerve ../../data/points/human.off 2 10 0.3`
+
+* Builds the Nerve of a point cloud sampled on a 3D human shape (human.off).
+The cover C comes from the preimages of intervals (10 intervals with gain 0.3) covering the height function (coordinate 2).
+
+`python KeplerMapperVisuFromTxtFile.py -f ../../data/points/human.off_sc.txt`
+
+* Constructs `human.off_sc.html` file. You can now use your favorite web browser to visualize it.
+
+## VoronoiGIC ##
+
+This util builds the Graph Induced Complex (GIC) of a point cloud.
+It subsamples *N* points in the point cloud, which act as seeds of a geodesic Voronoï diagram.
+Each cell of the diagram is then an element of C.
+
+The program also writes a file `*_sc.off`, that is an OFF file that can be visualized with GeomView.
+
+**Usage**
+
+`VoroniGIC <OFF input file> samples_number [-v]`
+
+where
+
+* `samples_number` is the number of samples to take from the point cloud
+* `-v` is optional, it activates verbose mode.
+
+**Example**
+
+`VoroniGIC ../../data/points/human.off 700`
+
+* Builds the Voronoi Graph Induced Complex with 700 subsamples from `human.off` file.
+`../../data/points/human_sc.off` can be visualized with GeomView.
+