From 47b758ff3831b689c20861b1795840b459cd1f36 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Loriot?= <sebastien.loriot@cgal.org>
Date: Mon, 20 Aug 2018 10:26:37 +0200
Subject: [PATCH] add an example showing how to dump into OFF visible facets of
 an alpha-shape

---
 .../visible_alpha_shape_facets_to_OFF.cpp     | 121 ++++++++++++++++++
 1 file changed, 121 insertions(+)
 create mode 100644 Alpha_shapes_3/examples/Alpha_shapes_3/visible_alpha_shape_facets_to_OFF.cpp

diff --git a/Alpha_shapes_3/examples/Alpha_shapes_3/visible_alpha_shape_facets_to_OFF.cpp b/Alpha_shapes_3/examples/Alpha_shapes_3/visible_alpha_shape_facets_to_OFF.cpp
new file mode 100644
index 00000000000..62258688fe4
--- /dev/null
+++ b/Alpha_shapes_3/examples/Alpha_shapes_3/visible_alpha_shape_facets_to_OFF.cpp
@@ -0,0 +1,121 @@
+#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
+
+#include <CGAL/Alpha_shape_3.h>
+#include <CGAL/Alpha_shape_cell_base_3.h>
+#include <CGAL/Alpha_shape_vertex_base_3.h>
+#include <CGAL/Delaunay_triangulation_3.h>
+
+#include <fstream>
+#include <vector>
+
+#include <boost/unordered_set.hpp>
+#include <boost/unordered_map.hpp>
+#include <boost/foreach.hpp>
+
+typedef CGAL::Exact_predicates_inexact_constructions_kernel Gt;
+
+typedef CGAL::Alpha_shape_vertex_base_3<Gt>          Vb;
+typedef CGAL::Alpha_shape_cell_base_3<Gt>            Fb;
+typedef CGAL::Triangulation_data_structure_3<Vb,Fb>  Tds;
+typedef CGAL::Delaunay_triangulation_3<Gt,Tds>       Triangulation_3;
+typedef CGAL::Alpha_shape_3<Triangulation_3>         Alpha_shape_3;
+
+typedef Gt::Point_3                                  Point;
+typedef Alpha_shape_3::Alpha_iterator                Alpha_iterator;
+
+int main()
+{
+  std::vector<Point> points;
+
+//read input
+  std::ifstream is("data/bunny_5000");
+  int n;
+  is >> n;
+  double x, y, z;
+  for (int i=0;i<n;++i)
+  {
+    is >> x >> y >> z;
+    points.push_back( Point(x, y, z) );
+  }
+
+  std::cerr << points.size() << " points read.\n";
+// compute alpha shape
+  Alpha_shape_3 as(points.begin(), points.end());
+  Alpha_shape_3::NT alpha_solid = as.find_alpha_solid();
+  as.set_alpha(alpha_solid);
+
+  std::cerr << "alpha_solid = " << alpha_solid << "\n";
+  std::cerr << as.number_of_solid_components() << " number of solid components\n";
+
+// collect alpha-shape facets accessible from the infinity
+  // marks the cells that are in the same component as the infinite vertex by flooding
+  boost::unordered_set< Alpha_shape_3::Cell_handle > marked_cells;
+  std::vector< Alpha_shape_3::Cell_handle > queue;
+  queue.push_back( as.infinite_cell() );
+
+  while(!queue.empty())
+  {
+    Alpha_shape_3::Cell_handle back = queue.back();
+    queue.pop_back();
+
+    if ( !marked_cells.insert(back).second ) continue; //already visited
+
+    for (int i=0; i<4; ++i)
+    {
+      if (as.classify(Alpha_shape_3::Facet(back, i))==Alpha_shape_3::EXTERIOR &&
+          marked_cells.count(back->neighbor(i))==0)
+        queue.push_back( back->neighbor(i) );
+    }
+  }
+
+  // filter regular facets to restrict them to those adjacent to a marked cell
+  std::vector< Alpha_shape_3::Facet > regular_facets;
+  as.get_alpha_shape_facets(std::back_inserter( regular_facets ), Alpha_shape_3::REGULAR );
+
+  std::vector<Alpha_shape_3::Facet> filtered_regular_facets;
+  BOOST_FOREACH(Alpha_shape_3::Facet f, regular_facets)
+  {
+    if ( marked_cells.count(f.first)==1 )
+      filtered_regular_facets.push_back(f);
+    else
+    {
+      f = as.mirror_facet(f);
+      if ( marked_cells.count(f.first)==1 )
+        filtered_regular_facets.push_back(f);
+    }
+  }
+
+// dump into OFF format
+  // assign an id per vertex
+  boost::unordered_map< Alpha_shape_3::Vertex_handle, int> vids;
+  points.clear();
+
+  BOOST_FOREACH(Alpha_shape_3::Facet f, filtered_regular_facets)
+  {
+    for (int i=1;i<4; ++i)
+    {
+      Alpha_shape_3::Vertex_handle vh = f.first->vertex((f.second+i)%4);
+      auto p = vids.insert( std::make_pair(vh, points.size()) );
+      if (p.second)
+        points.push_back( vh->point() );
+    }
+  }
+
+  // writing
+  std::ofstream output("out.off");
+  output << "OFF\n " << points.size() << " " << filtered_regular_facets.size() << " 0\n";
+  std::copy(points.begin(), points.end(), std::ostream_iterator<Point>(output, "\n"));
+  BOOST_FOREACH(const Alpha_shape_3::Facet& f, filtered_regular_facets)
+  {
+    output << 3;
+
+    for (std::size_t i=0;i<3; ++i)
+    {
+      Alpha_shape_3::Vertex_handle vh = f.first->vertex( as.vertex_triple_index(f.second, i) );
+      output << " " << vids[vh];
+    }
+    output << "\n";
+  }
+
+  return 0;
+}