Add a helper function to visualize a Cartesian_grid_3 object

Isosurfacing_3/include/CGAL/Isosurfacing_3/Cartesian_grid_3.h

@@ -18,9 +18,12 @@
 
 #include <CGAL/assertions.h>
 #include <CGAL/Bbox_3.h>
+#include <CGAL/boost/graph/named_params_helper.h>
+#include <CGAL/Named_function_parameters.h>
 #include <CGAL/Image_3.h>
 
 #include <array>
+#include <fstream>
 #include <type_traits>
 #include <vector>
 
@@ -368,6 +371,62 @@ operator Image_3() const
   return Image_3{ im, Image_3::OWN_THE_DATA };
 }
 
+namespace IO {
+
+template <typename GeomTraits,
+          typename NamedParameters = parameters::Default_named_parameters>
+bool write_OBJ(const std::string& filename,
+               const Cartesian_grid_3<GeomTraits>& grid,
+               const NamedParameters& np = parameters::default_values())
+{
+  using Point_3 = typename GeomTraits::Point_3;
+
+  auto x_coord = grid.geom_traits().compute_x_3_object();
+  auto y_coord = grid.geom_traits().compute_y_3_object();
+  auto z_coord = grid.geom_traits().compute_z_3_object();
+  auto vertex = grid.geom_traits().construct_vertex_3_object();
+
+  std::ofstream out(filename);
+  set_ascii_mode(out); // obj is ASCII only
+
+  set_stream_precision_from_NP(out, np);
+
+  if(out.fail())
+    return false;
+
+
+  // write vertices
+  for(std::size_t x=0; x<grid.xdim(); ++x) {
+    for(std::size_t y=0; y<grid.ydim(); ++y) {
+      for(std::size_t z=0; z<grid.zdim(); ++z)
+      {
+        const Point_3& p = vertex(grid.bbox(), 0);
+        const double x_coord_d = CGAL::to_double(x_coord(p) + x * grid.spacing()[0]);
+        const double y_coord_d = CGAL::to_double(y_coord(p) + y * grid.spacing()[1]);
+        const double z_coord_d = CGAL::to_double(z_coord(p) + z * grid.spacing()[2]);
+        out << "v " << x_coord_d << " " << y_coord_d << " " << z_coord_d << std::endl;
+      }
+    }
+  }
+
+  // write faces
+  for(std::size_t x=0; x<grid.xdim()-1; ++x) {
+    for(std::size_t y=0; y<grid.ydim()-1; ++y) {
+      for(std::size_t z=0; z<grid.zdim()-1; ++z)
+      {
+        const std::size_t v0 = (z * grid.ydim() + y) * grid.xdim() + x;
+        const std::size_t v1 = (z * grid.ydim() + y + 1) * grid.xdim() + x;
+        const std::size_t v2 = (z * grid.ydim() + y + 1) * grid.xdim() + x + 1;
+        const std::size_t v3 = (z * grid.ydim() + y) * grid.xdim() + x + 1;
+        out << "f " << v0+1 << " " << v1+1 << " " << v2+1 << " " << v3+1 << std::endl;
+      }
+    }
+  }
+
+  return out.good();
+}
+
+} // namespace IO
 } // namespace Isosurfacing
 } // namespace CGAL