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Hello everybody, I am really new to CGAL and now I try to learn 3D mesh
generation of the segmented 3D images. 
In the manual (Chapter 49: 3D Mesh Generation) I have found the following
sentence :
" The current implementation provides classes to represent domains bounded by
isosurfaces of implicit functions, polyhedral domains and domains defined
through 3D labeled images. Currently, 1-dimensional features may be defined as
segments and polyline segments. "

Please, does it mean that a 1D-feature (line) can be preserved also using
meshing of segmented 3D image? 
I've tried it without success.

I run succesfully two examples shown in the manual: "mesh_3D_image.cpp" and 
"mesh_two_implicit_spheres_with_balls.cpp", in the latter it is demonstrated
how an edge (polyline) can be preserved. Then I've tried to modify  the file
"mesh_3D_image.cpp" to implement 1D sharp feature when using the segmented
image. .
For that I use input image: "domain.inr", which describes a simple cube of
sizes 128x128x128 (one material only). 
(The header of domain.inr is:
#INRIMAGE-4#{
XDIM=128
YDIM=128
ZDIM=128
VDIM=1
TYPE=unsigned fixed
PIXSIZE=8 bits
CPU=pc
VX=1
VY=1
VZ=1
....
##}
)
Then I attempted to add 1D feature (such that one edge of the cube should be
preserved, i.e. sharp) to the  "mesh_3D_image.cpp". It was done consulting the
code in 
"mesh_two_implicit_spheres_with_balls.cpp".
The final code is compiled without errors, mesh of the cube is generated, but
the 1D feature IS NOT preserved.

My complete code is:
****************

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>

#include <CGAL/Mesh_triangulation_3.h>
#include <CGAL/Mesh_complex_3_in_triangulation_3.h>
#include <CGAL/Mesh_criteria_3.h>

#include <CGAL/Labeled_image_mesh_domain_3.h>
#include <CGAL/Mesh_domain_with_polyline_features_3.h> // added
#include <CGAL/make_mesh_3.h>
#include <CGAL/Image_3.h>

// Domain
typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
typedef K::Point_3 Point;  // added
typedef CGAL::Mesh_domain_with_polyline_features_3<
    CGAL::Labeled_image_mesh_domain_3<CGAL::Image_3,K> >    Mesh_domain;
//modified

// Polyline     //added
typedef std::vector<Point>        Polyline;
typedef std::list<Polyline>       Polylines;

// Triangulation
typedef CGAL::Mesh_triangulation_3<Mesh_domain>::type Tr;
typedef CGAL::Mesh_complex_3_in_triangulation_3<
    Tr,Mesh_domain::Corner_index,Mesh_domain::Curve_segment_index> C3t3;
//modified

// Criteria
typedef CGAL::Mesh_criteria_3<Tr> Mesh_criteria;

// To avoid verbose function and named parameters call
using namespace CGAL::parameters;

int main()
{
  // Loads image
  CGAL::Image_3 image;
  image.read("data/domain.inr");

  // Domain
  Mesh_domain domain(image);

  // Mesh criteria
  Mesh_criteria criteria(facet_angle=30, facet_size=6, facet_distance=4,
                         cell_radius_edge=3, cell_size=4);

// Create edge that we want to preserve
  Polylines polylines (1);
  Polyline& polyline = polylines.front();

    Point p=Point(0, 0, 0);
    polyline.push_back(p);
    Point p1=Point(127, 0, 0);
    polyline.push_back(p1);

 // Insert edge in domain
  domain.add_features(polylines.begin(), polylines.end());

  // Meshing
  C3t3 c3t3 = CGAL::make_mesh_3<C3t3>(domain, criteria);

  // Output
  std::ofstream medit_file("out.mesh");
  c3t3.output_to_medit(medit_file);

  return 0;
}

*************************

Please, can you see what I am missing, or some hints or suggestions?
Thank you, Ivan.