add custom initialization of gray image to user manual

and re-organize to have both custom initializations (gray level and
segmented) together

Mesh_3/doc/Mesh_3/Mesh_3.txt

@@ -573,13 +573,6 @@ constructor of the `Mesh_criteria` instance.
 Cut view of a 3D mesh produced from an implicit domain
 \cgalFigureEnd
 
-\subsubsection Mesh_33DDomainsGrayImageIsosurfaces 3D Domains Bounded by Isosurfaces in 3D Gray-Level Images
-
-The following example produces a 3D mesh for a domain whose boundary surface
-is the isosurface associated to an isovalue inside the input gray-level
-3D image. In the distribution you can also find the example \ref Mesh_3/mesh_3D_gray_vtk_image.cpp which can deal with DICOM files as input.
-
-\cgalExample{Mesh_3/mesh_3D_gray_image.cpp}
 
 \subsection Mesh_3MeshingMultipleDomains Meshing Multiple Domains
 
@@ -682,8 +675,18 @@ This allows to remesh a surface, and is equivalent to the function `make_surface
 View of a remeshed surface. (Left) input mesh (Right) output mesh. Code from subsection \ref Mesh_3RemeshingPolyhedralSurface generates the file.
 \cgalFigureEnd
 
-\subsection Mesh_3DomainsFromSegmented3DImages Domains From Segmented 3D Images
+\subsection Mesh_3DomainsFrom3DImages Domains From 3D Images
 
+\subsubsection Mesh_33DDomainsGrayImageIsosurfaces 3D Domains Bounded by Isosurfaces in 3D Gray-Level Images
+
+The following example produces a 3D mesh for a domain whose boundary surface
+is the isosurface associated to an isovalue inside the input gray-level
+3D image. In the distribution you can also find the example \ref Mesh_3/mesh_3D_gray_vtk_image.cpp which can deal with DICOM files as input.
+
+\cgalExample{Mesh_3/mesh_3D_gray_image.cpp}
+
+
+\subsubsection Mesh_3DomainsFromSegmented3DImages Domains From Segmented 3D Images
 \anchor Mesh_3_subsection_examples_3d_image
 The following code produces a 3D mesh from
 a 3D image. The image is a segmented medical image in which each
@@ -703,7 +706,7 @@ The resulting mesh is shown in \cgalFigureRef{figureliver_3d_image_mesh}.
 Cut view of a 3D mesh produced from a segmented liver image. Code from subsection \ref Mesh_3_subsection_examples_3d_image generates this file.
 \cgalFigureEnd
 
-\subsubsection Mesh_3DomainsFromSegmented3DImagesWithCustomInitialization Domains From Segmented 3D Images, with a Custom Initialization
+\subsubsection Mesh_3DomainsFrom3DImagesWithCustomInitialization Domains From 3D Images, with a Custom Initialization
 
 The example \ref Mesh_3/mesh_3D_image_with_custom_initialization.cpp is a modification
 of \ref Mesh_3/mesh_3D_image.cpp. The goal of that example is to show how
@@ -788,6 +791,25 @@ create a 3D image using the undocumented API of CGAL_ImageIO.
 The code of the function `%random_labeled_image()` is in the header file \ref
 Mesh_3/random_labeled_image.h\.
 
+
+The example \ref Mesh_3/mesh_3D_gray_image_with_custom_initialization.cpp is another
+custom initialization example, for meshing of 3D gray-level images. Similarly to
+the segmented image example above, the code consists in:
+  -# the creation of an empty `%c3t3` object,
+  -# a call to a non-documented function
+    `initialize_triangulation_from_gray_image()` that inserts points in
+    the triangulation,
+  -# then the call to `refine_mesh_3()`.
+
+\snippet Mesh_3/mesh_3D_gray_image_with_custom_initialization.cpp Meshing
+
+The code of the function `initialize_triangulation_from_gray_image()` is
+in the non-documented header \ref
+CGAL/Mesh_3/initialize_triangulation_from_gray_image.h\. As it is
+undocumented and may be removed or modified at any time, if you wish to
+use it then you should copy-paste it to your user code.
+
+
 \subsection Mesh_3UsingVariableSizingField Using Variable Sizing Field
 
 \subsubsection Mesh_3SizingFieldasanAnalyticalFunction Sizing Field as an Analytical Function

Mesh_3/doc/Mesh_3/examples.txt

@@ -1,9 +1,11 @@
 /*!
 \example Mesh_3/implicit_functions.cpp
 \example Mesh_3/mesh_3D_image.cpp
+\example Mesh_3/mesh_3D_gray_image_with_custom_initialization.cpp
 \example Mesh_3/mesh_3D_image_with_features.cpp
 \example Mesh_3/mesh_3D_image_with_custom_initialization.cpp
 \example Mesh_3/random_labeled_image.h
+\example CGAL/Mesh_3/initialize_triangulation_from_gray_image.h
 \example CGAL/Mesh_3/initialize_triangulation_from_labeled_image.h
 \example Mesh_3/mesh_3D_image_variable_size.cpp
 \example Mesh_3/mesh_hybrid_mesh_domain.cpp

Mesh_3/examples/Mesh_3/mesh_3D_gray_image_with_custom_initialization.cpp

@@ -59,7 +59,7 @@ int main(int argc, char* argv[])
                                            domain,
                                            image,
                                            criteria,
-                                           2.9f,
+                                           2.9f,//isolevel
                                            Image_word_type(0));
   CGAL::refine_mesh_3(c3t3, domain, criteria);
   /// [Meshing]