CGAL users discussion list

[Manuel Caroli <>]

Hello Hui Ding,

>     I have some data files that contain the values 3D points of the
>     object 3D, and I'd like to use CGAL to represent the object. I just
>     looked into the chapter22 of manuel, and i have some questions about
>     both concept of science and use of library, I will appreciate it, if
>     someone gives some helps:
I hope you are talking about chapter 27.
>      
>     1) I can see that a vertex has /0/-face, the edge has 1 face, and a
>     facet has 2 faces, but I don't understand how a cell has 3 face? why
>     not 4, if a cell is a 3 simplex(tetrahedron)?
The numbers actually indicate the dimension.
Thus a cell IS a 3-face, whereas a facet IS a 2-face, an edge a 1-face 
and a vertex a 0-face.

>     2) From some basic exemple of chapiter 22, i saw that we get the
>     triangulation information by using "/ostream/", but I dont
>     understand what every value stands for. (the dimention, number of
>     finite, non-combinatorial, the number of cells are clear, but the
>     others i cant figure out, anyone can help?).
btw. it would be helpful to be a little more precise. I assume you are 
talking about example 27.6.1, right? I'm not sure having understood your 
question in the right way. In this example the triangulation is written 
to a file via a std::ofstream using the output operator << .
This is described under section "I/O" in 
http://www.cgal.org/Manual/3.3/doc_html/cgal_manual/Triangulation_3_ref/Class_Triangulation_3.html


>     3) Could I easily put my 3D points data to Triangulation_3, and then
>     use the ostream to output the triangulation information of this
>     object? and is there any "Viewer" can read this information and show
>     the object in 3D? Or do i need to write it myself what chapiter do i
>     need to read?
See the link above, it is suggested to use GeomView.

>     4) Once I construct the triangulation of my object 3D, I need the
>     normal of every facet of every cell, is there already some functions
>     who do that? or i need to calculate myself? how?
I'm currently not aware of any such function.
>     5) I read only chapter 22 for my work right now(cause someone told
>     me so), but if anyone has some conseils to learn the 3D
>     triangulations like some chapiters or some tuturials
>     corresponding to 3D triangulation or something else, please tell me?
You can have a look into the first introductory chapters about kernels 
and number types... apart from that chapter 27 (and maybe 28) should be 
fine.

Best

Manuel