I find your mail a bit rude !
--- On Tue, 9/2/08, Jernej Barbic <> wrote:
From: Jernej Barbic <>
Subject: Re: [cgal-discuss] my experience with CGAL
To:
Date: Tuesday, September 2, 2008, 1:43 PM
Hi Greg,
This should be very easy to do for any person proficient in C++. I
looked into Complex_2_in_triangulation_3_file_writer.h and redirected
the output to my memory buffer as opposed to an output stream.
As per releasing my code: sorry, I don't have the time to clean my
code and release it publicly (I am not a CGAL developer).
Regards,
Jernej
On Tue, Sep 2, 2008 at 4:09 PM, Greg Fokum <>
wrote:
> hello there
>
> Saw your correspondence and struggles with CGAL. Could you please share
your
> code on how you got to obtain the vertices and faces of the triangulation
> using Complex_2_in_triangulation_3_file_writer.h as suggested ?
>
> Regards
>
> GT
>
> --- On Tue, 9/2/08, Jernej Barbic <> wrote:
>
> From: Jernej Barbic <>
> Subject: [cgal-discuss] my experience with CGAL
> To:
> Date: Tuesday, September 2, 2008, 11:11 AM
>
> Dear CGAL,
>
> I am posting these two message by invitation of Dr. Pierre Alliez. I
> attended the CGAL course at SIGGRAPH in LA, and then tried using CGAL
> to do surface mesh generation. I then had an email discussion
> with
> Pierre, relating my experience with CGAL. Also, please note that
(due
> to other work) it might be difficult for me to keep up with the
> follow-ups; however, I wanted to share my experience, so I wrote the
> emails below. Best of luck to CGAL!
>
> Regards,
> Jernej Barbic
> Post-doc associate, Computer Science and Artificial Intelligence Lab,
> Massachusetts Institute of Technology
> http://people.csail.mit.edu/barbic/
>
> The first email message:
> ===================
>
> Dear Pierre,
>
> I am a computer graphics post-doc at MIT, and I saw your presentation
> in this year's SIGGRAPH class on CGAL in Los Angeles.
>
> I was impressed with the presentation and the capabilities of CGAL, so
> I tried using it as a tool in my project. Specifically, I wanted to
> use it to generate good quality triangulations of isosurfaces, as in
> the "skull"
>
example:
>
http://www.cgal.org/Manual/3.3/doc_html/cgal_manual/Surface_mesher/Chapter_main.html
>
> I managed to compile and run the "skull" example (under Mac OS
X).
> However, I then ran into a problem which will most likely seem quite
> trivial to you: I was unable to adapt the example so that I could
> actually capture the output. That is, CGAL will generate an output
> mesh; I then wanted to convert this mesh to my own in-memory
> representation. This representation would be very simple: a list of
> vertices, each given as three double precision values, followed by a
> list of faces (triangles), each a set of three integer indices into
> the preceding table of vertices. I was ultimately unable to figure out
> how to do this.
>
> I listened to your presentation at SIGGRAPH, and then read the CGAL
> manual several times. I spent two days looking through
many CGAL
> header files (about 8-10 hours of work total), following
> seemingly
> endless templated (and cryptic at times) arguments, in an attempt to
> find clues as per how to access vertices of triangulations, and how to
> obtain the integer indices of the vertices of every face. For example,
> I "chased" the definition of "Vertex", over many
header
> files,
> following (many) typedefs and templates, and did not succeed in
> finding a final answer.
>
> I program in C++ all the time, and am quite proficient with templates
> and STL. I could not find a final answer - and got a bit frustrated
> given how simple and trivial the task. If I was on the CGAL team, I am
> pretty sure I could just ask somebody and get the answer very quickly,
> but with the information available to me, things were difficult.
>
> Here is what I did figure out. One can
print out the vertices like this:
>
> for ( C2t3::Vertex_iterator v = c2t3.vertices_begin(); v !=
> c2t3.vertices_end(); ++v)
> {
> std::cout << "v
> " << v->point() <<
> std::endl;
> }
>
> (but printing is not what I really wanted; it would be better if I
> could store v->point() into a C array such as "double [3]")
>
> And one can traverse the faces (but unsure if these are tet mesh
> cells, or surface mesh triangles, or surface mesh
> triangles+edges+vertices) like this:
>
> int count = 0;
> for(C2t3::Facet_iterator f = c2t3.facets_begin(); f != c2t3.facets_end();
> f++)
> {
> //printf("%d\n", f->second); // this I am guessing is
the
> dimensionality of the facet
> if (f->second == 3)
> {
> std::cout << f->first->vertex(0)->point()
<<
"
> ";
> std::cout << f->first->vertex(1)->point() <<
"
> ";
> std::cout << f->first->vertex(2)->point();
> std::cout << std::endl;
> count++;
> }
> }
>
> However, here I don't know what to do with
> f->first->vertex to pull
> out the global integer index of that vertex. As you can see, I figured
> out how to convert the vertex to its 3D location (3 coordinates); but
> getting the global integer index would be much better. Also, I am not
> sure if this method will give me the triangle faces of the surface
> mesh, or will it give me the entire tet mesh. And if I run it on the
> (unmodified, as shipped with CGAL) skull example, I get many 4-tuples
> (I assume tets from the output tetrahedralization) that look ok, but
> at the end there are several that have
three zero entries and one
> entry equalling 1. So if those are tets, unsure what they mean.
>
> I think CGAL is great: it really implements many useful algorithms and
> the ability to have exact arithmetic is very good. Based on my
> experience, may I add a suggestion: right now, the manual talks a lot
> about templates and about computational geometry algorithms, which
> is
> all good. Templates certainly make CGAL more powerful. However, at
> times the manual reads like the art of templatization was the main
> point of CGAL; there is a lot of template lingo. This will be good for
> somebody who is much into templates, but might be difficult to read
> for other people. I would add even more concrete examples: whenever
> you present some generic datastructure or algorithm (such as the
> kernel, for example), consider giving concrete examples of templated
>
arguments that one can pass to those datastructures or algorithms. And
> as per triangulations, it would be good to have a manual page listing
> the methods available to vertex iterators, face iterators, cell
> iterators; the difference between face, facet, cell; and more
> documentation on input and output.
>
> Thank you for creating CGAL, and for your help,
> Jernej Barbic
>
> The second email message:
> ======================
>
>>>dear
> Jernej,
>>>
>>>thanks for your positive feedback, that's very useful and
> encouraging.
>>>
>>>as I am still on holidays, the short answer can be found in file
> Complex_2_in_triangulation_3_file_writer.h
>>>
>>>I invite you to post your detailed email to the CGAL discuss list,
so
> that all developers see the trend (more and more people
start using that
> surface
> mesh generator, that's good to know).
>>>
>>>thanks again for your time,
>>>
>>>Pierre
>
> Hi Pierre,
>
> Thank you very much for the pointer to
> Complex_2_in_triangulation_3_file_writer.h. I was able to
> "reverse-engineer" that file and now the mesher works fine for
me. I
> already meshed several isosurfaces. The mesher is very good. I am not
> aware of any other mesher that can achieve similar results.
>
> One minor issue that I noticed is that there is sometimes a spot on
> the output mesh
> where for some reason the vertex density is
> unnecessarily high. If there is such a spot, there is always exactly
> one. It's not a big deal, but it does require either removing it
> manually, or running several runs, guessing parameters until it
> disappears. The location
of this spot might be related to the location
> of the center of the bounding sphere (user-supplied parameter), and to
> how deep that center is inside the volume enclosed by the isosurface.
> But not 100% sure.
>
> Also, documentation says that the sphere center must be inside the
> volume enclosed by the isosurface - but I noticed that the mesher
> produces a plausible result even if it is not. And, I am not
> completely sure if the bounding sphere has to cover the entire volume
> where the grayscale image is defined, or only the output isosurface.
> Also, I don't exactly understand the difference between the
"distance
> bound" and "radius bound" (inputs to the
> mesher); I read it
> several
> times in the manual and from that text I don't see the difference (and
> I know they must be different, or else you would not have two separate
>
parameters).
>
> FYI: I am combining your surface mesher with "tetgen" to
generate
> volumetric tet meshes (I do so since I am assuming that your mesher
> doesn't generate a (quality) tet mesh, only a quality surface mesh). I
> am getting very good quality meshes using this process.
>
> Again, my suggestion to CGAL would be to boost the documentation. Much
> of the documentation talks about "concepts" and how these
concepts
> are
> implemented in "models". This is interesting on its own,
however, it
> might be hard for a CGAL novice to parse this. For example, I'd try to
> lookup "Vertex", expecting to see a concrete class, and a list
of
> methods that can be used with this class. Instead, there would be lots
> of typedefs and talk about how this is a model of
> some concept and how
> it has to implement certain things. And then I'd wonder why
those
> typedefs are the way they are, and what alternative choices would be
> and where I would look up those. And whether the class that I am
> looking at (Vertex) is an actual class that I can use, or is it just
> an abstract concept for me to implement (or partially implement), etc.
> All this is just my 5 cents of course, take my criticism lightly. I
> realize templatization has big benefits.
>
> As per posting to the CGAL discussion list, please go ahead and post
> my previous email, in my name (it is easier for me if you post it,
> than if I post it). You can also post this email if you wish.
>
> Thank you for your help.
> Jernej Barbic
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