CGAL users discussion list

[Andreas Fabri <>]

Hi Alvaro,

alvaro cuno wrote:
> Hi Andreas,
>
> At the moment, I am using this alternative:
> - I have derived a SurfaceMesh class from
> CGAL::HalfedgeDS_vector<Kernel, HDSItems>. As a member of this class I
> have a linear array storing the vertex positions.
> - For my Vertex structure I avoid to instance a position (without
> Point). With this I avoid wasting memory.
> - I send the array of positions to the OpenGL as the third parameter
> of the function glVertexPointer.
> - As you can see, I am using the HDS only for the connectivity information.
>
> This option works very good. However, I don't like because is not
> elegant. Requires some additional tricks. For example, It is necessary
> to code I/O operators, etc.
>
> For my case (the number (2)), I think there is a more elegant solution
> using the Polyhedron_3 directly.  But I have some doubts:
>
> - Using the HalfedgeDS_vector it is possible to assert that the
> structures will be in consecutive order in the memory?

It's guaranteed that all vertices will be consecutive.
That's what it is about.

> - The coordinates of a Point_3 are in consecutive order?
Yes, but they are not necessarily inside the Point object.
This depends on the kernel: If you use CGAL::Cartesian, the points are
reference counted, that is the real x,y,z coordinates are somewhere
in memory, but certainly not in the vector of vertices holding
the points.

So it needs Simple_cartesian<double> or Filtered_kernel< 
Simple_cartesian<double> >.
Also the Exact_predicates_inexact_constructions_kernel should be ok.

> - For computing the STRIDE value I have to use "sizeof(Vertex)" or
> "sizeof(PolyhedronItems_3)" or ... ?

sizeof(Vertex) might be fine, but I didn't try it out.

andreas

> I think that for fast rendering of Polyhedron_3 using opengl we need only:
> 1- a pointer to the first position of vertex positions
> 2- these positions must be in consecutive order in the system memory, and
> 3- a STRIDE value
>
> What do you think? Could you give more suggestions, please?
>
> Alvaro Cuno
> COPPE-UFRJ
>
> PD: I have the same question of Laurent Rineau. For example, the
> rendering of sharp features requires normals per vertex. Anyone know
> to do this for OpenGL efficiently?
>
> On 07/11/2007, Andreas Fabri 
> <>
>  wrote:
> > Hello,
> >
> > I see two options. By default the polyhedron uses a halfedge data structure
> > where the vertices, halfedges, and faces are stored in a list.  For a list
> > of vertices a stride will not do it, as the list elements can be anywhere
> > in memory. But you could have your own Vertex class, and an array for your
> > points coordinates, and you let your vertex have a pointer in the array
> > instead of directly storing the point.
> >
> > The second option is to use the fact that the halfedge data structure
> > is a template parameter of the Polyhedron, and there is a class 
> > HalfedgeDS_vector.
> > It stores the vertices, halfedges, and faces in std::vectors. The limitation
> > is that you cannot delete vertices,  but you should be able to compute the 
> > stride
> > even  if this is not offered by the API.  For more information on  
> > HalfedgeDS_vector:
> > http://www.cgal.org/Manual/3.3/doc_html/cgal_manual/Polyhedron/Chapter_main.html#Subsection_12.3.5
> >
> > I would be glad to get your feedback once it works.
> >
> > andreas
> >
> >
> >
> >  wrote:
> > > Dear all,
> > >
> > > I would like to render a Polyhedron_3 mesh with +100K vertices in interactive 
> > > FPS using OpenGL.
> > >
> > > (1) If the mesh is static, there is not problem:
> > > - I can use a display list, build vertex arrays or use VBO (looping on the 
> > > Vertex_iterator, Facet_iterator). This is not my case.
> > >
> > > (2) If the mesh geometry is dynamic (but the connectivity is static)
> > > - It is possible to build a vertex array or VBO each time the geometry is 
> > > modified. However, I wouldn't like to loop on the iterators to build the new 
> > > opengl vertex array every time because is not efficient. I would like to pass 
> > > the pointer to the mesh geometry directly to OpenGL. I would like something 
> > > similar to (mesh is a pointer to a Polyhedron_3 object):
> > >
> > > ###############################
> > > typedef Polyhedron_3::Point_iterator Point_iterator;
> > > Point_iterator pi = mesh->points_begin();
> > >
> > > glEnableClientState(GL_VERTEX_ARRAY);
> > > glVertexPointer(3, GL_FLOAT, sizeof(SOMESTRIDEVALUE), &(*pi)[0]);
> > > glDrawElements(GL_TRIANGLES, 3*mesh->nfaces(), GL_UNSIGNED_INT, &faces[0]);
> > > ###############################
> > >
> > > ..... It is possible to do this? how to determine the STRIDE value?
> > >
> > >
> > > (3) If the geometry and the connectivity is dynamic
> > > - This is not my case
> > >
> > > Thanks in advance,
> > >
> > > Alvaro Cuno
> > > COPPE-UFRJ
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