CGAL users discussion list

["alvaro cuno" <>]

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?
- The coordinates of a Point_3 are in consecutive order?
- For computing the STRIDE value I have to use "sizeof(Vertex)" or
"sizeof(PolyhedronItems_3)" or ... ?

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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