CGAL users discussion list

[mikef <>]

Laurent Rineau (CGAL/GeometryFactory) wrote
> Le Friday, April 14, 2017 5:05:36 AM CEST mikef a ?crit :
>> I am using CGAL  "make_surface_mesh" to create a mesh out of an ellipsoid
>> function, then using OpenGL to draw it by plotting triangle by triangle.
>> I
>> have no clue what is going on...
>> 
>> *Problem:* the plotted surface is weird, some triangle has color, some
>> not,
>> some are like holes.
>> 
>> *My codes for mesh generation:*
>>         Surface_3 surface(earth_function,             // pointer to
>> function Sphere_3(CGAL::ORIGIN, 7000*7000));
>>         // bounding sphere
>>         // Note that "2." above is the *squared* radius of the bounding
>> sphere!
>>         // defining meshing criteria
>>         CGAL::Surface_mesh_default_criteria_3 criteria(30.,  // angular
>> bound
>>                 500,  // radius bound
>>                 50); // distance bound
>>                          // meshing surface
>>         CGAL::make_surface_mesh(c2t3, surface, criteria ,
>> CGAL::Manifold_tag());
>> 
>> *My codes for mesh plotting: *
>>         glNewList(EARTH_S, GL_COMPILE);
>> 
>>         Point_3 pt0, pt1, pt2, pt3;
>>         for (auto it = c2t3.facets_begin(); it != c2t3.facets_end();
>> it++)
>> {
>> 
>>                 pt0 = it->first->vertex((it->second) & 3)->point();
>>                 pt1 = it->first->vertex((it->second + 1) & 3)->point();
>>                 pt2 = it->first->vertex((it->second + 2) & 3)->point();
>>                 pt3 = it->first->vertex((it->second + 3) & 3)->point();
>> 
>>                 auto vv = pt1 - pt0;
>>                 auto n = CGAL::normal(pt1, pt2, pt3);
>>                 if (CGAL::angle(n, vv) == CGAL::ACUTE) { n = -n; }
>>                 //auto n = ((it->second % 2 ) == 1)?CGAL::normal(pt1,
>> pt2,
>> pt3): CGAL::normal(pt1, pt3, pt2);
> 
> First: to get the oriented list of three points of a facets, you should
> use 
> the function tr.vertex_triple_index(i, j), a function in a base class of
> the 
> triangulation.
> 
> http://doc.cgal.org/latest/TDS_3/
> classCGAL_1_1Triangulation__utils__3.html#a71288c8c2833f954a73090336ed60054
> 
> Second: the facets of a c2t3 are actually not oriented, and that means
> that 
> their orientation is random. That probably explains your rendering issues. 
> You should use the OpenGL lightening model "two sides", and disable the
> back-
> culling.
> 
> -- 
> Laurent Rineau, PhD
> R&D Engineer at GeometryFactory           http://www.geometryfactory.com/
> Release Manager of the CGAL Project       http://www.cgal.org/
> 
> 
> -- 
> You are currently subscribed to cgal-discuss.
> To unsubscribe or access the archives, go to
> https://sympa.inria.fr/sympa/info/cgal-discuss


Laurent Rineau (CGAL/GeometryFactory) wrote
> Le Friday, April 14, 2017 5:05:36 AM CEST mikef a ?crit :
>> I am using CGAL  "make_surface_mesh" to create a mesh out of an ellipsoid
>> function, then using OpenGL to draw it by plotting triangle by triangle.
>> I
>> have no clue what is going on...
>> 
>> *Problem:* the plotted surface is weird, some triangle has color, some
>> not,
>> some are like holes.
>> 
>> *My codes for mesh generation:*
>>         Surface_3 surface(earth_function,             // pointer to
>> function Sphere_3(CGAL::ORIGIN, 7000*7000));
>>         // bounding sphere
>>         // Note that "2." above is the *squared* radius of the bounding
>> sphere!
>>         // defining meshing criteria
>>         CGAL::Surface_mesh_default_criteria_3 criteria(30.,  // angular
>> bound
>>                 500,  // radius bound
>>                 50); // distance bound
>>                          // meshing surface
>>         CGAL::make_surface_mesh(c2t3, surface, criteria ,
>> CGAL::Manifold_tag());
>> 
>> *My codes for mesh plotting: *
>>         glNewList(EARTH_S, GL_COMPILE);
>> 
>>         Point_3 pt0, pt1, pt2, pt3;
>>         for (auto it = c2t3.facets_begin(); it != c2t3.facets_end();
>> it++)
>> {
>> 
>>                 pt0 = it->first->vertex((it->second) & 3)->point();
>>                 pt1 = it->first->vertex((it->second + 1) & 3)->point();
>>                 pt2 = it->first->vertex((it->second + 2) & 3)->point();
>>                 pt3 = it->first->vertex((it->second + 3) & 3)->point();
>> 
>>                 auto vv = pt1 - pt0;
>>                 auto n = CGAL::normal(pt1, pt2, pt3);
>>                 if (CGAL::angle(n, vv) == CGAL::ACUTE) { n = -n; }
>>                 //auto n = ((it->second % 2 ) == 1)?CGAL::normal(pt1,
>> pt2,
>> pt3): CGAL::normal(pt1, pt3, pt2);
> 
> First: to get the oriented list of three points of a facets, you should
> use 
> the function tr.vertex_triple_index(i, j), a function in a base class of
> the 
> triangulation.
> 
> http://doc.cgal.org/latest/TDS_3/
> classCGAL_1_1Triangulation__utils__3.html#a71288c8c2833f954a73090336ed60054
> 
> Second: the facets of a c2t3 are actually not oriented, and that means
> that 
> their orientation is random. That probably explains your rendering issues. 
> You should use the OpenGL lightening model "two sides", and disable the
> back-
> culling.
> 
> -- 
> Laurent Rineau, PhD
> R&D Engineer at GeometryFactory           http://www.geometryfactory.com/
> Release Manager of the CGAL Project       http://www.cgal.org/
> 
> 
> -- 
> You are currently subscribed to cgal-discuss.
> To unsubscribe or access the archives, go to
> https://sympa.inria.fr/sympa/info/cgal-discuss


Laurent Rineau (CGAL/GeometryFactory) wrote
> Le Friday, April 14, 2017 5:05:36 AM CEST mikef a ?crit :
>> I am using CGAL  "make_surface_mesh" to create a mesh out of an ellipsoid
>> function, then using OpenGL to draw it by plotting triangle by triangle.
>> I
>> have no clue what is going on...
>> 
>> *Problem:* the plotted surface is weird, some triangle has color, some
>> not,
>> some are like holes.
>> 
>> *My codes for mesh generation:*
>>         Surface_3 surface(earth_function,             // pointer to
>> function Sphere_3(CGAL::ORIGIN, 7000*7000));
>>         // bounding sphere
>>         // Note that "2." above is the *squared* radius of the bounding
>> sphere!
>>         // defining meshing criteria
>>         CGAL::Surface_mesh_default_criteria_3 criteria(30.,  // angular
>> bound
>>                 500,  // radius bound
>>                 50); // distance bound
>>                          // meshing surface
>>         CGAL::make_surface_mesh(c2t3, surface, criteria ,
>> CGAL::Manifold_tag());
>> 
>> *My codes for mesh plotting: *
>>         glNewList(EARTH_S, GL_COMPILE);
>> 
>>         Point_3 pt0, pt1, pt2, pt3;
>>         for (auto it = c2t3.facets_begin(); it != c2t3.facets_end();
>> it++)
>> {
>> 
>>                 pt0 = it->first->vertex((it->second) & 3)->point();
>>                 pt1 = it->first->vertex((it->second + 1) & 3)->point();
>>                 pt2 = it->first->vertex((it->second + 2) & 3)->point();
>>                 pt3 = it->first->vertex((it->second + 3) & 3)->point();
>> 
>>                 auto vv = pt1 - pt0;
>>                 auto n = CGAL::normal(pt1, pt2, pt3);
>>                 if (CGAL::angle(n, vv) == CGAL::ACUTE) { n = -n; }
>>                 //auto n = ((it->second % 2 ) == 1)?CGAL::normal(pt1,
>> pt2,
>> pt3): CGAL::normal(pt1, pt3, pt2);
> 
> First: to get the oriented list of three points of a facets, you should
> use 
> the function tr.vertex_triple_index(i, j), a function in a base class of
> the 
> triangulation.
> 
> http://doc.cgal.org/latest/TDS_3/
> classCGAL_1_1Triangulation__utils__3.html#a71288c8c2833f954a73090336ed60054
> 
> Second: the facets of a c2t3 are actually not oriented, and that means
> that 
> their orientation is random. That probably explains your rendering issues. 
> You should use the OpenGL lightening model "two sides", and disable the
> back-
> culling.
> 
> -- 
> Laurent Rineau, PhD
> R&D Engineer at GeometryFactory           http://www.geometryfactory.com/
> Release Manager of the CGAL Project       http://www.cgal.org/
> 
> 
> -- 
> You are currently subscribed to cgal-discuss.
> To unsubscribe or access the archives, go to
> https://sympa.inria.fr/sympa/info/cgal-discuss

Thanks for reply.

I tried using vertex_triple_index in this way. It doesn't help.

                pt0 = ch->vertex(i&3)->point();
                pt1 = ch->vertex(tr.vertex_triple_index(i, 0))->point();
                pt2 = ch->vertex(tr.vertex_triple_index(i, 1))->point();
                pt3 = ch->vertex(tr.vertex_triple_index(i, 2))->point();

I added glLightModeli(GL_LIGHT_MODEL_TWO_SIDE, GL_TRUE), still no luck.

I saw your post somewhere else about the coherent normal of faces, so I
added the following codes in hope of fixing the 2nd issue in your reply. But
it doesn't do any good. 
                auto vv = pt1 - pt0;
                auto n = CGAL::normal(pt1, pt2, pt3);
                if (CGAL::angle(n, vv) == CGAL::ACUTE) { n = -n; }




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