CGAL users discussion list

[Peter Hachenberger <>]

> >> >Remember to use the SNC_indexed_items together with every kernel you
> >> >use. That makes it faster. So
> >> >
> >> >typedef Nef_polyhedron_3<Kernel, SNC_indexed_items> Nef_3;
> >> >
> >> >I guess you are already doing that, aren't you? 
> >> 
> >> If your have told me the difference in efficiency of the default items 
> >> type
> >> and the indexed items, I should have been using the indexed one. :-)
> >> 
> >> But the fact is 'no'.   :-(
> >> 
> >> The reason should be either I haven't been told of this or I have tried 
> >> with it but have met some difficulties.
> >> 
> >> Anyway, I shifted, back or not, :-), to the indexed items type. But I 
> >> found 
> >> the program behaves differently - it can even not run correctly, error 
> >> arises 
> >> before it ends.
> >> 
> >> I will debug the program and report more details.
> >
> >The system is supposed to behave the same way with the indexed items.
> >The only difference should be in input and output of coordinates. If you
> >have bugs, then send them to me. I will do the debugging. It's important
> >for me to make the indexed items as reliable as possible.
> 
> The context of the error is as the attached sreenshot.
> If it's not enough, I'll prepare a minimum program that can reproduce the 
> problem. 

That would be good. I'm curious about the problem. 

> One thing to note: I'm reading a nef_3 from a .nef file. 
> And I've refreshed the .nef file from an identical .off model (very simple)
> using the nef_3 type with the indexed items type. (I've not found any 
> significant 
> difference in format from the two .nef files written from two identical 
> nef_3
> with different items types)

What is the question of this paragraph? Are you wondering that the nef
files are the same although you use different kernels? That's intended.
I spent lots of time to write input and output functions that convert
and normalize coordinates. Actually I have code that even sorts the
output such that it is always the same, but I only use that for testing
purpose.

> >> >Explain your scenario. Then I might can help you to improve. Do you
> >> >always intersect a polyhedron with a plane? Do the polyhedra have
> >> >special properties? Are they small or large? Are they convex? Are they
> >> >two-manifold? What do you do with the resulting polygon?
> >> 
> >> Here are some aspects of my scenario:
> >> - I'm dealing with float type coordinates of vertices of the polyhedrons
> >> - I often intersect a nef_3 with a plane with such idiom:
> >>    Plane_3 p;
> >>    Nef_3 N = Nef_3(p).intersection(p).intersection(p.opposite());
> 
> Sorry, it's a typo, it should be:
>     Plane_3 p;
>       Nef_3 N;
>     Nef_3 result = N.intersection(p).intersection(p.opposite());

I got that fault in the first place. My comments where on the corrected
version. Do you already have an impression of how much the indexed items
help you? Is Nef still a bad bottleneck? How much of a speed up do you
need?

Peter