Got a problem with periodic nurbs, e.g. a cylinder. I get the definition of the nurbs face of a cylinder. Then I get two trimming loops, which define the upper and lower border of the cylinder in (u,v)-space. Problem is, that opengl displays nothing when I try to apply this. Documentation tells me, that a trimming loop has to be closed. But thats not possible with a periodic face. When you unroll the cylinder face to a flat surface, then the two trimming loops will appear as lines in (u,v)-space. I tried to close them by giving opengl the coordinates of the first point as the last point -> didn’t work.
The cylinder is just an example. I could get any periodic surface.
Any suggestions?
Thanks,
Kilam.
[This message has been edited by Kilam Malik (edited 08-07-2000).]
I don’t know if I understand the question entirely, but I will have a shot at what I think is an answer.
Are you trying to cut a hole in the surface of a nurbs object that loops around onto itself??
if so, it could be
1: Your outside trimming loop is listed clockwise instead of counterclockwise. This will mean that everything inside the loop will be trimmed out.
2: Your inner loop (if you have one) is wound counter instead of clockwise, trimming everything outside the loop.
If you are actually trying to alter the “LENGTH” of the cylinder, or other periodic surface, it may be better to just set it with control points and leave trimming out of it.
I am pretty sure that trimming wiht nurbs surfaces need to be wound the right way AND closed loop AND within the bounds of the surface, meaning that you would need to alter the control points or tesselate the surface and trim triangles. I don’t thing that what you are trying to do with the NURBS functions is actually possible.
Uh, now I feel really stupid. But if I am completely wrong as to what I think you mean, let me know and I will delete this post.
Does that help??
Nate<–desperately hoping he is right.
[This message has been edited by subpoposaur (edited 08-08-2000).]
> Are you trying to cut a hole in the surface
> of a nurbs object that loops around onto
> itself??
I try to make an outside trimming loop. The inner ones are working. But I can’t close it as its periodic. I can provide you with a picture if my descriptions are not clear enough.
> If you are actually trying to alter
> the “LENGTH” of the cylinder, or other
> periodic surface, it may be better to just
> set it with control points and leave
> trimming out of it.
Thats the problem… I get the data from a CAD system and they always define there surfaces bigger and cut them down by outer loops (internal a CAD system uses infinite surfaces).
Kilam.