# How can I offset a sphere in spheres in Alias

Dear all,

When using the offset tool in Alias, I have no problems offsetting normal surfaces(planes, birails, skins) but when I try to offseet surfaces that have been created from a sphere moving its cv´s, I cant offset it. Does any one know the solution?

The most likely answer has to do with how spheres are constructed and the different between “rational” and “non rational” geometry. Effectively Alias has different ways of looking at the math behind a circle/sphere and assigning different values to how much each CV effects the shape.

Because of the math behind everything in Alias theres two conditions for creating primitives like a sphere the “rational” and “non-rational” way. I’ve attached the picture above. You can see that if I create a non-rational sphere (Preferences->construction options->Rational Flags->Uncheck Primatives) like on the left, it should offset but will create a painfully complex object. If I were to offset with explicit control on to keep the geometry simpler, it stops being a sphere.

The real answer to this question is it’s best practice not try to offset a sphere. Either copy your object and scale it to create the offset, or build your object with a revolve. If you’re just doing it to create a picture it’s OK, but the geometry of a sphere is inherently a mess.

If theres a more specific problem you can try posting a screen shot and I can let you know a better way to build it. You should still be able to offset your geometry, but it will be rather nasty. You could probably clean up the poles of the geometry but it will still be a mess.

Hello CyberDemon,

Thanks for the advice. I have attached the shape I am talking about. I started using a sphere because I can control the surface very well moving the cv´s. This is the bestway (at least to my knoledge) to really control all areas of the surface for this kind of shapes. The problem comes when I have to give it wall thickness, when I use the offset tool it does not work. I have tried scaling it but somehow when doing a proportinal scale, wall thicknesses are not constant.

I suppose now that I have the desired shape I could go back, throw some lines and rebuild it using birails, that will definitely offset but I will have to do all the work again.

What do you suggest I should do?

If you’re fairly happy with the trimmed back surface, you could simplify it’s original construction by breaking the surface up outside where the visible surface finishes (the cv’s show that the underlying surface still remains part of the maths even though it’s been trimmed)

If you break it up using the ‘detach’ tool , you’ll be left with something visibly 4 sided that’s far more likely to offset than the sphere-based surface. This is because all surfaces are inherently 4 sided - with spherical surfaces created by making two of the sides have a length of zero - this could be causing a problem for the offsetting maths.
hope this makes sense, and helps.

Are you trying to create your trimmed area? (The helmet shape) If so you sould be able to do it much cleaner and simpler while still pulling the CV’s.

If you just need that top area of the surface, detatch the front, bottom, and rear. This should leave you with a clean mesh that should offset better.

One reason that it may give you a bad result is offsetting works by moving each CV by your offset distance in its normal direction. In your case, you have a LOT more CV’s then you need so they may start overlapping if you’re offsetting it too far. A simpler surface with fewer CV’s will help - you don’t necessarily need to redo everything, just detach the surface areas you don’t need, and then do a “Rebuild Surface” and play with decreasing your spans till you get something that falls within your tolerance and is simpler. It looks like you should be able to build that design pretty easily with something in the 2-3 span range.

I want to thank you all, your advice worked perfectly, I rebuilt the surface with less spans and detached not only the parts that I didnt need but also the useful surface by each of its spans, it now ofsetts perfectly.

Thanks again!