In NURBS you can add another isopram similar to loop-cut in Blender for subdiv.
the difference is that the new iso is placed in a way the original surface curvature
is remained.
In Rhino for example after inserting a u or v iso the control cage is changed to maintain
the surface.
would that be possible with subsurf as well? it is a pain when you have to add another
loop at one point and the hole geometry changes.
claas
Interesting. Though I’m trying to figure out just how to do it, because if you have a subsurfed cube and you put a loop cut on it it’s very hard to get back to a round shape, the subsurf just doesn’t want to go as smooth.
I’m sure there’s a paper on it somewhere though.
It crossed my mind a couple of weeks ago, as something that would be a very unique feature. So I second that proposal.
when you get enough user support, feature requests can be posted here http://www.blender.org/forum/viewforum.php?f=8
I know exactly what you mean - this would be incredible useful.
It seems like the kind of thing we could flesh out in a python script first. Even if what we end up doing is iteratively moving the control points until the lim surface it matches the previous state, It probably wouldn’t be too slow. Certainly better than doing it manually.
Press ctrl+r. Then press S. This turns on loop smoothing. Tada.
I know - any opportunity to show your l33t blender skills is irresistable - however, you obviously didn’t understand the request.:no: Now go away…
Um, just to make sure I understand, , like(say), when you add an edge loop to a subsurfed cube, and it makes the edges harder? Or, when you subdivide a sphere(that is subsurfed) and the edges get harsher?
If thats what you mean, then this would be great. If not, sorry, misunderstood.
Maybe you should take a look at this thread. By the way, I don’t see you even attempting to contribute to this thread in a positive way? Nope. Instead, you decided to attack me with sarcasm and make BA look a little more like a troll community. Oh, and if that’s what what they want, maybe you can tell me?
I did, and got really inspired:D However, I’m sorry if you took that too seriously. I could write an entire essay on why I’m in a bad mood today, and it does relate to this community, but my common sense tells me to refrain…
And I did contribute. I said I liked the idea, right 
We all have bad days, and I understand that. Thanks for apoligizing about it (I believe you one as well; sorry!). Ah, I love this community…
cekuhnen: I’m not sure how complex the geometry you have in mind is, but there’s some possibilities with the lattice modifier. In this example, placing the lattice modifier after the subsurf modifier allows you to further control the resulting shape without having to use edge creases or introducing additional edgeloops into the geometry.
The lattice subdivisions can be increased as needed in the editing panel (F9 KEY) for finer control. Afterwards, the lattice’s verts and edges can be selected and moved accordingly.
In this example, I “bevel” the side and back edge of a simple subsurfed cube by bringing inner loops of the lattice closer to the outer edges. The result is pictured on the left side, and shown in edit mode (as a simple cube) on the right side.
The modifier is disabled in edit mode here but can be enabled so the mesh retains its lattice deformation even while being edited.
If this doesn’t work out for what you need, hopefully this information can be useful to someone else out there 
RobertT
for those who did not understand what I proposed here.
I am not asking for add a loop cut to make an edge sharp.
I am asking for the opposite.
Add a loop and do NOT affect the geometry curvature.
crtl - r + s does NOT do that.Isoprams left and right have to be changed
to remain the curvature of the surface.
RobertT
the objects I do are very complex. Sometimes
I need to add another loop cut and that is the time when I need
the surface be rebuild like in Rhino.
cekuhnen: I see If the surfaces you are working with are physically separate (or can be separated for individualized poly work) or aren’t too high-poly already and/or are part of a larger model that can suffer possibly another level of subsurfacing in order for you to gain more geometry without sacrificing the current shape, you can try applying one (or more) levels of a subsurf modifier to the existing mesh. If the overall form your project requires is more important than economy of geometry, this might work, or not depending on your situation/requirements/poly budget. I just figured I’d throw that out there if it might work for you Good luck with your project!
RobertT
In trying to adjust manually, I realized that writing a script to perform this is probably a lot harder than I originally thought it might be.
I know this is possible in at least one case: when the loop is complete (i.e. either connects with itself or ends at a terminating edge) and doesn’t involve any extraordinary vertices. That covers a good deal of cases, especially considering what loop cutting is.
But I’m not entirely certain if that covers all cases, and–if it doesn’t–whether other cases are possible as well. My gut tells me it’s probably possible, but I’m not 100% sure.
cessen,
I try to model first as the mesh would be a nurbs patch.
So I have a very clean mesh structure.
I don’t think this is possible simply due to the way subdivs are calculated. You’d have to modify the loops next to the new edge loop and subsequently that knocks the curvature off the next again loops. It would be a nice feature though.
i might be mistaken and completely confused about what you are talking about (very very likely) but what i normally do is do the main shape, subsurf, when i’m happy with it i click apply, then put the sharp edges and stuff.
Indeed:yes: