Insert Isoparm questions

[withnail]

when insert isoparm with nurbs first i select the isoparm
where i want to add the cuvs, and them this isoparm would become yellow line ,them just hit the insert isoparm tools and the new cvs would created , but why the new add cvs hull
always can not match where i have pick the isoparm(the yellow one), the new hull of cv would have little distance from the isoparm where i want i know the insert isopram have the insert at selection and between selected options
but the nwe add hull cvs cant match the location where i
pick the lsoparm???
could some one help me or explain why it so that ????

[positron]

I don't know the internal math of why this is, but the isoparm just describes the overall visual topography of the nurb, whereas the cv/hulls are the actual geometry, like vertices on a poly object. On a parametric surface like a nurb, inserting an isoparm has the effect of adding CV to the deformable surface without changing the shape of the surface. Thus where it actually places the new CVs has to be averaged out because the isoparms don't actually describe the position of CVs, just the overall surface shape.
If you want more accuracy, sometimes inserting more than one isoparm (the multiplicity option), or picking two isoparms close to each other and using the 'insert between' option will place the new hull more where you want it.
If you absolutely need to have the new hull exactly somewhere, after you create it, you can select the hull, then select the move tool (modify>transform tools>move tool ) and switch to 'normal' mode. The move tool will then allow you to slide the hull up & down along U or V without focking up the surface. (TIP: moving along the 'N' in this mode will let you 'scale' the hull outward uniformly to the surface direction!). Just don't forget to switch your move tool back to Local or World when you exit component selection mode or you won't be able to move objects.