Hulls do not necessarily line up with the iso params. The hulls are rows and columns of CV's and they do not necessarily lie on the surface but rather influence the surface.


What you do need to know though is as you add iso params you will effect the parameterization of the surface.

What does this mean you ask? Well look at the images.

1. I start with two nurbs circles and loft and you see I get a surface with a parameterization of (8,1) so you see the u range of 0-8 and v range of 0-1. (look in the channel box to the right).

What this means is there are 8 uniform spans in U and 1 uniform span in V. Where uniform mean each span is 1 unit in length.

2. also notice in this next image that there are 4 hulls. Recall that a degree 3 surface must have a minimum of (degree+1) or 4 cv's to define the curve on that surface.

3. Now I added an iso param and note that the hull does not line up with the iso param. Think of it this way. The hull represents a row of cv's on the surface and as we know cv's do not correspond to vertices which lie on the surface for polygons. Cv's represent points of influence to the surface (like little gravity wells pulling the surface towards them) and so just as they do not lay on a curve they also do not lay on a surface.

4. Now look at the parameterization. It still says 8 spans in U and 1 span in V but there are now clearly 2 spans in V.

This means the parameterization is no longer UNIFORM and this can cause issues when you perform other nurbs operations with this surface. Or in other words the first span now goes from [0 to some fraction x] and span 2 goes from just past the fraction (x to 1].

The general "rule" (or "guideline") is that you should rebuild nurbs surfaces after modifying them which includes inserting iso params, or attaching and detaching surfaces. I keep the rebuild surface and rebuild surface options tools on my shelf.

5. In this last image I simply rebuilt the surface with the default settings and use CV's.

The overall shape has not changed (although the inserted hull and iso will shift because by default the rebuild option for cv distribution is "uniform"). So after rebuilding the parameterization shows 8 spans in U and 2 spans in V and all the spans are uniform meaning they are one unit in length.

This has been a public announcement from your friendly SM nurbs buddy. I hope I have not confused you.