Attaching NURBS Surfaces Cleanly
General Maya Tips and Strategies #1

These general tips and strategies are tutorials for beginners or intermediate users of Maya, and may hold information advanced Maya users don't already know... but you never know. :)

Anyway, for this lesson, I'd like to go over attaching NURBS surfaces and the many general things an artist needs to look out for to ensure a nice, clean attachment.

So, let's take a look at our scene (Fig 1). Here, we've got 2 nurb surfaces created by lofting and inserting isoparms, much like one would do when creating a complicated object. The one on the left has 3 spans in the U direction, and 3 in the V direction. The one on the right has 6 in the U, 3 in the V.Figure 1
Our objective is to attach these two surfaces, and also to ensure a clean attachment. Right now, if I were to select the left surface, then the right surface, and select Edit NURBS>Attach Surfaces, I get the surface seen in Fig 2. This surface now has 8 spans in U and 6 spans in V. Not a very clean attachment.

So, when something like this happens, undo the attachment and take a look at what you've got in your 2 surfaces.

Figure 2
DELETE HISTORY - First and foremost, it's a good idea to delete the history of the objects. Edit>Delete By Type> History.

MATCH SPANS - In order to have a good, clean connection, you almost always have to have the spans of both sides match before attaching. There are a number of ways you can do this. First, let's simply insert isoparms (Fig 3).

Figure 3
REBUILD SURFACE - When inserting isoparms, however, you always need to watch out for what I call surface background noise. Background noise in film and video can cause distorted sounds or a constant background static. It doesn't necessarily ruin the video, but it can be an annoyance. Take a look at Fig 4. This is the Attribute Editor display for the surface I just added isoparms to. Note that the Spans UV are 6 and 3, just like we want, but take a look at the Min and Max Range U and V. 0 and 1? You usually want to have your Min and Max match your Spans. This may not seem like a big deal and you might even skip this part and see that your model seems to work fine, but later on, if and when you try to animate, you will more than likely run into difficulty and won't know why. So, in order to deal with surface background noise, simply rebuild both surfaces with the settings seen in Fig 5. Edit NURBS>Rebuild Surfaces>Options. Your spans will shift some, but they shouldn't move so much as to be a problem. If you find that your model changes too drasticly for your liking, you may want to try experimenting with the Rebuild Type options available to you.

While were talking about rebuilding surfaces, you may notice that you can change the number of spans your surface has by rebuilding. In order to not worry too much about the surface background noise, you could change your surface spans simply by rebuilding. This normally keeps the change relatively clean. Always check, though.

Figure 4Figure 5
SURFACE DIRECTION - Surface direction is important. Take a look at Fig.6. OUCH! That twisting is caused by the difference of the two surfaces directions! Now, take a look at Fig 7... ah, much better. Let's take a look at the Edit NURBS>Reverse Surface Direction>Option box. It's pretty straight forward... you can reverse your surface only in the U direction, only in the V direction, or in both directions. You can also do what's called Swap. The Swap command flips your spans. So, our 6Ux3V surface would change to a 3Ux6V surface.Figure 6Figure 7
FREEZE TRANSFORMATIONS - If none of the above seems to help, you may need to Freeze your Transformations. This is only for very rare occasions, however, and be wary that this may mess with any animation done previously. Freeze Transformations can be found under the Modify menu in the Modeling MenuSubset. Freeze Transform will reset your Channel Box items back to their default positions without changing anything with your model's position or form. Sometimes, a previous action that somehow relys on an item from your Channel Box may prevent a clean attachment.

Well, that's all for this little lesson. I hope it helped some of you understand attaching NURBS surfaces better. If anyone has anything to add to this, anything I forgot or whatever, don't hesitate to Email me!

Michael McKinley

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