black outline around avatar?

I've seen a couple all-prim avatars with a black outline around them. Looks like a drawing with an ink outline on the edge of the avatar, but not the individual prims. Any idea how this is done?

Cel shading; there are a few ways to do it, but one way is to take a hollow sphere, and set it to all black, then apply a 100% transparent texture to the outer face. This will just leave the inside visible. You can create another sphere, slightly smaller, and put it inside, and the black outline will show all around it from any angle.

While DanielFox's idea is helpful to think about, in the case of a sphere, all you have to do is make one single Sphere prim, hollow it, twist it 180 & 180, and there you have it, an outlined ball for you to color the surface and the outline of (and if you outline in any color other than black, be sure to hit the Full Bright checkbox on that surface). A similar cylinder can be made by twisting a Tube 90 degrees and applying just the right amount of Profile Cut.

Good prim-saving tips, Al. I must say though, I've never been 100% comfortable with these twisting methods. I have a hard time believing that reversing normals like this was an intended result of the twist function. It's much more likely that it's a bug, which means one day it might get "fixed", breaking all the objects that were made with it.

Maybe I'm just a little overly fearful, but I feel much safer with the double-prim method. It most definitely demonstrates intended functionality, so there's no reason to think it might ever stop working.

In the here and now though, either method works just fine.

So, the whole thing has to be built that way. I was hoping there was an easier way. Ideally, a switch you could throw

I'd be very surprised if someone hasn't come up with a script to automate it.