Smartskin Ain't That Hard

Here's a basic arm joint. Nuthin flash, just a tube with two bones. Don't mind the fact that the model is called 'arm smartskin', it could be named 'Phoebe' for all it matters.

The hardest part about smartskin is working out which axis you want it to work on.

That cross doo-dad shows the local x, y, and z headings for the bone. From this, you can generally assume that

• z runs down the length of the bone, so a z rotation rolls the bone.
• y runs parallel to the roll handle, so rotating around y moves the bone up and down (if viewed from the front in this case)
• x runs perpendicular to both the above, so in this case rotating around x moves it toward and away from the front viewport.

Seeing as how I made this arm with the elbow pit pointing towards the front, we want this arm to rotate around the x axis. With that in mind, we can now set up the smartskin. Right click on the bone you wish to define it for (in this case the forearm bone), and choose "new smartskin / x-rotate":

The next thing you'll see is the smartskin window. It looks just like an action window, because it basically is an action window.

Notice that the bone has already been rotated 90 degrees in x. It looks fine from the front, why don't we move to a top view...

Eww, lots of weirdo stuff going on here. This is where you get creative, and start pulling points around. Just grab cp's and shunt them around until you get a smooth looking bend.

Better. Notice I also added a bicep bulge. This is a very cool feature of smartskin, the points affected by smartskin aren't limited to those attached to the bone in question. Save your model, and open an action window to test your smartskin.

Here's what it looks like. Grab the forearm bone and rotate it in x, you'll see that it smoothly morphs into the position we set at it's 90 degree rotation. Magic!

Where to go from here

• If you have more detail in your model, you can add tendons that raise as you move the bones around.
• Handy for blinking eyelids, so that you just rotate a blink bone in your choreography, and the eyelids will fold and crease smoothly and convincingly. Mike Hoveland has created an example project that does this. Thanks goes to Tony Bayat for the link!
• You can set as many keys as you like in a smartskin, so if you need the arm to bend past 90 degrees, you can set a key at, say, 120 degrees, and it will smoothly morph between positions as you move the bone
• It allows you to move a lot of controls that you'd normally put in a pose slider back into your skeleton, leaving you with a cleaner interface
• If you drag-n-drop an action you designed for one creature onto another, but it doesn't quite look right (pinching and poking and whatnot), fix the joints of the model using smartskin, and you can leave the action untouched, which is great for reuseability.
• Facial animation can have lots of control embedded in bones too, rotating the jawbone open can pull tendons in the neck and pull the lips open, rotating the jaw closed can puff the cheeks.

This is one of those killer features that just waiting for a cool exploit. To do something similar in Max or even Softimage would require expressions or morph targets, and neither would really give you the freedom or ease of use of smartskin. Raf has already been gettin' jiggy wit' smartskin with his Dennis project.