Class 1 - Week 5 - Rotation Through an Obstacle Course

When playing around with the new ball rig, you may notice that if you
stretch it then try to rotate, the ball will feel like an American football,
making it impossible to roll across a surface in a nice smooth way.

While this is simple enough to avoid when the ball is flying through the air,
it can lead to issues when you want the ball to interact with the environment,
like a ramp. The ball should roll across the surface but then that might lead
to spacing issues if the ball is traveling at high speeds. So what do you do?
You forget about the rotation! If the ball is traveling that fast, just have it
stay stretched as it goes across the ramp and don't worry about rotating it.
It will give the animation a smoother feel and your eyes probably won't even
notice that the rotation is missing.

One thing to watch out for when the ball stretches across a ramp is how it
contacts the surface. If you add the stretch to the ball after making sure
it moves nicely across the ramp, the ball will start to leave the surface
of the ramp, creating a floaty feeling, kind of like the ball is flying
over the surface instead of rolling across. To fix this, you'll need to
tweak the keys so that the ball is in contact with the ramp at all times.
Sadly, this can lead to the spacing of the ball not being quite as smooth
as you would like. One thing I found extremely useful for this is the
AC3D tool in the arAnimTools shelf. It will create an interactive curve that
you can use to make sure your spacing and arcs are nice and smooth.

AM has a great tutorial explaining how to use it.
AC3D Tool Tutorial

Now, if you're feeling fancy, you can mix and match as well. If the spacing is
small enough try zeroing out the stretch and rotate the ball as it goes across
the ramp then stretch it back out afterwards. If done properly, your eyes won't
even notice the change but you will still feel the rotation giving a nice
variation to the animation.

I would also recommend planning when and how the ball will rotate through the
environment. It doesn't need to be anything complex, but knowing the orientation
the ball needs to be for all the contacts helps quite a bit.

And remember while Squash & Stretch is crucial to giving life and character to
whatever you animate, it does have a bit of a Dark Side. In general, it should
be felt and not seen.

I hope that helps and as usual, if you have any comments or requests for tips,
just send us a comment or PM.


  1. These tips on rotation helped A LOT in my planning. Thanks!!!

  2. Wow thanks! really useful info!!