Understand global and relative space -- and you'll avoid a lot of time problem-solving, fixing rigs, re-working animation.
Relative space in Rigging, Animation, and General Problem-Solving.
First, a quick summary from Einstein and Bradley Gabe:: (*)
Bradley is running. How fast is he running?
Bradley is running on a train that's going 60 miles per hour.
How fast is he running now?
Are you asking speed relative to the train? or relative to the ground?
Then consider the earth is moving too.
And the earth is moving within a solar system -- which sits in the universe!
Relative space applies to our CG scenes, rigs, controllers, and animation in many ways -- and the more complex our scenes get, the more clean your values|structures need to be. Putting these tools to work will help problems when::
Creating complex camera moves.
Gimbal lock with arms.
Repurposing rigs or parts of rigs.
Setting Expressions or Connections.
Adjusting previously animated objects.
Adjusting [dirty] cameras from tracking scenes.
Reverse engineering motion control object to camera relationships.
Exporting/Importing rigs for motion capture and clean animation afterwards.
The list goes on.
Remember:: Many common problems from relative space occur because artists quickly "zero out" transforms of a group|node|controller with Freeze Transforms (maya) or Set Neutral Pose (xsi). As a general rule, do not use Freeze Transforms on controllers/nodes/groups. Both maya and xsi remember the original values of a node deeper in the software and will cause problems in more complex situations (some listed above). Many problems can easily be avoided by creating a relative or local "zeroed-out" value by giving your node a parent with the same transformations (the parent takes on the transformations, the child is (0,0,0) relative to it's parent. You can then move the parent anywhere you want and the child stays at (0,0,0).)
“Based on an actual conversation and questioning from Bradley Gabe -- thanks Bradley for helping paint the picture.”