Retargeting Motion to New Characters By Dr. Michael Gleicher Aaron CrickenbergerCS551 Spring ‘02

Introduction Adapting a motion from one character to another –Characters are of the same form (in this case, all are bipeds) –Differ in terms of proportions and dimensions IK is used to some degree but really, it’s… Yet Another Spacetime Constraints Paper

Why do this? Motion capture “Humanize” creatures Slight changes have been made to character after it has already been animated Come on.. IK, frequency domain, spacetime constraints, B-splines… why wouldn’t Brogan love this?

What’s been done before? Character Studio adjusts keyframes to maintain footplants & balance Adjusting physical parameters to create a new motion for a new character (whereas we are now looking to re-use the old motion with a new character) Motion in the frequency domain

What’s been done before? Spacetime constraints allowed motions to be adjusted for the same character to do new things. A paper was written (Spacetime Constraints Revisited) and wasn’t really about spacetime constraints at all.

Mathy stuff q t – the configuration of our character (at time t) m(t) = m o (t) + d(t) –Retargeted motion is the original motion plus differences f i (t) – constraints on the motion –Footplants –Contact with a box

Getting it right How to deal with skating or intersecting joints? Let’s try IK! Whoops… IK will give us jittery motion. But some of the jittery motion may already have been there… OK, so let’s IK and then lowpass-filter our motion! Well crap, now we’re violating constraints again…

The big picture The problem: up until now, considering individual frames The solution: consider them all at once How? Spacetime constraints!

We’ll never get tired of them Generally define a constraint as: –f(q t i, q t j )  c,  ={>,<,=} Examples: –Parameter’s value in a range –Point on character in a specified location or region –Point stays in the same location or region for specified time –Point is a specified distance from another point –Vector between two points has a specified orientation

But math still sucks We can’t really define “grace” as a constraint The objective function (variance over the whole motion) doesn’t consider the motion as a whole (doesn’t want to waste motion to prepare to meet a goal). Oh heck, let’s use it anyway and hack around elsewhere to fix this…

B-Splines Restrict how close the control points for B- splines can be, and you indirectly control the highest possible frequency in your motion. Express d(t) as B-splines to smooth it out, but the original motion can still have snaps to it (ie: karate kick)

But wait! There’s more! We need to scale the motion just as we have scaled the figure… Now we may also need to translate the motion to put the character back where we want them We’ll use interpolation to get rid of high- frequencies caused by adding these changes to d(t)

Step by step 1.Start with initial motion, initial constraints 2.Translate/scale the motion w/ interpolation 3.Frequency decomposition 4.Non-linear spacetime constraints for a displacement that when added to step 2 gives us what we want. 5.If we’re not there, use the result of step 4 as the input to step 1.

What’s good? This looks at an entire motion, and so is able to create anticipated motions. –Eg: person slowly reaching for a box before they get to it It’s computed in near-real time, which allows the animator to tweak constraints to Get It Right

What’s bad? When it comes to constraints, limited just to “things we can solve.” Physics isn’t taken into account, so things could look unnatural

Pretty things

Yeah that’s pretty much it