Animation Retargeting is a process by which animation data from one skeleton is applied to another skeleton. Sometimes the skeletons are identical. Other times, the skeletons can have different scales but the same orientations of their bones and bone hierarchies. And then there are times when the skeletons that are to share animations are completely different. The situation will determine the method of retargeting.
Auto Retargeting
Retargeting animation for skeletons that are exactly the same is known as “Auto Retargeting”. This is the simplest form of porting animation data from one skeleton to another. The data can be baked in one key per frame, or actual controller key values can be used. Whenever you see multiple copies of the same character in a game you are looking at auto retargeting. The animation data is applied to a copy of a skeleton that is identical in every way to the source skeleton of which the animation data was originally created on. Of course, this applies to the skeleton - multiple character meshes can use the same skeleton. For the source side of animation work, this method is particularly useful for cinematics.
Base Retargeting
The term “Base Retargeting” is used for skeletons of the same hierarchal chain and orientations but differing in proportions. 3D Studio Max’s Biped is a classic example of base retargeting. All joint orientations are the same, but proportions can be dramatically different. This is one of the most widely used type of retargeting as it is very easy to implement. As above, you can simply take the rotational part as is, then multiply up or down the positional and scale portion of a transform by the difference in proportion. Another benefit of this method is that your base pose does not have to be the same for skeletons involved in the retargeting process. However, there are several problems can occur using this method… IK retargeting is not always perfect, especially in regards to where hands grip weapons. Other common problems with this method include initial offset errors in orientation for the clavicles, spine, arms, hands and fingers. These errors can appear as super shrugged shoulders and hunched backs, as examples.
Mapped Retargeting
“Mapped Retargeting” is used for retargeting animation data with skeletons that are completely different. This is the most difficult type of retargeting to master. It almost always involves a mapping process by which one obtains a relative offset from one bone to another for each bone in the entire hierarchy. Using a version of target skeleton constrained to a version of the source skeleton is an easy way to derive the proper transform to values. The mapped method can also alleviate weapon grip issues if you are constraining IK Goals instead of arm bone orientations.
Here at Spacetime Studios, we currently use both Base and Mapped retargeting. We use mapped Retargeting for porting our Autoskels animation data onto Bipeds, then we use Base retargeting to transfer the biped data to the other characters for a quick base set of animations from which to work with. This process gets a fully functional character into the game engine without any delay, thus allowing the animators to create final stylized versions of the animations at their leisure, without holding up other departments requiring the asset.