The Difference between "Solving" and "Retargeting"

This question and the use of these terms comes up a lot in conversation, with new clients and industry folks. We mention it briefly in the FAQ’s, and I thought I’d cover it here as well.  “Solving” is taking typically optical mocap data, marker “cloud” files such as .c3d, .trc and others, and effectively translating it to create skeletal animation for a CG character. A breakdown for those not familiar with optical mocap:

 An optical mocap system captures the movements of small markers placed at key positions all over an actor’s body. The resulting data files contain a moving “cloud” of points; representing the motion of each marker on the actor, and named accordingly by body part and location. A CG skeleton is fitted inside and constrained to the cloud of points, placed where the now missing actor was, and the skeleton is then driven to move like the original actor.

In a quicker nutshell:
The actor’s motion moves the markers.
The markers motion is captured, a moving cloud of points remains with the actor removed.
A CG skeleton is placed where the human actor was and moved by the dots, recreating the actor’s original motion.

 “Solving” is sometimes used interchangeably with the term “retargeting”, which more specifically means translating and fitting animation between two separate and possibly different skeletons. Using an FBX skeleton animation and transferring it to another character’s skeleton, Character>Character in Motionbuilder for example, is retargeting.

 Many industry folks use the term retargeting for most any translation of motion from one character, face or other rig to another; whether it’s marker cloud, skeletal, mathematical, etc.  Retargeting does describe well any number of processes, and so be it. Potāto, Potäto as they say.

 - Crispin