The  articulated body  models used  to represent  human motion  typically  have many degrees  of  freedom,  usually  expressed   as  joint  angles  that  are  highly correlated.  The  true range  of motion can  therefore be represented  by latent variables that span a low-dimensional space.
This has often been used to  make motion tracking easier.  However, learning the latent space in a problem-independent way makes it non trivial to initialize the tracking process by picking appropriate initial values for the latent variables, and thus for the pose.  In  this paper, we  show that by directly  using observable quantities as our  latent variables, we eliminate this  problem and achieve full
automation given only modest amounts of training data.
More specifically, we exploit the fact that the trajectory of a person's feet or hands  strongly constrains  body pose  in motions  such as  skating,  skiing, or golfing. These trajectories are easy to  compute and to parameterize us...