ToonFace is a considerable simplification of FACS, but it is precisely for this reason that it is an attractive
alternative. The head motions of humans have three degrees of freedom: head turn, medial (forward-backward) head
tilt , lateral (side to side) head tilt. ToonFace simplifies this into two degrees of freedom, eliminating the lateral
head tilt. For the upper face, the only features that are identical between the two are the eyes, which have 2 df each.
Action unit (AU) 1 (inner brow raiser) and AU 4 (inner brow lowerer) are represented in ToonFace by Bm, with AU
4 approximated by motor Bm having an extended range downward (this depends on the particular face design). AU 2
(outer brow raiser) is approximated by motors Bc and Bl, which also help in capturing motions involving AU 1.
Eu, or Eu and El together, approximates the following AUs: AU 5 (upper lid raiser), AU 7 (lid tightener), AU 41