Shape and texture coefficients i and i in our morphable face
model do not correspond to the facial attributes used in human language.
While some facial attributes can easily be related to biophysical
measurements [13, 10], such as the width of the mouth, others
such as facial femininity or being more or less bony can hardly be
described by numbers. In this section, we describe a method for
mapping facial attributes, defined by a hand-labeled set of example
faces, to the parameter space of our morphable model. At each position
in face space (that is for any possible face), we define shape
and texture vectors that, when added to or subtracted from a face,
will manipulate a specific attribute while keeping all other attributes
as constant as possible.