  Interaction methods based on computer-vision hold the potential to become the
next powerful technology to support breakthroughs in the field of
human-computer interaction. Non-invasive vision-based techniques permit
unconventional interaction methods to be considered, including use of movements
of the face and head for intentional gestural control of computer systems.
Facial gesture interfaces open new possibilities for assistive input
technologies. This chapter gives an overview of research aimed at developing
vision-based head and face-tracking interfaces. This work has important
implications for future assistive input devices. To illustrate this concretely
we describe work from our own research in which we developed two vision-based
facial feature tracking algorithms for human computer interaction and assistive
input. Evaluation forms a critical component of this research and we provide
examples of new quantitative evaluation tasks as well as the use of model
real-world applications for the qualitative evaluation of new interaction
styles.
