Reorganize the document without substantially changing the content.

Divide the techniques discussed into "legacy" and "state-of-the-art"
approaches. Rewrite the introductory material accordingly.

Among the state-of-the-art approaches, distinguish between interactive
and non-interactive methods.

These classifications may need to be changed further; this is only an
initial attempt at a reorganization of the document.

Author: jasonjgw