A few questions.

1) They refer a couple times to the "direct method"; what is the direct method?

2) Why do they require the the shape maps for background layers to sum to 1? It seems like they could have removed this restriction with no harm done.

3) How well could layering methods be adopted to non-stationary cameras, where parallax would create non-uniform motion among all the background layers?

These questions are probably pretty stupid as I didn't read the paper as closely as I would have liked (starting before midnight would help :)).