I was curious about their use of 2d motion models. They say that background motion is described with a 2d affine model, and foreground layers undergo 2d rigid motion. This sounds like there would be some types of motion that would confuse the algorithm, including some camera motions - am I right, or is 2d enough? I just wasn't sure if they made that choice because it's enough or because it was some kind of tradeoff...