In the first paper, I'm wondering how well the "automatic plane detection" works. They argue that the best homography for the whole image will correspond to the homography for the largest plane in the image, but I'm not sure how robust this is. I'm also a little confused about how much hand initialization is going on. They say they specify the plane for the initial homography, but then they say something about setting "the boundary to the convex hull of the 2D points" for initialization...

And I guess my answer to Matt's question about where this will fail: scenes without planar surfaces, scenes where the planar surface has no texture (no corners will be detected), sequences where the initial plane moves out of view before a second plane becomes visible...

The outer space scenario is somewhat interesting. Since any translational motion will be miniscule compared to the distances between stars, all motion will be approximately rotational, meaning a homography will exist. The problem of course is that this doesn't really provide a frame of reference for synthesized objects since translation can't be detected.