Estimating planar homography from an occluded rectangle?

Estimating planar homography from an occluded rectangle?

Computer vision newbie here. For a project I am working on, I have to
estimate a planar homography from the image plane to a given rectangle's
(paper sheet) plane. The rectangle is occluded, so two of the corners are
not visible on the image. If it weren't for the occlusion, the homography
could've been easily calculated from the four corner points.
Sample image is available here: http://i.stack.imgur.com/IhSfT.jpg
There should be enough constraints (two lines are parallel, third is
orthogonal to both, and lengths of all sides of rectangle are known, ...)
to fix the 8 degrees of freedom needed, however I am not sure how to apply
them in this problem. Any help is much appreciated.