  Digitally unwrapping images of paper sheets is crucial for accurate document
scanning and text recognition. This paper presents a method for automatically
rectifying curved or folded paper sheets from a few images captured from
multiple viewpoints. Prior methods either need expensive 3D scanners or model
deformable surfaces using over-simplified parametric representations. In
contrast, our method uses regular images and is based on general developable
surface models that can represent a wide variety of paper deformations. Our
main contribution is a new robust rectification method based on ridge-aware 3D
reconstruction of a paper sheet and unwrapping the reconstructed surface using
properties of developable surfaces via $\ell_1$ conformal mapping. We present
results on several examples including book pages, folded letters and shopping
receipts.
