  Can we capture shape and reflectance in stealth? Such capability would be
valuable for many application domains in vision, xR, robotics, and HCI. We
introduce structured polarization for invisible depth and reflectance sensing
(SPIDeRS), the first depth and reflectance sensing method using patterns of
polarized light. The key idea is to modulate the angle of linear polarization
(AoLP) of projected light at each pixel. The use of polarization makes it
invisible and lets us recover not only depth but also directly surface normals
and even reflectance. We implement SPIDeRS with a liquid crystal spatial light
modulator (SLM) and a polarimetric camera. We derive a novel method for
robustly extracting the projected structured polarization pattern from the
polarimetric object appearance. We evaluate the effectiveness of SPIDeRS by
applying it to a number of real-world objects. The results show that our method
successfully reconstructs object shapes of various materials and is robust to
diffuse reflection and ambient light. We also demonstrate relighting using
recovered surface normals and reflectance. We believe SPIDeRS opens a new
avenue of polarization use in visual sensing.
