  Non-line-of-sight (NLOS) imaging methods are capable of reconstructing
complex scenes that are not visible to an observer using indirect illumination.
However, they assume only third-bounce illumination, so they are currently
limited to single-corner configurations, and present limited visibility when
imaging surfaces at certain orientations. To reason about and tackle these
limitations, we make the key observation that planar diffuse surfaces behave
specularly at wavelengths used in the computational wave-based NLOS imaging
domain. We call such surfaces virtual mirrors. We leverage this observation to
expand the capabilities of NLOS imaging using illumination beyond the third
bounce, addressing two problems: imaging single-corner objects at limited
visibility angles, and imaging objects hidden behind two corners. To image
objects at limited visibility angles, we first analyze the reflections of the
known illuminated point on surfaces of the scene as an estimator of the
position and orientation of objects with limited visibility. We then image
those limited visibility objects by computationally building secondary
apertures at other surfaces that observe the target object from a direct
visibility perspective. Beyond single-corner NLOS imaging, we exploit the
specular behavior of virtual mirrors to image objects hidden behind a second
corner by imaging the space behind such virtual mirrors, where the mirror image
of objects hidden around two corners is formed. No specular surfaces were
involved in the making of this paper.
