  Non-line-of-sight (NLOS) imaging and tracking is an emerging technology that
allows the shape or position of objects around corners or behind diffusers to
be recovered from transient, time-of-flight measurements. However, existing
NLOS approaches require the imaging system to scan a large area on a visible
surface, where the indirect light paths of hidden objects are sampled. In many
applications, such as robotic vision or autonomous driving, optical access to a
large scanning area may not be available, which severely limits the
practicality of existing NLOS techniques. Here, we propose a new approach,
dubbed keyhole imaging, that captures a sequence of transient measurements
along a single optical path, for example, through a keyhole. Assuming that the
hidden object of interest moves during the acquisition time, we effectively
capture a series of time-resolved projections of the object's shape from
unknown viewpoints. We derive inverse methods based on expectation-maximization
to recover the object's shape and location using these measurements. Then, with
the help of long exposure times and retroreflective tape, we demonstrate
successful experimental results with a prototype keyhole imaging system.
