  Developed at MIT CSAIL, the Wallcamera has captivated the public's
imagination. Here, we show that the key insight underlying the Wallcamera is
the same one that underpins the concept and the prototype of differential
imaging forensics (DIF), both of which were validated and reported several
years prior to the Wallcamera's debut. Rather than being the first to extract
and amplify invisible signals -- aka latent evidence in the forensics context
-- from wall reflections in a video, or the first to propose activity
recognition following that approach, the Wallcamera's actual innovation is
achieving activity recognition at a finer granularity than DIF demonstrated. In
addition to activity recognition, DIF as conceived has a number of other
applications in forensics, including 1) the recovery of a photographer's
personal identifiable information such as body width, height, and even the
color of their clothing, from a single photo, and 2) the detection of image
tampering and deepfake videos.
