For the past decade I’ve been developing forensic techniques for
determining if an image is a forgery. The general philosophy
that I have adopted is to first concede that there is no single
technique that can detect all forms of digital manipulation. I
have, therefore, been developing a number of different forensic
tools each tailored to detecting specific forms of photo
manipulation – some of these techniques operate on subtle pixel-
level statistics that are invisible to the human eye, and others
operate on geometric properties that can sometimes be seen with
a trained eye. For example, in the image shown below, the
bottle’s cast shadow is clearly incongruous with the shape of
the bottle (as is the shadow on this cover of Time Magazine).
Such obvious errors in a shadow are easy to spot, but more
subtle differences can be harder to detect. Shown below are two
images in which the bottle and its cast shadow are slightly
different (the rest of the scene is identical). Can you tell
which is consistent with the li