Home' Technology Review : February 2005 Contents 82
TECHNOLOGY REVIEW february 2005
EDITED BY MONYA BAKER
Each month brings new investigative tools, new ideas for revolutionary
technology, and revolutionary applications of existing technology.
No one can know today which will matter most tomorrow.
But these represent Technology Review’s best prediction.
An algorithm sniffs out
context: Photographs used to be a reli -
able source of evidence, but the advent of
digital cameras and photo-editing soft -
ware has made every picture a potential
scam. The skillful user of Adobe Photo -
shop and other tools can produce realistic
images of imaginary scenes. As Dart -
mouth University computer scientists Alin
Popescu and Hany Farid note, the Los An -
geles Times unwittingly ran an altered
photograph from the war in Iraq on its
front page. To help defend against these
types of forgeries, Popescu and Farid have
published a new image-processing algo -
rithm that detects photographic fakery.
methods and results: When forgers
modify an image, they often insert ele -
ments taken from other photographs or
from other sections of the same photo -
graph; these insertions need to be dis -
torted, resized, or rotated to �t in with the
rest of the image. Even when no new ele -
ments are added, digital manipulations
may leave telltale signs of “re-sampling.”
For example, to double an image’s size,
software inserts a new pixel between ev -
ery pair of neighboring pixels in the origi -
nal image. The new pixels are a combina -
tion of the pixels surrounding them in the
original image—the result of interpola -
tion. Such regularity rarely occurs in natu -
ral images and often produces patterns
that Popescu and Farid’s software can de -
tect, even when they’re unapparent to the
naked eye. In trials employing 50 images
selected at random from a database of
200, Popescu and Farid’s method found
nearly all cases of enlargement greater
than 1 percent and most cases of rotation
that required interpolation. Some cases of
shrinking could also be detected.
why it matters: Current forgery detec -
tion techniques, which are vital for screen -
ing news items and intelligence, leave
much to be desired. Digital watermarking
works only when someone has had the
foresight to insert hidden information
into an image �le to prevent tampering. In
contrast, Popescu and Farid’s method can
be applied automatically to any image �le.
However, the method is not foolproof: for
example, it cannot detect cases of shrink -
ing without interpolation. Also, data com -
pression, used in JPEG �les, and noise in -
terfere with the algorithm. Nonetheless,
the new software makes it harder for a
digital photograph to lie.
Source: Popescu, A. C. and H. Farid. 2005. Exposing
digital forgeries by detecting traces of re-sampling.
IEEE Transactions on Signal Processing(in press).
Hand gestures control
context: Video games, design software,
and scienti�c visualization technologies
routinely use 3-D graphics. Typically, us -
ers interact with 3-D graphics on �at com -
puter screens and cannot grab, move, or
rotate graphical representations as they
can real physical objects. Even the most
advanced stereoscopic displays, like those
used in virtual-reality systems, require
head-mounted displays, which restrict
viewers’ range of motion, obstruct pe -
ripheral vision, and cause discomfort.
Now, researchers at the University of To -
ronto have created a system that frees 3-D
graphics from such constraints.
methods and results: Key to the system
is a “volumetric” display, one that lets
multiple users view graphics from any
angle without wearing headgear. Tovi
Grossman and his colleagues used a
swept-volume display, which spins a se -
ries of 2-D images around an axis fast
enough that humans perceive them as a 3-
D image. The researchers created a way
for users to manually interact with their
display, which is housed in a clear plastic
dome. Cameras track special rings worn
by the users, who can select 3-D objects by
pointing at them and drag them by mov -
ing their �ngers across the display sphere.
Using both hands, users can stretch or
shrink objects, or specify the axis about
which an object should rotate.
why it matters: While the new display
does not provide sensory feedback, it per -
mits the control of 3-D graphics through
hand gestures similar to those whereby
people manipulate real objects. The tech -
nology eliminates the need for joysticks or
virtual representations of users and paves
the way for 3-D graphics applications that
anyone can use with minimal training.
Volumetric displays, once found only in
expensive research prototypes, have be -
come commercially available. The To -
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