Scouring to Reveal Hidden Clues - Software to Help Investigators Clearly Scrutinize Videos
Scouring to Reveal Hidden Clues -
Software to Help Investigators
Clearly Scrutinize Videos
By Paul Eng, ABC News
Videotapes from surveillance
cameras can provide criminal investigators
with a wealth of information — a getaway
car's license plate, the type of weapon used in
a holdup, and perhaps even the face of the
perpetrator of the nation's anthrax attacks.
ABCNEWS has learned that one law
enforcement agency has already keyed
its search to a select few tapes from
several post offices' tiny video
But to find that face or other clues
hidden within often grainy and poor-quality
videotapes, investigators are turning to a new
computer-based tool called dTective.
The computer program, produced by Ocean Systems
in Burtonsville, Md., works with a digital
video-editing system called Avid Xpress. That
machine, basically a high-powered computer, converts
images stored on the magnetic tape of a videocassette into digital form.
Video, at least in the United States, is typically
recorded at a rate of 30 frames, or images, a second.
Each frame of video is comprised of thousands of tiny
dots of light, or pixels. The Avid machine captures
and converts each one of those pixels for each frame
into the computer code of ones and zeros and stores it
on the machine's hard drive. Once on the hard drive,
investigators can use the dTective software to
mathematically clarify the pictures and perform other
Play It Really Slow
For example, most video surveillance systems will record at a much slower rate —
say, five frames a second — in order to store a whole day's worth of surveillance
on a single videotape. Using dTective software, investigators can automatically
slow the playback of the digital images it captured from the tape so that it appears
to investigators as if it had been shot at normal recording speeds.
This feature becomes even more useful if the videotape comes from a "multiplex"
surveillance system. In those setups, the videotape stores images from several
cameras on a rotating basis. The first frame, for example, may be from the camera
mounted above a store's checkout counter, the next frame from the camera above
the store's front door, and the next frame from a camera at the store's back door. If
a detective played such a tape back on a standard VCR, all they would see is a wild
blur of flashing images.
But since each frame is captured and defined as a separate and distinct image,
investigators can easily sort out only the frames they're interested in seeing. They
merely program the software with the number of frames per camera and how many
cameras were used in the original surveillance system and the dTective sorts the
images into the proper order.
Cleaning Up On Averages
What's more, since each pixel of each frame of video is stored as a mathematical
piece of data, dTective can use a routine known as image averaging to "clean up"
the images and allow detectives to see the footage more clearly.
How does it work? Footage with moving objects — raindrops, a speck of "noise"
— contain frames that have many pixels with wildly fluctuating mathematical
values. The software combines the information from a number of frames to
generate a set of averaged frame values. The resulting video frames created by
those averages filters out the moving objects, leaving behind only the images of
consistently non-moving objects within each averaged frame.
Since investigators can selectively set how many frames to average over the part of footage they wish to clarify, the results can be stunning. In some cases, objects that
were obscured in the original video can be clearly seen on the digital version
modified by dTective.
And such abilities of the dTective and Avid systems have helped local law
enforcement officers catch and convict criminals.
"Many times investigators will look at a videotape and conclude that there's no
evidence there," says Grant Fredericks, a forensic video specialist who teaches at
the FBI Academy in Quantico, Va., on how to use the systems. But with these
video tools, "We are clearing cases that we never thought were clearable in the
And it's possible dTective will give police a hand in the ongoing anthrax
investigations. "I have no doubt that the people who bought those stamps were
caught on video," says Fredericks. And if they were, officials and the public may
finally get a clear glimpse at the faces of those responsible for such the attacks.