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Machines Like Us

Video amplification makes the invisible visible

Friday, 22 June 2012
by Larry Hardesty

In these frames of video, a new algorithm amplifies the almost imperceptible change in skin color caused by the pumping of the blood. Credit: Michael Rubinstein

New software amplifies changes in successive frames of video that are too subtle for the naked eye.

At this summer's Siggraph — the premier computer-graphics conference — researchers from MIT's Computer Science and Artificial Intelligence Laboratory (CSAIL) will present new software that amplifies variations in successive frames of video that are imperceptible to the naked eye. So, for instance, the software makes it possible to actually "see" someone's pulse, as the skin reddens and pales with the flow of blood, and it can exaggerate tiny motions, making visible the vibrations of individual guitar strings or the breathing of a swaddled infant in a neonatal intensive care unit.

The system is somewhat akin to the equalizer in a stereo sound system, which boosts some frequencies and cuts others, except that the pertinent frequency is the frequency of color changes in a sequence of video frames, not the frequency of an audio signal. The prototype of the software allows the user to specify the frequency range of interest and the degree of amplification. The software works in real time and displays both the original video and the altered version of the video, with changes magnified.

Although the technique lends itself most naturally to phenomena that recur at regular intervals — such as the beating of a heart, the movement of a vibrating string or the inflation of the lungs — if the range of frequencies is wide enough, the system can amplify changes that occur only once. So, for instance, it could be used to compare different images of the same scene, allowing the user to easily pick out changes that might otherwise go unnoticed. In one set of experiments, the system was able to dramatically amplify the movement of shadows in a street scene photographed only twice, at an interval of about 15 seconds.