via NY Times:

Every time someone passes in front of a camera connected to the system, the software logs a separate “motion event.” The time and location of the event - along with a still picture - are sent to a security guard’s desktop computer. The guard can then browse through these pictures instead of staring at a bank of black-and-white monitors showing images that are constantly changing, waiting for something to happen….

The 3VR software assigns an identification number to every person a camera spots, and establishes a profile based largely on the geometry of the person’s face. Whenever the face is captured from a
different angle or in a different light, the system creates another mathematical model….

For now - and the foreseeable future - 3VR’s system is effective only in small, controlled environments where the lighting is consistent and only a few people pass in front of one camera at a time. Picking out criminal suspects on the street or in a crowd - as the city of Tampa, Fla., tried to do in its Ybor City district from 2001 to 2003 - is still beyond the ability of 3VR and every other surveillance system.