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W4 is a real time visual surveillance system for detecting and tracking people and monitoring their activities in an outdoor environment. It operates on monocular grayscale video imagery, or on video imagery from an IR camera.Unlike many of systems for tracking people, W4 makes no use of color cues. Instead, W4 employs a combination of shape analysis and tracking to create models of people's appearance so that they can be tracked through interactions such as occlusions. W4 is capable of simultaneously tracking multiple people even with occlusion. It runs at 20 Hz for 320 X 240 resolution images on a dual-pentium 200 PC.
Haritaoglu et al. (Sun,) studied this question.