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Video cameras are the eyes of these fast processors, capturing im- ages in digital form; that is, as streams of bits representing the position and color of each picture element or pixelin a video frame.A pixel is the smallest unit in an electronic display. It takes about a million pixels to form an image on a computer screen, just as a myriad of individual colored tiles forms a wall mosaic. (To be exact, computer monitors typically display 1,024 × 768 or 1,280 × 1,024 pixels hori- zontally and vertically, respectively). Even fewer pixels per frame is adequate for many uses, and that lower resolution is easily achieved with inexpensive Web cameras that routinely send video over the Internet.

Other approaches work differently from the eyes; they examine the environment actively rather than passively. One method employs low-power infrared lasers mounted on the artificial being.When the laser beams strike an object, they are reflected back to sensors mounted on the being, where their time of flight is analyzed to find the objects range and bearing.Another approach emulates the echolocation used by bats and porpoises.These creatures generate high-frequency (ultra- sonic) sound waves and listen for the echoes, which their brains ana- lyze to characterize their surroundings. A similar process operates in sonar (sound navigation ranging) as used by nuclear submarines, and some robots use sonar as well.

There are also new ways to interpret sensory data, such as the promising approach called probabilistic robotics. According to Sebastian Thrun (then at Carnegie Mellon University, and now at Stanford), it uses the fact that robots are inherently uncertain about the state of their environments,because of limitations in their sen- sors, random noise, and the unpredictability of the environments themselves, caused by, for example, the movement of people within the creatures visual field. Instead of calculating exactly what to do next, the being accommodates its uncertainty by determining a range of possibilities.As its sensors gather more data, the beings calculations converge to a high level of confidence about its physical location and other quantities.This method takes more computer time than direct approaches, but todays computers are up to the task.As Thrun notes, the payoff is that a probabilistic robot can

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