The iPhone popularized multitouch displays--touch screens that ­recognize more than one touch at a time. Then Microsoft brought them to the large screen with the Surface, a multitouch table. Now engineers at Nortd in New York have created a do-it-yourself, 18-by-27-inch multitouch screen. By using open-source software and selling kits for building the hardware, Nortd has significantly reduced the cost of owning a multitouch display, while enabling freelance programmers to develop novel applications.

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