Digitizers convert analog or physical input into digital images. This makes them related to both scanners and mice, although current digitizers serve completely different roles.
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Modern digitizers appear as flat scanning surfaces or tablets that connect to a computer workstation. The surface is touch-sensitive, sending signals to the software, which translates them into images on the screen.
Digitizers carry out important work in computer-aided design, graphics design and engineering. They also help convert hand-drawn images into textures and animation in video games and movie CGI.
In addition to the tablet itself, digitizers have an input stylus that acts as a pen. Mode of input does vary---earlier models relied on simple pressure and electrical impulses, while more advanced designs offer better accuracy with lasers and even camera pens.
Much like scanners and fax machines, digitizers trace their ancestry to the late 19th century, when they emerged from telegraph-related technology. Modern digitizers came about in the 1950s, but only gained popularity with the advent of 16 bit computers in the 1980s. Digitizers of that era were often confused with scanners, as in converting any image into "digital."
Important factors to consider when looking at digitizers are resolution, sensitivity and image recognition. While users can input any image, the tablet and software may not be able to convert it fully. Also, handwriting recognition and text auto-detect are popular features.
The largest maker of digitizers in the world is Wacom, a name synonymous with digitizing among graphics designers and game artists. Their product range includes smart pens and large tablet PCs, covering almost every conceivable use.