A tablet PC, or pen computer, is a notebook or slate-shaped mobile computer, equipped with a touch screen or graphics tablet/screen hybrid technology that allows the user to operate the computer with a stylus, digital pen, or fingertip instead of a keyboard or mouse. Tablet PCs offer a more natural form of input, as sketching and handwriting are a much more familiar form of input than a keyboard and mouse, especially for people who are new to computers. Tablet PCs can also be more accessible because those who are physically unable to type can utilize the additional features of a tablet PC to be able to interact with the electronic world.
Multi-touch (or multitouch) denotes a set of interaction techniques that allow computer users to control graphical applications using multiple fingers or input devices (e.g., stylus). Multi-touch implementations usually include touch hardware (e.g., a screen, table, wall, and so on) and software that recognizes multiple simultaneous touch points. Multi-touch stands in contrast to traditional touch screens (e.g., computer touchpad, ATM, shopping kiosk) that only recognize one touch point at a time. Multi-touch hardware can sense touches using heat, finger pressure, high capture rate cameras, infrared light, optic capture, tuned electromagnetic induction, ultrasonic receivers, transducer microphones, laser rangefinders, shadow capture, and other mechanisms. Many applications for multi-touch interfaces exist and application designers and users are proposing even more. Some uses are individualistic (e.g., Microsoft Surface, Apple iPhone, HTC Diamond). As a new input method, multi-touch offers the potential for new user experience paradigms.
An application cannot use multi-touch hardware without an interface for the application software to receive information from the multi-touch hardware. Unfortunately, each multi-touch hardware device includes its own proprietary interface and application authors must have specific knowledge of a hardware device to write software that works with the device. For example, a multi-touch hardware provider may provide a kernel-mode driver and a user-mode application interface through which user-mode software applications can communicate with the multi-touch hardware to receive touch information. An application author writes software that communicates with the user-mode application interface, but the application author's software works only with that multi-touch hardware. A computer user with a different multi-touch hardware device cannot use the application author's software unless the application author produces a different version of the software that operates correctly with the computer user's device. This produces a very limited potential market for application authors, reduces the incentive to write applications supporting multi-touch interactions, and keeps the cost of the most popular devices high for which the greatest number of applications is available.
Another problem is the difficulty for applications to determine a user's intentions based on touch input received from multi-touch hardware. Touch input may be received as a list of coordinates where the hardware senses touch input at any given time. Each application has to include software to interpret the coordinates and determine the user's intention. For example, if an application receives information about two different touches, a first touch at one position then later a second touch at a new position, it is up to the application to determine whether the user used one finger for the first touch and another for the second touch, or whether the user slid the same finger from one location to another location to produce the first touch and the second touch. Depending on the purpose of the application, these two different interpretations of the user input can have very different meanings.