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Apple Exploring Pressure Sensitive Touch Input
Apple describes that historically, purely pressure-sensitive screens offer little advantage as an input device. The resultant "stiction" created between the finger and the touch surface makes it difficult to operate smoothly.
Instead, resistive (Newton, Palm) touch displays and more recently capacitive (iPhone) displays have dominated the market. Neither of these touch screen technologies, however, measure the degree of pressure the user applies to the screen. By incorporating this pressure information in a hybrid capacitive + pressure display, Apple believes useful information could be gleaned to provide a better user experience.
This is not the first time that Apple has described the use of "pressure" information to aid in gathering information from the user. A patent application published in March 2006 also describes the use of pressure and length of time pressed as variables that might change action. An example provided is when the user is given the option of a "Delete" button, the user would have to press harder for the delete button to be acknowledged.