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When they’re not rolling sheets of metal into tubes and stuffing them with conductive foam to make iPhone styluses, the smart engineers at Ten One design are hacking away at the iPad. And quite miraculously, they have managed to turn the iPad’s screen into a pressure sensitive drawing tablet.
Proper graphics tablets like those from Wacom have pens which detect thousands of levels of pressure, but the iPad offers just two levels: one and zero. To get around this, the Ten One people, makers of the Pogo stylus, have hacked Apple’s private UIKit frameworks to enable the screen to detect pressure. The video above shows the test software in action (the delay in drawing is due to “an issue with [the] demo application code.”
Does this mean that the iPad’s screen is somehow able to know how hard you are pressing? Maybe not. Although the Ten One blog post doesn’t mention just how this information is gotten from the iPad, my guess would be that the size of the tip is being measured by the standard multi-touch detection. As you press on the foam, the tip grows. This also explains how the test software is able to ignore the side of the hand while still reacting to the pen.
Ten One plans to release its hack to the world for free inclusion in any software. This rests on Apple opening up the private software framework, which is a notoriously slow process. Still, it would be pretty awesome, and would add a lot of finesse to drawing and painting apps like the excellent Brushes.
Pressure-Sensitive Drawing on iPad [Ten One designers blog]
See Also:

Pogo Stick Turns Mac Touchpads into Graphics Tablets
Pogo iPhone Stylus Bounces Back. Again
Pogo Stylus: The Most Useless iPhone Accessory. Ever

Fuente: Gadget Lab