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I'm not sure, but maybe there's a way to determine the finger pressure on the iPhone screen? I know it has no pressure sensors, but it could be somehow calculated from the touch "footprint", if it was accessible in some way.
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Capacitive screens are made up of multiple layers of glass and plastic, coated with a conductor material like indium tin oxide or copper. This conductive material responds when contacted by another electrical conductor, like your bare finger.
Because the human body conducts electricity, a fingertip in close proximity to the glass will absorb the electrical charge and create a measurable disturbance in the field, alerting a grid of electrodes on the screen and enabling the phone to register the command.
The iPhone's screen detects touch through one of two methods: Mutual capacitance or self capacitance. In mutual capacitance, the capacitive circuitry requires two distinct layers of material. One houses driving lines, which carry current, and the other houses sensing lines, which detect the current at nodes.
Like @kendall said. You only get a point.
I would suggest that you do not look at pressure but you look at duration of the touch. This way you could translate this into increased pressure. Eg if you were making a painting application, the longer you hold your finger in one place the large the blob would appear compared to when moving your finger quickly to create a thin line.
Cheers, John,
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In the default SDK you are not given a touch area, but a point.
At first this seems less useful but the iPhone is doing a lot of work to determine where a user really means to press, based on the shape it sees. An interesting side effect is that if you try to use the iPhone upside down, it's not nearly as good at registering touches.
Using jailbroken libraries it may be possible to read the more raw touch data, but I've not seen any mention of that.
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The touch detection on the iPhone does not work with pressure but with capacitance afaik, so not directly. There might be ways of detecting the shape/area covered and then deducing pressure from that, though it would involve so many assumptions it's unlikely to be very accurate.
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