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Technical question about touchscreens.
Samstag:
The touch resolution on a phone is going to be way too low for fingerprint scanning. Using interpolation to give a rough guess of finger position lets them use much cheaper panels than you need for capacitive fingerprint scanning.
leapinlew:
Great info...
search term: raw touchscreen data pulled up some interesting stuff and was very useful for me.
I've been focusing on iDevices but it seems like Android may offer more access to this raw data. I'm inching toward the conclusion that the device is simply passing off X,Y coordinates to the software, I'd like to find some proof of this. Maybe I need to find out how a capacitance fingerprint scanner works to determine how much different it is than a capacitance touchscreen.
Howard_Casto:
I can give a LITTLE insight, but keep in mind I'm not an expert adn this is just info I've cobbled together via various projects over the years.
Resistive and capactive touch screens both work on the same concept, only the type of electrical field and screen is different.
Basically you've got voltage (or voltage sensors) flowing across the screen along both the x and y axis. When you touch it, you distort the field and sensors on the touch screen use that to determine x and y position. Imagine sticking your hand inside a grid of laser beams. Your hand will block the beams it is touching, the sensor on the other side of the laser won't get a beam and thus the program can determine the x and y value. Capacitive screens are a little better because they analyze a complex electrical field instead of just checking two resistance values. This allows for the famous "multi-touch" display.
Accuracy goes down dramatically when using multi-touch though. It's difficult for me to explain (or completley understand for that matter), but things get a bit wonky due to the gaps between the points. It's sorta like ghosting on keyboards.
The issue is sensitivity. I think the iphone can accurately track around 4 points at a time. Most android phones can track two. A decent fingerprint scanner I THINK tracks 64 to 200+ points. I'm pretty sure that was to do with the setup of the physical screen and not the software. A finger print scanner uses more of a pressure pad setup, while a touch screen uses a EM field setup. They are both measruing capacitance, but using different methods.
Long story short, I think it's possible, but not practical. Even if you manage to get it to work, I don't think that it's going to be accurate enough to be used for security.
lilshawn:
typical fingerprint scanners use a CCD to generate their data... newer technology scanners use capacitance.
http://computer.howstuffworks.com/fingerprint-scanner3.htm
a touch screen would be like 2 capacitance readings where the fingerprint scanners use hundreds or thousands per inch
older single touch capacitance touchscreens basically have one horizontal plate and one vertical plate. when you touch, the amount of capacitance (high or low) on each of these plates (horizontal and vertical) gives an x,y of where the touch is.
example: high on the vertical and low on the horizontal would mean the finger is someplace on the top left. high on the vertical and high on the horizontal would be somewhere on the top right.
the information you could obtain from the controller can basically tell you:
a: the position relative to the screen (x,y)
b: the size/pressure being used to press the screen (a light finger tip or the whole hand)
you can test this theory by trying to multi press a touchscreen that supports only single touch. pressing 2 spots will produce a touch either:
a: an area the sum of the capacitance of the 2 presses value away.
b: an area the sum of the capacitance of the 2 presses value away but is "outside" the screen that is ignored or simply parsed to the extreme edge of the touch area.
a multi touch screen might be able to tell you the actual size of the press (half a fingertip or a whole fingertip) but the resolution still isn't there.
fingertip scanners (new technology) would be like having 1000's of ipods arranged into a square and having a giant finger press down. the ridges of the fingerprint would be apparent in that some ipods would be pressed (ridges) and others wouldn't (valleys)