Technical question about touchscreens.

[leapinlew]

My company develops Biometric software which uses many metrics the iPhone/iPod/iPad touch to assess the identity of the user.

My task has been to try and find out if the touchscreen on these devices is sensitive enough to get any kind of fingerprint. Our software can accurately identify a fingerprint with only a partial print at 160dpi. After much searching and discussion, I am still at a loss. I have a few questions and was hopeful someone here could help answer.

1. How does the screen respond to a touch on the screen? Does it sense the capacitance difference between the valley/ridges of a finger print?

2. When I was reviewing capacitance fingerprint scanners, it appears to be the same technology. After a few conversations with touchscreen manufacturers, it seems the technology is very similar but a phone touchscreen has much fewer electrodes and uses interpolation and different to determine where on the screen is being pressed.
 
I know these are broad questions, but I'm having a difficult time even trying to figure out the right questions to ask. The basic question I have is:

Does a iDevice have enough sensitivity in the screen to pull a rough fingerprint image? If not, why not? If anyone could help me answer or point me in the right direction, I would greatly appreciate it. I've been searching for a while and haven't been able to locate a satisfactory answer.

[Mysterioii]

If it has fewer sensors and uses interpolation to make an educated guess as to where you're touching, that would explain why my droid razr maxx seems to do a crappy job of identifying which link I'm trying to click or which key I'm trying to press on the on-screen keyboard.     It could just be that the software isn't making the most of the hardware's capabilities, but based on my user experience I'd be shocked if this thing would have the capability to do what you're looking for.

[kahlid74]

My understanding is the technology used does not allow information like that to be gathered because it is only concerned with gathering position, not "what" is touching it.  My guess is you could hack one to do what you're looking for but the iOS may not have hooks in it capable of doing such natively.

[drventure]

First, I don't know ANYTHING about this particular area, so take my comments with a grain of salt.

Everything I've found tends to imply that the conversion of "touch" to x-y happens in hardware, well before you can get your hands on the raw data.

But I did find this link that talks about touchscreen kernel level filters being able to distinguish between finger, thumb, thumbnail and stylus.

http://wiki.openmoko.org/wiki/Touchscreen_Filters

Might be a place to start.

I googled "raw touchscreen data"

[knave]

I also have no education on how the data is handled, but from replacing smartphone screens I know that the digitizer sticks on the back of the touchscreen glass and connects with a ribbon cable to the phone hardware. Weather it gust reads x, y coordinates or is capable of sensing texure is the real question. My theory is that it reads when our touch interrupts the flow od current through the digitizer and then plots the coordinates from that. This would suggest that what you want to do is not possible. How do common biometric fingerprint scanners read the fingerprints? Light? Hmm...

A dongle might be an option...

...or find a way to use the camera...

[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)