The sensor forms an image and OpenCV detects the actual touch configuration. It appears you can use the raw data from the Arduino, too, but it might be a little harder.

At the heart is an Arduino compatible microcontroller (based on the Leonardo) with a few extras baked in, including a Freescale touch sensor connected to 12 electrodes and an audio processor for ...

Capacitive touch sensors are entirely in the domain of DIY, requiring little more than a carefully-chosen conductive surface ...

Touch Board works with any conductive material, and although it comes pre-programmed, it can also be programmed like any other Arduino.