What kind of sensor can i use to detect coordinate?

i am looking for a sensor which can detect the coordinate(X,Y) of an object from a fix point. let's say i fix the sensor at the corner of table, i put an object on the table then i can get the coordinate as output. as the title, what kind of sensor can i use?

i found one device named Note Taker. The sensor i want to find is some sort like the sensor used in Note Taker. What kind of sensor is that? Ultrasonic? Radar?

Can you put a pulsed IR LED on the object being tracked?
Can you put an ultrasonic emitter on the object being tracked?

It's very difficult to do well. There is a lot of research going on for multi-touch screen sensing.

thanks for your replies.
actually i wan to do something new and convenient for computer user as my final year project. after i see Nete Taker then i have an idea to virtualized mouse and keyboard. i start to do research on sensor, till, i found another new launched product named evoMouse yesterday. that is totally same as what i want to do. is the evoMouse using concept of image processing?