Comment 40 for bug 1287341

Still affects me on 14.04.

As it is already said, there is no reliable way to assign a touch device to a certain display device. So I suggest a two phase design:

1) Check if there is information for making an automatic assignment in some database. If there are more then one equal device, use display output and USB port id for identification.

2) If none is found, select the by some simple heuristics. Then launch a (to be made) settings program, that displays a centered button the first screen "PLEASE TOUCH HERE". If the button is pressed, the coordinates are checked against the center of all possible displays from 'xrandr' or gnome-display-settings information. If it matches, the assignment is stored to the local database. The procedure continues with the second screen, if there is a third one and so on.

If for some reason the database gets outdated (eg the user exchanges screens) phase 2 will be entered autmatically as you see.

The only caveat may be exchanging the output assignments of multiple equal display devices. For this case, the phase 2 program should be integrated into the system settings center, and easily be reached without proper touch operation (eg. launch via Unity by keyboard).