A webcam, interesting. Readily available, but I have no idea how to interface from a graphical device to digital output, maybe someone could come up with something there. On the line following robot idea, that kit I linked to earlier would just need two ADC controllers, the robot's drive motors input's connected to them, so the output would be digital into the parallel interface board. That gives you a two axis input, all that's needed. OK, so there's the electronic side of it, I don't know anything about writing a macro in Mach to run this "Visual digitising device"