Similarly to reading the mechanical digits on a utility meter?
If you can afford to use a PC for this, it would be quite easy to get something that worked under ideal conditions. Making it work reliably under less-than-ideal conditions may be very difficult, and there would always be some situations where it simply wouldn't work, or produced the wrong answer.
Tackling this with something less powerful than a PC would involve all the same problems, but they would be harder to solve since resource-intensive approaches would not be feasible.