Encoders on mecanum drive?

[jtrv]

Can someone explain how encoders work on a mecanum drive? I don't really get how its supposed to work. The only encoders I've worked with are 1 dimensional - we've always used tank drive (until this year) and we just put encoders on each side of the drive, and got 2 values for each side, either + or -. We could then convert it to tell how far we've traveled.

How on earth does this work with mecanum when you can travel at a 45 deg angle, and how do you measure left or right directional movement?