The ruler is effectively useless. You can set an origin and then measure the distance to some other point. The problem is if you set the origin, the initial value of W and H are 1 and 1 instead of 0 and 0! Move a pixel to the left, and it goes from 1 to -2. So, if you have positive measurements, you need to subtract one, but if you have negative measurements, add one. Sure you can do the math, but for $4.99 you shouldn't have to.