[stephencoyle]

You're right that it doesn't consider changing diameter, though I did contemplate some approaches for doing that. But, since it only needs whole-inch resolution, using the diameter of a half-full roll of tape gets close enough for novelty purposes.

[implements]

How about measuring the linear tape motion in a similar way that laser mice measure surface movement?

[stephencoyle]

That's a cool idea! It's all open source, and forks/remixes are encouraged, so feel free to give that a whirl. :)