it's not, the dispenser is a jab at the kind of trinket stalls that form around holy sites. what i don't understand is how the dispenser compensates for the changing radius of the tape roll in order to measure accurately. i suspect that it doesn't.

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.

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

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

Two ways to make it:

- know the total length of the tape, and when it's changed, and therefore predict the current radius of the roll

- a small sprung variable resistor pressing against the under side of the role

But that could be over-engineering something that almost more perfect if it's a little imperfect.

thought of a third way:

cellophane tape does emit xrays when unrolled, so correlate the strength of the radiation with the angular velocity of the roll to more precisely calculate how much tape was removed. I suspect a second photodiode may respond to the radiation.

Id just weigh the roll.

Divine intervention, presumably. They don't bless those tape dispensers for nothing.

Divine intervention sounds reasonable for an automated blessing device. Otherwise the issued blessings might be unauthorized or even in violation of copyright.