I would like to measure two points and tangents of a curve and transfer that information to a computer. Here is a picture of what I would like to measure:
Points (x1,y1), (x2,y2)
Tangents T1, T2 (or an approximation of the tangents)
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Very possible, and I think not that hard. With a few changes. 1. Hands are 3 dimensional and can point each end not in two directions but in 3-space. 2. Though it is easy to compute if you know the 3D coordinates of each end, you need to know the distance between the two ends of the blade.
Therefore I suggest these simplifications for simplicity and cost:
Recompute your script to use relative coordinates of only: the distance between the blade ends, the angle of T1 up from 0 pointing to hand 2, and the angle of T2 up from 0 pointing to T1.
Build a physical skeleton consisting of a metal ruler with a rotating potentiometer at each end (10k linear). The two pot knobs each hold an end of the blade, and restrict them to rotating in the same plane. The user adjusts the thing with his hands, first reading the distance between the ends. That gets entered into the Arduino program. The program reads the each pot to get the angles T1 and T2. Your script spits out the shape on a screen or little display.
Here's an all mechanical solution. Get two of the those metal T-square rulers that have a built in adjustable protractor. Unscrew the knobs and put both smaller protractor arms on the same long arm. Mount a blade end on each knob. You then physically adjust each angle and blade end, visually seeing the tangents since they are pieces and reading off the protractor angles.
Enter the two tangent angles and the distance between them, and if you calculated right, voila! Your script prints they shape of the reality you created!