Hi Joe

You just made my day.

Gee using a microscope with your software is a huge step, 3 um = 0.00011811 about a ten thousandth of an inch imperial. accuracy.

Using wire or fishing line allows the measured line to be accurately positioned. By anchoring each end of the wire to the machine frame precisely you can place the linear rail exactly where you want it. A lot easier than pointing a laser from one end. It also allows you to easily place two rails parallel to each other, by simply measuring the space between the ends.

I would really like to try your new software. In fact I am quite excited.
While tweaking it You mentioned that you used black fishing line piano wire is bright and I guess would require a dark background instead of the light background you used to enhance the contrast of the line you used in your experiments.
Maybe the ability to invert the pixel colour in the contrast discrimination code would be a useful addition.

Wire or mono filament fishing line? I can think of a number of applications where the the measuring system will need to be left in place for days or weeks when say a large machine installation or While a DIY machine is being built, All plastics will keep creeping and possibly break. Music wire will retain its elasticity indefinitely if it is not loaded above its elastic limit. I use Roslau music wire with great success. Size 0 is .009 Inches in diameter. I never reuse it (From the coil it is perfectly straight you do not want any kinks) a small coil will last for years of hobby use.

There is another use for this system! A non contact plumb line sensing system or maybe even a pair of plumb lines!!
Two suspended plumb lines will always remain parallel and perfectly vertical in relation to each other (Ignoring the curvature of the earth that is only measurable at great distances.)

The two wires define the edges of a rectangular flat plane that in this case is measurable to .0001 inches using wire and your software. I am going to think some more on this....... I have an idea.

Regards
John