I was thinking you could use webcam streaming video to provide servoing feedback on a hanging V-plotter. I mean, you could use it to provide servoing feedback for any kind of robot (at least for things that don’t need <30ms or so latency) but V-plotters (aka polargraphs, drawbots, wall-plotters, etc.) are particularly promising.
There’s a bunch of V-potter videos on YouTube, including some which are just video from a fixed point of view, in real time, without any change in point of view. These are not the most interesting or informative videos to watch as a human being, but they make it possible to test this concept before actually having built a V-plotter.
https://www.youtube.com/watch?v=HU9SaCFnCng shows a deformable, swinging V-plotter, more rapidly than real-time, with changes in lighting in the room, for a couple of minutes.
https://www.youtube.com/watch?v=WyLPpGdfR7s is 5 minutes from a fixed point of view of a somewhat wobbly V-plotter, from a fixed point of view, with lots of texture in the background and a few distractors. Midway through a guy walks in front of the camera, and the lighting changes. Toward the end, the lighting changes again.
https://www.youtube.com/watch?v=i5rxxGuWUo8 has a minute or so in the middle of a somewhat wobbly V-plotter drawing on a shiny whiteboard.
https://www.youtube.com/watch?v=ojcZ7kcklu4 is a sped-up video of a Polargraph drawing Spider-Man, from a single point of view.
https://www.youtube.com/watch?v=aiw3hkDvp-M has about a minute of a Polargraph drawing some classic art, sped up.
https://www.youtube.com/watch?v=qury-0BiURA has another few minutes of a Polargraph drawing. The camera moves a bit toward the end, and lighting flickers throughout.
https://www.youtube.com/watch?v=MbpT907qIHg has a couple of 1-minute-or-so sequences of time-lapse drawing, each from a different points of view. This one uses fishing line for the V, which means the strings won’t interfere with tracking.
https://www.youtube.com/watch?v=MG70TvuRU9Y has a couple of 30-second-or-so sped-up segments of a V-plotter drawing from fixed points of view, one with changing lighting.
https://www.youtube.com/watch?v=5z8LTj74uiE is almost a minute of sped-up footage of a V-plotter with a novel design and substantial parallax.
https://www.youtube.com/watch?v=0BOuqLPaEMc is a very short video of a V-plotter made out of a binder clip.
https://www.youtube.com/watch?v=iE7sCiv7VTE is a particularly tricky case: the V-plotter gondola is made from a CD-R, which means that its reflection is almost entirely specular, and furthermore has diffraction rainbows all over it. The video runs in apparently real time for about six minutes.
https://www.youtube.com/watch?v=FmCWx30g7Ks is a whiteboard V-plotter in real time for about two minutes. It’s rather blurry.
https://www.youtube.com/watch?v=jknm5qtgO1A is another blurry whiteboard V-plotter in real time (except for occasional jumps), with conspicuous hanging cables.
https://www.youtube.com/watch?v=u4KyPN0sw0A is a V-plotter using a spray-paint can that appears to be using the Hektor constant-speed path-design software. Its speed varies.
https://www.youtube.com/watch?v=OselyTkA6wU seems to be Hektor itself, running in real time for some minutes.
On another topic, https://www.youtube.com/watch?v=ycrGUTGlzik shows the design of the PolargraphSD gondola that keeps its wobble so low. It uses cartridge bearings to pivot the two strings around the precise XY center of the gondola.