Peter Stewart's Plotter Art

My grandfather left behind a binder full of artistic experiments he carried out with a computer and electronic plotter in the early 1980's. The most common image was "a kind of bird" produced with lines that connected iterated trigonometric functions.

He also included some faded printouts of the code he had written to generate these images.

The code is quite simple, so I decided to try to replicate his plotter images using a modern web browser. I took my best guesses about what his HP calculator was doing in its BASIC-like language and simulated it in Javascript.

Unfortunately, the results of my simulation do not quite match his bird image. So in an attempt to better understand the math behind the algorithm, I began adding interactive controls that make it possible to adjust the various parameters in real time.

So far, I haven't figured out why my simulation is producing different results than Peter's HP calculator, but I've had a lot of fun watching the image subtly morph as I adjust the sliders. You can try it yourself below. Let me know if you come up with any ideas about why my simulation doesn't match the historical version!