Duality is the geometric concept of defining a mapping between points in one plane (the "primal plane") and lines in another (the "dual plane"). The basic idea is that a point (a, b) maps to a line y = ax - b in the dual plane, and vice-versa (y = ax + b <-> (a, -b)) Believe it or not, this is both cool and useful!
The demo is an interactive visualization that lets you create points and lines in either of two planes, drag them around, and watch as the dual transforms according to the primal-dual mapping. You can create points and lines of different colors to help visualize different scenarios, and you can delete objects.
The idea of duality can be very helpful in approaching 2-D geometric problems, but it's often hard to visualize how a set of points in the plane will translate to a dual arrangement. There exists at least one java applet that demonstrates this, along with extra functionality, but many of us now have Java disabled in our browsers. This was a quick and dirty attempt to get something I could play with when trying to develop algorithms for my computational geometry homework. I hope that someone else will find it useful as well.
The demo is a self-contained client-side web application written in JavaScript using the Fabric.js canvas library. I hacked about with Fabric enough to make it handle plane objects the way I wanted it, then wrote my own custom color picker. (Yes, I know there are a million out there, but I wanted one with a few discrete color choices.) The JS is pretty standard; don't expect it to work on your old version of IE or Mosaic or WorldWideWeb.
This code is released under the terms of the MIT License.