Figures have been drawn using the free software, IPE:

  https://ipe.otfried.org/

It was developed and maintained by Otfried Cheong (one of the authors of our
textbook).

Note that IPE stores files as pdfs, but (if generated by IPE) these pdfs can
be opened in IPE. IPE also supports a plain text format, as .ipe files.

IPE has a nice programmable feature in the form of scripts, called ipelets.
There are ipelets for many handy constructions in computational geometry, such
as Voronoi diagrams and Delaunay triangulations.
