Just as the title says, this is an exercise(look up 'etude' and you'll know what does it mean), and 's-plane' means the complex plane.
In brief, we have a f(z) in the main.cc, which represents a complex function, and out.png(use make out.png to get it, and you should have ffmpeg installed) shows what this function looks like. For example, the image of f(z) = log(z) is shown as below:
We'll not go any further into details because it is pretty easy for the program to be understood.
Here's some references for how this process is done:
This repo is in the public domain.