Intro page needs to be more of a sales page. "This series is your introduction to the simple equations that can produce images like <image>, <complicated Lissajou image>, <beautiful image>, and more!" Not "Let me start by regretting that I didn't care enough to make this a book, and then show how to draw a straight line." I only clicked another chapter heading because of the HN endorsement.
Lissajous curves have some interesting mathematical properties [1] as well some practical applications in the physical world (as many other mathematical concepts do). I just looked at the Wikipedia article about them again, after some time, and found it cool.
I had done a good amount of elementary 2D graphics programming earlier, on various models of computers and in a few different programming languages, as a hobby, not for work. As part of that tinkering, while plotting various combinations of trigonometric expressions, one of my programs generated what looked like a Lissajous figure, which I recognized from high school math. Fun.
[1] The article says that circles, ellipses, parabolas and lines are special cases of Lissajous figures, based on the values of the parameters in the equations.
Oscilloscopes can be made to generate Lissajous figures based on the inputs you give them.
