Here is a good list.
https://github.com/kosmos/awesome-generative-art
Also… Shaders ! Is a great way to start.
Thats my work. All realtime.
A collection of my generative artwork, mostly with Processing in Python mode https://github.com/aaronpenne/generative_art
cutterkom/generativeart: Create Generative Art with R https://github.com/cutterkom/generativeart
kosmos/awesome-generative-art: Awesome generative art https://github.com/kosmos/awesome-generative-art
Programming Graphics I: Introduction to Generative Art | Joshua Davis | Skillshare https://www.skillshare.com/classes/Programming-Graphics-I-In...
OpenFrameworks and Cinder are probably a bit more powerful, but more difficult to use (built around c++).
I also like PaperJS, which is built around JavaScript, but doesn't have the performance of Processing and OpenFrameworks.
Finally, you can just use raw drawing APIs (such as Canvas in the browser).
Good list of resources here: https://github.com/kosmos/awesome-generative-art