Blender Extended Expressive Renderer (BEER) is a free and open source realtime non-photoreal (NPR) rendering engine. The main feature of BEER is the ability to extend the rendering capability from the ease of adding custom shaders to a customizable shader graph. Since NPR is a journey of endless styles which never settle, we need a render engine which will fulfill all the stylized needs.
https://www.youtube.com/watch?v=g9NHz9mzuBc
https://docs.google.com/spreadsheets/d/1PzEgDB8d6vHqGSSmOa-_IF8u5USBEzD0ycEW-LztSOc/edit?usp=sharing
https://github.com/BlenderNPR/BEER/blob/master/BlenderMalt/readme.md