Thinking about GI again. Found a paper online for understanding the core of global illumination, light scattering:

Lars Øgendal. Light Scattering Demystified Theory and Practice. https://www.nbi.dk/~ogendal/personal/lho/lightscattering_theory_and_practice.pdf

  • quantitative light scattering computation: use complex number; simply and elegantly; page 28, 2.6 Scattering from one small, composite object