1.To learn the Differential Geometry and try to treat the Steradian as the 2-Manifold which reduces the dimensions from 3(R^3) to 2(S^2) and simplifies the integral over the sphere surface when one calculates the lighting.
Try to understand the following papers from the perspective of the 2-Manifold:
An Introduction to Manifolds / Example 23.11 (Integral over a sphere)
[Tu 2011] Loring Tu. "An Introduction to Manifolds, Second Edition." Springer 2011.
An Introduction to Manifolds / Example 23.11 (Integral over a sphere)
[Tu 2011] Loring Tu. "An Introduction to Manifolds, Second Edition." Springer 2011.
To my pleasure, someone seems to have the similar idea which I find on the Google Scholar:
[Herholz 2018] Sebastian Herholz, Oskar Elek, Jens Schindel, Jaroslav krivanek, Hendrik Lensch. "A Unified Manifold Framework for Efficient BRDF Sampling based on Parametric Mixture Models." EGSR 2018.
The traditional Euclidean-Space method may be replaced by the efficient 2-Manifold method in the next few years.
Try to understand the following papers from the perspective of the 2-Manifold: