Energy conservation. I could follow the rules. I couldn't explain why breaking them breaks everything downstream.
Where I stood on Day 1
Lighting has never been my strongest area. I know how materials respond to it: how roughness, metalness, and F0 determine what gets reflected where. But I hadn't dived into the rendering side properly in a long time. I wrote custom viewport shaders, specific conversion nodes, etc. I'd dabbled with concepts like energy conservation, Fresnel, masking-shadowing. I hadn't gotten my hands into the actual math for over 3-5 years now, and only slightly understood why the rules are the rules.
That gap is part of why the rendering block of this series was the hardest to write. It's also where I learned the most.
What the concept actually connects
The principle: a surface cannot reflect more light than it receives. Diffuse and specular are complementary. As specular rises, diffuse falls. That constraint isn't arbitrary. It's the condition under which every downstream system produces predictable results.
A material that violates energy conservation injects extra energy into every bounce. Indirect light brightens in ways that are hard to trace. The error doesn't stay in the material slot. It propagates through the whole lighting chain.
This is why the albedo range exists. Why metalness is binary. Why F0 for dielectrics sits around 0.04. The rules are load-bearing. They're the conditions under which the lighting math works correctly, from the material outward through global illumination.
Where it clicked
Deeply investigating IBL and Energy Conservation made this concrete. The split-sum approximation precomputes the specular response under a fixed environment, and that derivation is built on the same energy conservation and Fresnel assumptions that PBR materials follow. The two systems are designed together.
A non-compliant material fed into a physically correct IBL produces incorrect output even if the environment capture was perfect. Writing Day 21 - 🔬 Deep dive - PBR Compliant Work and Day 22 - 🔬 Deep dive - How IBL Works in sequence made that connection visible. Trace the energy from the material through the BRDF through the precomputed LUT to the final pixel and you can see how each step depends on the previous one being correct.
Practical takeaway
Knowing how light and materials interact at this level isn't just academic. It explains why certain lighting conditions work better for specific material types, how the BRDF constrains what's artistically possible and where limitations come from. Understanding the rules tells you which ones can be bent and which ones break the whole chain when you push past them.
© 2026 Stefan Groenewoud. All views are my own, not those of my employer.