Day 13 told you what broken PBR looks like. This post explains why the rules exist in the first place.
PBR-Compliant vs PBR-Using
When authoring shaders and materials, a PBR ruleset ensures that whatever goes into the shader is read out correctly by the lighting system. But the system, the BRDF shading model, also needs to be physically based. Otherwise whatever energy a material carries could be assigned or read out incorrectly, even with the right shader in place.
This distinction is crucial: having a PBR shader doesn’t guarantee physically accurate results. The artwork it processes must adhere to the same rules. A metalness value of 0.5 applied to a physically based BRDF still produces an impossible result, the shader can’t rectify the texture’s error. This is the framework for everything that follows. The sections below delve into the implications, beginning with the physics that necessitates these rules.
Energy Conservation
The unifying principle behind all PBR rules: a surface cannot reflect more light than it receives. Diffuse and specular are complementary, as one goes up, the other comes down. This constraint is what makes PBR self-consistent and is the reason the albedo range, metalness binary, and F0 values are what they are.
Whenever a material receives light and scatters it, energy is lost. A bright material like snow scatters more light than a dark material like charcoal, not further, but more of it. This doesn't just affect the material itself; it propagates through the scene via GI light bounces, which is why a non-conserving material can throw off the entire lighting environment around it.
F0 - Reflectance at Normal Incidence
A surface is the interface between the surrounding medium (typically air, with a refractive index of approximately 1.0) and the object's substance. When light hits that interface, the Fresnel equations describe how it splits: some reflects, the rest refracts and enters the material. The proportion that reflects is the Fresnel reflectance F, and it varies with the angle of incidence.
At normal incidence (light arriving perpendicular to the surface at θ = 0°) reflectance reaches its minimum. This baseline value is F0, and it's a fixed property of the material. What makes it useful is how predictable it is: almost every dielectric lands near 0.04 (4%), with a realistic range of roughly 2–6%. Water, plastic, skin, wood, glass: visually very different materials that all sit in roughly the same place. Their differences come from their diffuse response, not their specular.
As the angle increases toward grazing (θ → 90°), F climbs toward 1.0 for all materials and all frequencies. That's the physics behind the edge brightening visible on almost every surface; at grazing angles, everything becomes a mirror.
Metals are the exception. Their F0 is high (typically 0.5 to 1.0) and tinted across the visible spectrum. A gold surface reflects red and green more than blue, giving its reflections their characteristic color. For metals, F0 is the specular color, which is why a PBR workflow stores that color in the albedo map for metallic materials.
This is the physics behind the metalness binary rule. A value of 0.5 describes no real substance; real materials are either dielectrics (F0 ≈ 0.04, uncolored) or metals (F0 high, tinted). The gap between those two ranges is where physically impossible materials live.
The Cook-Torrance BRDF
The Cook-Torrance model splits the surface response into diffuse and specular. The diffuse term (typically a Lambertian or Burley approximation) handles light that enters the material, scatters internally, and exits in a random direction. The specular term handles light that reflects at the surface without entering, and this is where the physical complexity lives.
The specular lobe is the product of three terms.
The Normal Distribution Function (GGX is the standard implementation) describes the statistical distribution of microfacet orientations: microscopic surface irregularities that each reflect light like tiny mirrors. Roughness is the parameter feeding this distribution. A low roughness concentrates the NDF into a tight peak, producing a sharp highlight. A high roughness spreads it wide. This is why a flat grey roughness map reads as artificial: it collapses the NDF to a single value across the entire surface, something no real material does.
The Geometry/Masking term (Schlick's approximation is common) accounts for microfacets that shadow or obscure each other at grazing angles. Without it, the specular response would be unrealistically bright when a surface is viewed or lit obliquely.
The Fresnel term ties directly to F0. Reflectance isn't constant; it scales from F0 at normal incidence up toward 1.0 at grazing angles. The Fresnel term drives this within the shader, so a surface automatically brightens at glancing angles regardless of material type. At perpendicular incidence you see F0; at grazing you see full reflectance. This is what makes PBR materials behave correctly across changing view angles without manual adjustment.
Together, the three terms produce a specular lobe that is energy-conserving by construction: it distributes incoming light, it doesn't create any.
In-Engine Validation
The albedo range and the light test are covered in Day 13 - ⚡️ Quick - How to Spot Broken Materials, this section focuses on what those checks miss, and where the BRDF debug views pick up the slack.
Having ways to do custom Metal or Roughness override debug view can help isolate the specular response and expose metalness values that aren't binary. What a metalness override at 0.5 actually looks like vs 0 or 1, the wrong specular tint is readable immediately.
Roughness override: how a flat grey roughness map collapses the NDF to a single value and makes that obvious. Why these views catch compliance breaks that the albedo overlay doesn't see, you can have a correct albedo and still have a broken specular response. Also: the difference between checking under a neutral lighting vs a directional light for catching roughness issues specifically.
Practical Takeaway
The F0 values are the most practically useful thing to take from this post. Every dielectric sits near 0.04, you don't need to guess or measure. If you're authoring plastic, stone, or skin, the specular sits around 4% and the albedo carries the diffuse color. For metals, F0 is the specular color, so the characteristic tint goes into the albedo map and metalness goes to 1.0: not 0.9, not 0.5, not somewhere in between.
Violating energy conservation has consequences that extend beyond the material itself. A surface that reflects more light than it receives injects extra energy into every GI bounce in the scene, brightening indirect light in ways that are hard to track down. The error doesn't stay local.
The rules in PBR aren't arbitrary restrictions. They're the conditions under which the lighting math, the BRDF, the IBL precomputation, the tone mapping downstream, all produce predictable results. Stay within the ranges and the system behaves correctly. Step outside them and the shader can't correct for it.
© 2026 Stefan Groenewoud - All views are my own, not those of my employer.