refactor(pbr): clear up semantics

[jiayev]

This pull request refactors the PBR (Physically Based Rendering) shader code to standardize terminology and clarify the use of Fresnel reflectance (F0) across the codebase. The main focus is on replacing ambiguous specularColor parameters with the more physically accurate F0, updating function and variable names, and ensuring consistency in the implementation of specular BRDFs. This improves code clarity, physical correctness, and maintainability, while updating all usages and tests to match the new conventions.

Refactoring and Terminology Standardization:

• Replaced all instances of the specularColor parameter with F0 (Fresnel reflectance at normal incidence) in specular BRDF functions and throughout the code, aligning with standard PBR terminology. [1] [2] [3] [4] [5]
• Renamed core functions for clarity: GetSpecularDirectLightMultiplierMicrofacet → SpecularMicrofacet and GetSpecularDirectLightMultiplierMicroflakes → SpecularMicroflakes. Updated all usages accordingly. [1] [2] [3] [4] [5] [6] [7] [8]

Physical Correctness and Consistency:

• Updated the Schlick Fresnel implementation and all its usages to consistently use F0 as the input, improving physical accuracy in specular calculations. [1] [2] [3] [4]
• Improved comments and parameter documentation to reflect the new terminology and clarify the physical meaning of each parameter.

Lighting and Energy Conservation:

• Clarified separation between physical Fresnel and effective contribution in wetness and coat lighting calculations, and ensured energy conservation by consistently attenuating diffuse/specular by the appropriate Fresnel terms. [1] [2] [3] [4] [5] [6]

Test Updates:

• Updated all PBR test code to use the new function names and F0 terminology, ensuring tests reflect the refactored code and continue to verify physical constraints and expected behavior. [1] [2] [3] [4] [5]

These changes modernize the shader codebase, make it easier to reason about physical properties, and reduce ambiguity in future development.

Summary by CodeRabbit

• Refactor

  • Improved hair specular lighting calculations with refined Fresnel computations
  • Enhanced wetness surface effects through optimized lighting evaluation
  • Streamlined physically-based rendering shader structure and helper functions
  • Consolidated and renamed specular computation utilities

• Tests

  • Updated shader validation tests for rendering accuracy

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR refactors Fresnel handling across the shader codebase by renaming parameter semantics from specularColor to F0 in function signatures, shortening function names (e.g., GetSpecularDirectLightMultiplier* to Specular*), and updating intermediate variable names (e.g., specular to Fr, coatSpecular to coatFr) throughout BRDF, PBR, and lighting evaluation functions.

Changes

Cohort / File(s)	Summary
BRDF Fresnel Function package/Shaders/Common/BRDF.hlsli	Updated F_Schlick(float3 specularColor, float VdotH) signature to accept F0 instead, with return expression adjusted from Fc + (1 - Fc) * specularColor to Fc + (1 - Fc) * F0.
Hair Specular features/Hair Specular/Shaders/Hair/Hair.hlsli	Refactored Hair::D_Marschner to replace specularColor parameter with local float3 F0 = HairF0(), passing F0 to subsequent BRDF::F_Schlick calls for R, TT, and TRT lobes.
PBR Microfacet Helpers package/Shaders/Common/PBRMath.hlsli	Renamed GetSpecularDirectLightMultiplierMicrofacet(…, specularColor, …) to SpecularMicrofacet(…, F0, …) and GetSpecularDirectLightMultiplierMicroflakes(…, specularColor, …) to SpecularMicroflakes(…, F0, …), updating Fresnel computation to use F0 parameter.
PBR Direct & Indirect Lighting package/Shaders/Common/PBR.hlsli	Renamed GetSpecularDirectLightMultiplierMicrofacetWithGlint(…, specularColor, …) to SpecularMicrofacetWithGlint(…, F0, …); renamed specular accumulators from specular/coatSpecular/fuzzSpecular to Fr/coatFr variants; updated hair Fresnel to compute F0 = IORToF0(hairIOR) and streamlined diffuse energy conservation in GetIndirectLobeWeights(…).
Wetness Lighting Computation package/Shaders/Common/LightingEval.hlsli	Introduced intermediate wetnessF = F * wetnessStrength to consistently apply strength-weighted Fresnel to both specular and attenuation computations, replacing in-place scaling of F.
PBR Unit Tests package/Shaders/Tests/TestPBR.hlsl	Updated test invocations to use renamed PBR::SpecularMicrofacet(…) and PBR::SpecularMicroflakes(…) functions; renamed test parameter from specularColor to F0 and adjusted all assertions accordingly.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• refactor(hair): enhance hair specular visual, change default hair settings and feature info #1203: Modifies Hair.hlsli Marschner scattering to switch Fresnel input from specularColor to F0 using BRDF::F_Schlick, directly overlapping with this PR's hair specular refactoring.
• refactor: restructure lighting #1633: Refactors Fresnel handling by renaming function signatures from specularColor to F0 across PBR/BRDF/PBRMath/LightingEval, with overlapping changes to BRDF::F_Schlick and helper functions.
• refactor: move BRDFs to separate file #1161: Touches BRDF Schlick Fresnel functions and their microfacet call sites, directly related to the Fresnel parameter semantic changes throughout this PR.

Suggested reviewers

• alandtse
• doodlum
• davo0411

Poem

🐰 With whiskers twitching, I hop with glee,
F0 now flows through shader spree!
From specularColor to brighter names,
Our code hops cleaner, light reframes. ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title accurately summarizes the main objective of the changeset: refactoring PBR shader code to clarify Fresnel reflectance semantics by renaming variables and functions.

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✨ Finishing Touches

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• Create PR with unit tests

📝 Coding Plan

• Generate coding plan for human review comments

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[github-actions]

✅ A pre-release build is available for this PR:
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