fix(hair): typo in code

[jiayev]

This pull request corrects a calculation in both the hair and PBR shader code to ensure the view vector is projected correctly onto the tangent plane. The change updates the computation of the projected view vector, which should improve the accuracy of specular reflection calculations.

Shader math correction:

• In both features/Hair Specular/Shaders/Hair/Hair.hlsli (namespace Hair) and package/Shaders/Common/PBR.hlsli (namespace PBR), the calculation of the projected view vector Vp was changed from V - NdotL * N to V - NdotV * N. This ensures the view vector is projected using its own normal component rather than the light's, fixing a subtle bug in the specular reflection math. [1] [2]

Summary by CodeRabbit

• New Features

  • Improved hair rendering realism with refined lighting response in the Marschner model.

• Bug Fixes

  • Corrected the hair shading calculation to use a view-based projection, improving highlight placement and color fidelity.
  • Reduces shading artifacts and enhances consistency across different viewing and lighting angles.

• Chores

  • Internal shader adjustments with no changes to public APIs.

[coderabbitai]

Caution

Review failed

The pull request is closed.

Walkthrough

Updated Marschner hair shading math by changing the Vp projection from V − (N·L)N to V − (N·V)N in two shader files. This affects cosPhi and downstream BRDF terms in the Marschner diffuse/specular computations. No public interfaces were changed.

Changes

Cohort / File(s)	Summary
Marschner Vp projection update features/Hair Specular/Shaders/Hair/Hair.hlsli, package/Shaders/Common/PBR.hlsli	Replace Vp computation: from V − (N·L)N to V − (N·V)N, altering cosPhi via dot(Lp, Vp) and impacting subsequent Marschner BRDF terms (Mp, Np, Fp, etc.). No signature changes.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• feat(hair): hair self-shadow, finished marschner model and hair flow map support #1225 — Adjusts Marschner Vp projection affecting cosPhi and downstream BRDF terms in the same shading path.
• refactor: move BRDFs to separate file #1161 — Modifies GetHairDiffuseColorMarschner in PBR.hlsli, overlapping with the Marschner hair diffuse computations changed here.
• refactor(hair): enhance hair specular visual, change default hair settings and feature info #1203 — Updates Marschner BRDF logic, including Vp/projection handling related to cosPhi.

Suggested reviewers

• alandtse
• doodlum

Poem

A whisk of light on silky threads, I hop and tweak the gleam—
Vp now follows N·V, a subtle shining theme.
CosPhi pivots, highlights sing, along each radiant strand,
I thump the ground—small change, bright land!
Hare today, hair to stay—BRDFs in hand. 🐇✨

✨ Finishing touches

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment

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📥 Commits

Reviewing files that changed from the base of the PR and between cf9c400 and c4f75c9.

📒 Files selected for processing (2)

• features/Hair Specular/Shaders/Hair/Hair.hlsli (1 hunks)
• package/Shaders/Common/PBR.hlsli (1 hunks)

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[github-actions]

Using provided base ref: cf9c400
Using base ref: cf9c400
Base commit date: 2025-09-23T17:08:54+01:00 (Tuesday, September 23, 2025 05:08 PM)
No actionable suggestions for changed features.

[github-actions]

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