fix: fog flicker in unified water

[davo0411]

fixes flicker, also results in better final fog colour with less "glow"
before/after

Summary by CodeRabbit

• Bug Fixes
  • Improved water fog rendering by updating fog distance calculations for more accurate visual representation.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Modified the fog distance calculation in a water shader by replacing the W-component of world position with the 3D Euclidean distance magnitude. This changes how fog intensity is evaluated based on camera distance rather than a transformed scalar value.

Changes

Cohort / File(s)	Summary
Water Shader Fog Calculation package/Shaders/Water.hlsl	Replaced input.WPosition.w with length(input.WPosition.xyz) in fog distance factor computation (2 locations), switching from W-component scalar to 3D magnitude-based distance evaluation

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested reviewers

• jiayev
• doodlum

Poem

🐰 A shimmer of water so blue and so bright,
Distance now measured by true spatial sight,
No W-component shall cloud up this stream,
Length-based fog paints a realistic dream,
Two tiny tweaks make the water pristine! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and clearly describes the main change: fixing fog flicker in unified water, which matches the primary objective of the pull request.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[github-actions]

Using provided base ref: 50e2929
Using base ref: 50e2929
Base commit date: 2026-01-25T17:17:09-08:00 (Sunday, January 25, 2026 05:17 PM)
No actionable suggestions for changed features.

[github-actions]

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

[alandtse]

What's the reasoning for the changes? Are we changing the algorithm significantly? In other words why are the parameters used before being dropped?

[davo0411]

What's the reasoning for the changes? Are we changing the algorithm significantly? In other words why are the parameters used before being dropped?

the old input.WPosition.w was distance calculated per-vertex then interpolated, which broke with unified water.
length(input.WPosition.xyz) recalculates distance per-pixel from the interpolated position which is stable for uw

all the other fog params are identical (FogParam.x/y, FogFarColor.w, FresnelRI.y).
The fog looks a bit different because length(lerp(A,B)) is not the same as lerp(length(A), length(B)), but that's actually more accurate and it doesn't flicker anymore.

[alandtse]

Please also consider the commit message.
fix(unified water): fog flicker