fix: terrain variation height blend fix

[davo0411]

last commit broke height blend operator on PBR landscapes. This restores the backup luminence blend calculations.

Summary by CodeRabbit

• Improvements
  • Enhanced terrain texture blending for more natural transitions by implementing full height-based blending with improved contrast and influence controls.
  • Terrain visuals now benefit from smoother and more detailed texture transitions, especially in areas with varied elevation.

[coderabbitai]

Walkthrough

The blending logic in the StochasticEffect function within TerrainVariation.hlsli was reworked to implement a comprehensive height-based blending approach for terrain textures. This includes new contrast adjustment for blend weights, a unified luminance/alpha-based height calculation, and removal of mip-level-dependent conditional branches.

Changes

Cohort / File(s)	Change Summary
Terrain Variation Shader Logic features/Terrain Variation/Shaders/TerrainVariation/TerrainVariation.hlsli	Refactored StochasticEffect to use full height-based blending, contrast-adjusted weights, unified luminance/alpha height calculation, and simplified normalization logic. Removed mip-level-based conditional branches.

Sequence Diagram(s)

sequenceDiagram
    participant CPU
    participant GPU
    participant StochasticEffect

    CPU->>GPU: Dispatch terrain rendering
    GPU->>StochasticEffect: Call with texture samples, weights, mip level
    StochasticEffect->>StochasticEffect: Compute luminance/alpha-based heights
    StochasticEffect->>StochasticEffect: Adjust blend weights with contrast
    StochasticEffect->>StochasticEffect: Apply height influence to weights
    StochasticEffect->>StochasticEffect: Normalize weights
    StochasticEffect->>GPU: Output blended terrain color

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Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• perf(Terrain Variation): feature rework, miplevel & sample optimisation #1224: Refactors and optimizes the same StochasticEffect function and terrain height blending logic.
• fix: terrain variation broken parallax shadows & incorrect mipmapping. #1124: Modifies texture sampling in an early exit path of StochasticEffect, related to blending logic changes.
• perf(terrain variation): custom mip levels & parallax function #1246: Alters StochasticEffect focusing on mip level calculation and blending weight simplification.

Suggested reviewers

• doodlum

Poem

A rabbit hopped through code so bright,
Blending heights with newfound might.
Contrast sharpened, colors fused,
Luminance and alpha, wisely used.
No more branches—logic clean!
The terrain now blends serene.
🐇✨

Note

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📜 Recent review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 6f18aee and 4034729.

📒 Files selected for processing (1)

• features/Terrain Variation/Shaders/TerrainVariation/TerrainVariation.hlsli (1 hunks)

🧰 Additional context used

🧠 Learnings (2)

📓 Common learnings

Learnt from: alandtse
PR: doodlum/skyrim-community-shaders#0
File: :0-0
Timestamp: 2025-06-24T07:17:36.604Z
Learning: When reviewing PRs, always clarify the scope if there are multiple related features or dependencies. WeatherPicker was a separate PR that was already merged, while this PR focuses specifically on WetnessEffects climate preset system enhancements.

Learnt from: alandtse
PR: doodlum/skyrim-community-shaders#0
File: :0-0
Timestamp: 2025-07-05T05:20:45.823Z
Learning: In the skyrim-community-shaders repository, file deletion error handling improvements that replace existence checks and try-catch blocks with std::filesystem::remove error-code-based approaches are considered bug fixes rather than refactoring, as they address inadequate error handling and misleading log messages.

Learnt from: jiayev
PR: doodlum/skyrim-community-shaders#0
File: :0-0
Timestamp: 2025-07-18T15:21:03.641Z
Learning: In the skyrim-community-shaders rendering pipeline, materials with alpha < 1 or alpha blending enabled are rendered in non-deferred mode rather than deferred mode. This means issues with dynamic cubemaps on transparent materials are actually non-deferred rendering issues.

Learnt from: alandtse
PR: doodlum/skyrim-community-shaders#577
File: features/Wetness Effects/Shaders/WetnessEffects/WetnessEffects.hlsli:57-61
Timestamp: 2025-06-17T05:40:22.785Z
Learning: Default parameter values are supported in the HLSL compiler used by the skyrim-community-shaders project, contrary to standard HLSL (FXC/DXC) limitations.

features/Terrain Variation/Shaders/TerrainVariation/TerrainVariation.hlsli (1)

Learnt from: alandtse
PR: #577
File: features/Wetness Effects/Shaders/WetnessEffects/WetnessEffects.hlsli:57-61
Timestamp: 2025-06-17T05:40:22.785Z
Learning: Default parameter values are supported in the HLSL compiler used by the skyrim-community-shaders project, contrary to standard HLSL (FXC/DXC) limitations.

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (3)

• GitHub Check: Validate shader compilation (VR, .github/configs/shader-validation-vr.yaml)
• GitHub Check: Validate shader compilation (Flatrim, .github/configs/shader-validation.yaml)
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🔇 Additional comments (1)

features/Terrain Variation/Shaders/TerrainVariation/TerrainVariation.hlsli (1)

151-170: LGTM! Solid implementation of height-based terrain blending.

This implementation correctly restores the luminance-based height calculations as mentioned in the PR objectives. The approach cleverly combines luminance-derived heights from RGB channels with alpha channel data when available, using a proper mask-based selection. The mathematical formulation is sound:

• Contrast adjustment using pow(saturate(offsets.weights), contrastFactor)
• Proper sRGB luminance weights (0.2126, 0.7152, 0.0722)
• Smart alpha/luminance blending with step(0.001, alphaValues) mask
• Correct final weight normalization and height influence integration

The fix addresses the broken height blend operator on PBR landscapes mentioned in the PR description.

✨ Finishing Touches

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[github-actions]

Using provided base ref: 6f18aee
Using base ref: 6f18aee
Base commit date: 2025-07-27T15:38:04+01:00 (Sunday, July 27, 2025 03:38 PM)
No actionable suggestions for changed features.

[github-actions]

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