chore: improve contrast function

[doodlum]

Summary by CodeRabbit

• New Features
  • Enhanced HDR tone mapping for a more cinematic look with improved contrast.
  • Better preservation of shadow detail and highlight control across varied scenes.
  • Smoother, more natural color transitions without requiring user setting changes.

[coderabbitai]

Walkthrough

Introduces a two-path contrast mapping in ISHDR.hlsl: computes contrastOriginal via lerp and contrastShadows via a pow-based curve, then selects between them per-channel based on a comparison with hdrColor. Function signatures and external interfaces remain unchanged.

Changes

Cohort / File(s)	Summary
HDR Contrast Mapping Update package/Shaders/ISHDR.hlsl	Replaces single-line HDR remap with dual-path contrast: contrastOriginal = lerp(avgValue.x, hdrColor, Cinematic.z) and contrastShadows = pow(abs(hdrColor)/avgValue.x, Cinematic.z)*avgValue.x*sign(hdrColor), choosing between them with a ternary comparison against hdrColor. No public API changes.

Sequence Diagram(s)

sequenceDiagram
  autonumber
  participant Scene as Scene Input
  participant ISHDR as ISHDR.hlsl
  participant Output as Color Out

  Scene->>ISHDR: hdrColor, avgValue.x, Cinematic.z
  rect rgba(200,230,255,0.25)
    note over ISHDR: Compute two contrast paths
    ISHDR->>ISHDR: contrastOriginal = lerp(avgValue.x, hdrColor, Cinematic.z)
    ISHDR->>ISHDR: contrastShadows = pow(abs(hdrColor)/avgValue.x, Cinematic.z)\n * avgValue.x * sign(hdrColor)
  end
  alt contrastOriginal < hdrColor
    ISHDR->>ISHDR: hdrColor = contrastShadows
  else
    ISHDR->>ISHDR: hdrColor = contrastOriginal
  end
  ISHDR-->>Output: hdrColor
  note over Output: Public interface unchanged

Loading

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• feat: improved hdr post processing #1445 — Also modifies ISHDR.hlsl to add a Cinematic.z-driven pow-based contrast path with a ternary selection, mirroring the approach here.

Suggested reviewers

• jiayev

Poem

I tweaked the tones with gentle care,
Two paths for light, a bunny’s flair—
A pow for shadows, lerp for glow,
We pick the best, then let it flow.
Hop hop—contrast finds its art,
In pixels bright, a rabbit’s heart. 🐇✨

Pre-merge checks and finishing touches

✅ 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 "chore: improve contrast function" accurately and concisely reflects the main change in the diff — updates to the HDR contrast/remap logic in package/Shaders/ISHDR.hlsl introducing a two-path contrast strategy — and is clear enough for a teammate scanning history to understand the primary intent.
Docstring Coverage	✅ Passed	No functions found in the changes. Docstring coverage check skipped.

✨ Finishing touches

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch contrast-bettere

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📜 Recent review details

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Review profile: CHILL

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📥 Commits

Reviewing files that changed from the base of the PR and between 05f5c40 and 1030cea.

📒 Files selected for processing (1)

• package/Shaders/ISHDR.hlsl (1 hunks)

🧰 Additional context used

📓 Path-based instructions (1)

**/*.{cpp,cxx,cc,c,h,hpp,hxx,hlsl,hlsli,fx,fxh,py}

📄 CodeRabbit inference engine (.github/copilot-instructions.md)

Do not include TODO/FIXME placeholders; provide complete, working solutions

Files:

• package/Shaders/ISHDR.hlsl

🧠 Learnings (2)

📓 Common learnings

Learnt from: jiayev
PR: doodlum/skyrim-community-shaders#0
File: :0-0
Timestamp: 2025-08-03T18:37:19.690Z
Learning: ISReflectionsRayTracing.hlsl and ISWorldMap.hlsl in the skyrim-community-shaders repository are image-space post-processing shaders that perform color sampling and blending operations that need proper linear color space handling for the linear lighting system. ISReflectionsRayTracing handles screen-space reflections and should use conditional Color::IrradianceToLinear/Gamma conversions similar to ISCompositeLensFlareVolumetricLighting.hlsl. ISWorldMap performs 7x7 color accumulation that should be done in linear space similar to the pattern used in ISSAOComposite.hlsl.

📚 Learning: 2025-08-03T18:37:19.690Z

Learnt from: jiayev
PR: doodlum/skyrim-community-shaders#0
File: :0-0
Timestamp: 2025-08-03T18:37:19.690Z
Learning: ISReflectionsRayTracing.hlsl and ISWorldMap.hlsl in the skyrim-community-shaders repository are image-space post-processing shaders that perform color sampling and blending operations that need proper linear color space handling for the linear lighting system. ISReflectionsRayTracing handles screen-space reflections and should use conditional Color::IrradianceToLinear/Gamma conversions similar to ISCompositeLensFlareVolumetricLighting.hlsl. ISWorldMap performs 7x7 color accumulation that should be done in linear space similar to the pattern used in ISSAOComposite.hlsl.

Applied to files:

• package/Shaders/ISHDR.hlsl

⏰ 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)
• GitHub Check: Build plugin and addons

🔇 Additional comments (1)

package/Shaders/ISHDR.hlsl (1)

119-121: Fix component-wise selection and guard against divide-by-zero in contrast step

• Use per-channel selection (mask + lerp), explicit broadcast for avgValue, and add an epsilon to avoid division-by-zero.

-	float3 contrastOriginal = lerp(avgValue.x, hdrColor, Cinematic.z);
-	float3 contrastShadows = pow(abs(hdrColor) / avgValue.x, Cinematic.z) * avgValue.x * sign(hdrColor);
-	hdrColor = contrastOriginal < hdrColor ? contrastShadows : contrastOriginal;
+	// Use explicit broadcast, add epsilon for robustness, and select per-channel via mask
+	float  avg = max(avgValue.x, 1e-6);
+	float3 contrastOriginal = lerp(float3(avg, avg, avg), hdrColor, Cinematic.z);
+	float3 contrastShadows  = pow(abs(hdrColor) / avg, Cinematic.z) * avg * sign(hdrColor);
+	float3 mask = saturate(sign(hdrColor - contrastOriginal)); // 1 where hdrColor > contrastOriginal, else 0
+	hdrColor = lerp(contrastOriginal, contrastShadows, mask);

• Optional perf notes: use rcp(avg) instead of division, consider approximating pow if hot, and clamp Cinematic.z with max(Cinematic.z, 0.0) if negatives are possible.

Verification: compile-check failed in the provided environment — "dxc: command not found". Run the dxc compile-check in your local/CI environment to confirm.

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[github-actions]

Using provided base ref: 05f5c40
Using base ref: 05f5c40
Base commit date: 2025-09-13T01:06:55+01:00 (Saturday, September 13, 2025 01:06 AM)
No actionable suggestions for changed features.

[github-actions]

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