fix(ISHDR): guard contrastedColorModified against zero avgValue

[mvanhorn]

Summary

Guard the contrast modification path in ISHDR.hlsl against division by zero when avgValue.x is 0.

Why this matters

Line 178 divides tintedColor by avgValue.x without checking for zero. When avgValue.x is 0, the division produces INF, which propagates through pow() and contaminates outputColor. The earlier exposure path (lines ~168-169) already guards this same value with if (avgValue.x != 0), but the contrast modification skips the check. Flagged by @alandtse during review of PR #2103.

Changes

package/Shaders/ISHDR.hlsl line 178: Replace raw avgValue.x with max(avgValue.x, 1e-6) (epsilon clamp). Both the divisor and the multiplier use the clamped value so the result stays mathematically consistent.

Testing

HLSL static analysis. The change is a single float substitution with no control flow changes.

Fixes #2104

This contribution was developed with AI assistance (Claude Code).

Summary by CodeRabbit

• Bug Fixes
  • Prevented rare division-by-zero that could produce extreme or unstable shadow contrast in certain lighting, improving rendering stability.
  • Ensured more consistent shadow-contrast behavior in low-intensity/near-zero average lighting scenarios.

[coderabbitai]

📝 Walkthrough

Walkthrough

The shader package/Shaders/ISHDR.hlsl now includes Common/Math.hlsli and replaces direct uses of avgValue.x in the shadow-contrast computation with a clamped safeAvgValue = max(avgValue.x, EPSILON_DIVISION), preventing division-by-zero/near-zero in contrast math.

Changes

Cohort / File(s)	Summary
Shader Safety & Includes package/Shaders/ISHDR.hlsl, Common/Math.hlsli	Added Common/Math.hlsli include and replaced direct avgValue.x usage with safeAvgValue = max(avgValue.x, EPSILON_DIVISION) in the shadow-contrast calculation; used safeAvgValue for divisions, scaling, and exponentiation inputs to avoid division-by-zero/near-zero issues.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related issues

• Standardize epsilon values with domain-specific constants in HLSL shaders #1227: Standardizes use of EPSILON_DIVISION in ISHDR.hlsl to guard divisions — aligns with this change.

Possibly related PRs

• chore: improve contrast function #1470: Introduced the shadow-contrast path in ISHDR.hlsl that this PR hardens against division-by-zero.
• refactor(epsilon constants): ♻️ standardize epsilon constants in HLSL shaders (#1227) #1992: Added Common/Math.hlsli and used EPSILON_DIVISION for numerical safety in shader code.
• build: fix warnings #1505: Also modified shadow/contrast normalization/exponentiation in ISHDR.hlsl (related code area).

Suggested reviewers

• alandtse
• doodlum

Poem

🐰 In shader fields where shadows play,

A tiny zero crept one day.
I clamped it safe, with EPSILON fine,
Now contrast hums along the line.
Hooray — no NaNs, the pixels shine! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: guarding the contrast modification against division by zero when avgValue is zero.
Linked Issues check	✅ Passed	The PR implements all coding requirements from issue #2104: replaces avgValue.x with a clamped safeAvgValue in the contrastedColorModified computation and uses EPSILON_DIVISION constant for consistency.
Out of Scope Changes check	✅ Passed	The changes are narrowly scoped to the stated objective: adding a safe average value computation and including Math.hlsli for EPSILON_DIVISION, with no extraneous modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[github-actions]

No actionable suggestions for changed features.

[github-actions]

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

[alandtse]

Minor correction to avoid a hardcoded magic number.

[coderabbitai]

🧹 Nitpick comments (1)

package/Shaders/ISHDR.hlsl (1)

1-1: Optional: tighten PR title to match repo convention limits.

Suggested title: fix(ishdr): guard contrast path for zero avg value
As per coding guidelines, commit titles should follow type(scope): description, use lowercase description, and stay within 50 characters.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@package/Shaders/ISHDR.hlsl` at line 1, Update the PR title to follow the repo
convention "type(scope): description" in lowercase and within ~50 characters;
replace the current title with the suggested one "fix(ishdr): guard contrast
path for zero avg value" so it matches the required format and succinctly
describes the change (scope: ISHDR).

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@package/Shaders/ISHDR.hlsl`:
- Line 1: Update the PR title to follow the repo convention "type(scope):
description" in lowercase and within ~50 characters; replace the current title
with the suggested one "fix(ishdr): guard contrast path for zero avg value" so
it matches the required format and succinctly describes the change (scope:
ISHDR).

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 7307c682-e4a1-431d-8c1d-931e48ce8e82

📥 Commits

Reviewing files that changed from the base of the PR and between 6dc9262 and a13eab5.

📒 Files selected for processing (1)

• package/Shaders/ISHDR.hlsl