fix(color): use the sRGB exponent when linearising linear-raw files

[pluja]

srgb_to_linear in src-tauri/src/raw_processing.rs raises to the power of 3.0. The inverse sRGB transfer function uses 2.4:

C_linear = ((C_srgb + 0.055) / 1.055) ^ 2.4

sRGB is: a linear segment below 0.04045 and this power curve above it. The two have to meet at that threshold. With 2.4 they do. With 3.0 the curve jumps 76% at the join. The constants already in the function (0.055, 1.055, 12.92, 0.04045) come from the sRGB definition and only produce a continuous curve with exponent 2.4.

The rest of the codebase already uses 2.4. Same formula, same constants:

• src-tauri/src/image_loader.rs:60
• src-tauri/src/image_processing.rs:1215

Only raw_processing.rs differs.

This only affects files where is_linear_raw_format() is true (RawPhotometricInterpretation::LinearRaw) and apply_ungamma is set, so the gamma and gamma_skip_calib values of the linear_raw_mode setting. Bayer and X-Trans files never reach this branch.

Note: This was introduced in f00ea84, which added the linearisation step rather than tuning an existing one, so the exponent looks like a slip rather than a deliberate curve.

Type of Change

• Bug fix
• New feature
• Breaking change
• Performance improvement
• Code refactoring
• Documentation update
• UI/UX improvement
• Build/CI or Dependency update

Changes Made

One line in src-tauri/src/raw_processing.rs:

-        ((value + 0.055) / 1.055).powf(3.0)
+        ((value + 0.055) / 1.055).powf(2.4)

Screenshots/Videos

Not applicable, no UI change.

Testing

• These changes were tested locally by a human and confirmed to work.
• I haven't added any automated tests to the code because the codebase currently lacks a test suite.

cargo check --no-default-features --all-targets passes with no new warnings.

I could not verify this against a real LinearRaw file, since I do not have one. The change is a correction to a published constant rather than a behavioural guess, but someone with a LinearRaw source should confirm it before merging.

Test Configuration:

• OS: Arch Linux
• Hardware: N/A

Checklist

• My code follows the project's code style
• I haven't added unnecessary AI-generated code comments
• My changes generate no new warnings or errors

AI Disclaimer:

Please state the involvement of AI in this PR:

• This PR is created by an AI agent
• This PR is mostly AI-generated but edited/merged together by a human
• This PR was handwritten with AI assistance (spell check, logic suggestions, error resolving)
• This PR contains only blood, sweat, and coffee (AI-free)

Claude found the incorrect exponent and made the change. I reviewed it, verified it builds, and am submitting it.