fix white channel for "Rainbow" effect

[chschu]

This PR fixes an issue when using the "Rainbow" effect on RGBW LEDs:

• The CRGBW instance allocated on the stack in Segment::color_wheel is not initialized, so all its color values may (and will) contain garbage.
• The subsequent assignment from a CHSV32 instance calls hsv2rgb_rainbow, which doesn't clear the white value either.
• So even if the white value of the primary color is zero, the "Rainbow" effect produces RGBW colors with a non-zero white value.
• The final OR'ing with the primary color's white value in Segment::color_wheel produces (at least to a human eye) unexpected results.

The issue has been introduced in WLED v16.0.0 by the FastLED replacement in #4615.

The important part of the fix is to properly clear the white value in hsv2rgb_rainbow, because HSV only maps to RGB (not W). The line is already there, and I don't know why it had been commented out in the first place.

The change in Segment::color_wheel is merely cosmetic. Setting the white value produces the same result as OR'ing it to a (now guaranteed) zero value.

Summary by CodeRabbit

• Bug Fixes
  • Fixed white channel handling in color wheel calculations to properly derive from primary color values
  • Fixed white channel initialization in HSV to RGBW color space conversions to prevent uninitialized values

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 9a1a4a27-9c5c-4c2d-9103-4e8225a6a8c6

📥 Commits

Reviewing files that changed from the base of the PR and between 221964e and 105f427.

📒 Files selected for processing (2)

• wled00/FX_fcn.cpp
• wled00/src/dependencies/fastled_slim/fastled_slim.cpp

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Walkthrough

Two bug fixes for RGBW white channel handling: Segment::color_wheel() now assigns the white byte via a CRGBW struct member (rgb.w) instead of a bit-shift/OR expression, and hsv2rgb_rainbow explicitly zeroes rgbdata[3] in the isRGBW branch instead of leaving it as a commented-out/uninitialized value.

Changes

RGBW White Channel Fixes

Layer / File(s)	Summary
hsv2rgb_rainbow white channel zero-initialization wled00/src/dependencies/fastled_slim/fastled_slim.cpp	In the isRGBW branch, rgbdata[3] is now explicitly assigned 0, preventing the white channel from being uninitialized in RGBW HSV conversions.
color_wheel() CRGBW struct packing wled00/FX_fcn.cpp	color_wheel() populates a CRGBW temporary from CHSV32(...), assigns rgb.w from W(getCurrentColor(0)), and returns rgb.color32 — replacing the previous bit-shift/OR approach for combining the white byte.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

Possibly related PRs

• wled/WLED#3681: Modifies Segment::color_wheel() in wled00/FX_fcn.cpp to change how the white channel is sourced from the current segment color and packed into the returned color32.
• wled/WLED#4615: Touches both hsv2rgb_rainbow in fastled_slim.cpp and the CRGBW-based color_wheel() logic in FX_fcn.cpp, directly overlapping with both files changed here.

Suggested labels

bug

Suggested reviewers

• DedeHai
• TripleWhy

🚥 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 'fix white channel for "Rainbow" effect' accurately describes the main change—fixing a white channel rendering issue in the Rainbow effect on RGBW LEDs.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[DedeHai]

this was intentional, I can't recall why I commented it as it does not make sense to me now to split the two but do the same... maybe some conversion issue somewhere else

edit:
please revert this, I need to check what the proper way is, either remove the isRGBW or set it to zero

[chschu]

Done.

The other part of the PR fixes the issue as well, because now it simply overwrites the garbage value.

[DedeHai]

good call, thanks for uncovering this. we changed this function a few times over so something must have gotten lost somewhere along the way.