feat(color): add ktsu.Semantics.Color (linear-RGB color science)#104
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Cross-task netstandard2.0 fixes in Color.Conversions.cs and Semantics.Color.csproj: - Replace byte.Parse(AsSpan) with Convert.ToByte(Substring, 16) (no CA1846, works on all TFMs) - Add System.Numerics.Vectors package reference for netstandard2.0 (Vector3/Vector4) - Add System.Numerics.Vectors version to Directory.Packages.props All 5 TFMs (net10.0, net9.0, net8.0, netstandard2.1, netstandard2.0) build clean; 39 Color tests pass.
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Summary
Adds
ktsu.Semantics.Color— a new dependency-light color-science package (multi-targetnet10/9/8+netstandard2.0/2.1), the first shippable unit of the color consolidation described indocs/roadmap-semantic-domains.md.The canonical type is
readonly partial record struct Color(double R, double G, double B, double A)storing linear RGB + straight alpha. Around it:Srgb,Hsl,Hsv,Oklab,Oklch; hex (#RGB/#RRGGBB/#RRGGBBAA) and 8-bit byte parsing (sRGB-interpreted).RelativeLuminance,ContrastRatio,AccessibilityLevel,AdjustForContrast.DistanceTo(Oklab),MixOklab,Lerp,Gradient.NamedColors(CSS/X11 subset) with case-insensitive lookup.The correctness fix
The existing ThemeProvider/ImGuiStyler color code fed sRGB values straight into the Oklab matrix and WCAG luminance (the structs were labelled "linear" but never gamma-decoded). This package confines the gamma transfer to the single
Srgb↔Colorboundary, so perceptual math always runs on linear values and "feed sRGB into Oklab" is unrepresentable through the API.hex → linear → sRGBround-trips to identity (pinned byGammaRegressionTests).ToSrgbVector4()(never a raw linear cast).Scope
Core package only. The
ktsu.ImGui.Coloradapter (ToImColor/ToImGuiVector4/ImGuiVector4, ImGuiApp repo) and thektsu.ThemeProvider/ktsu.ImGui.Stylermigrations are intentionally separate follow-on PRs in their own repos, once this publishes. Spec:docs/superpowers/specs/2026-06-29-semantics-color-design.md; plan:docs/superpowers/plans/2026-06-29-semantics-color.md.Verification
net10.0,net9.0,net8.0,netstandard2.1,netstandard2.0.netstandard2.0needs a conditionalSystem.Numerics.Vectorsreference (forVector3/4) and a sign-awareCbrt(noMath.Cbrt).Notes
1e-6, not1e-9: the published Ottosson forward/inverse matrices are independently-rounded and don't invert to exact identity (~1e-7). Non-Oklab conversions round-trip to ~1e-9.Follow-ups (non-blocking)
AdjustForContrasttest (lighten branch); gradient out-of-gamut doc/clamp decision; cacheNamedColorsas static readonly; broaden a few R-only test asserts.ktsu.ImGui.Colorlands: a cross-package smoke test thatToSrgbVector4is what reaches ImGui.🤖 Generated with Claude Code