fix(core): compute contrast from unrounded values

[pbroom]

relativeLuminance() and contrastAPCA() measured colors through toRgb(),
i.e. 8-bit rounded, clipped sRGB: oklch(0.6 0.1 200) vs
oklch(0.95 0.02 90) gave WCAG 3.2520 where colorjs.io gives 3.2635, and
the APCA field was stepped (5 distinct values over 200 samples spanning
dL = 0.004). Contrast regions meanwhile used unclamped luminance for
WCAG, so points just inside an AA region could still fail
contrastRatio() >= 4.5.

• Contrast metrics now measure the unrounded sRGB channels clipped to
  the display gamut. Clipping (not gamut mapping) was chosen because it
  is what toRgb()/toHex() render, it is exact and equal to colorjs.io for
  every in-gamut color, it is O(1) with no dependence on a mapping
  method, and WCAG/APCA are defined on displayable [0, 1] channels.
• New optional ContrastOptions on relativeLuminance, contrastRatio,
  contrastAPCA, meetsAA and meetsAAA: gamut ('srgb' default, or
  'display-p3' to clip in P3 and measure the extended sRGB result) and
  precision ('exact' default, or '8bit' to quantize like hex-based
  tools; reproduces color-kit 0.1 results bit for bit).
• Contrast region fields (hybrid and legacy, WCAG and APCA) evaluate the
  public contrastRatio/contrastAPCA with the region's gamut, so region
  boundaries sit on the check's own boundary. The internal
  relativeLuminanceUnclamped/contrastRatioUnclamped are removed.
• toHsl/toHsv/toHct no longer round to 8-bit first (new
  conversion/unrounded.ts; toHct uses Material's CAM16 on the unrounded
  clipped color). rgbToHsl/rgbToHsv treat channel spreads <= 1e-10 as
  gray so float noise does not produce an arbitrary hue; 8-bit inputs
  are unaffected. This touches three one-line bodies in
  conversion/index.ts.
• 8-bit inputs: default results match the old values to float precision
  and pass/fail decisions are unchanged; '8bit' mode is bit-identical.
• Tests: colorjs.io reference for unrounded/out-of-gamut OKLCH pairs
  (vs the clipped color), fast-check property that contour vertices
  nudged 0.0005 L either side split across the public check (about half
  failed with 8-bit checks), APCA/WCAG continuity, P3 gamut option, and
  HCT/HSL parity with the 8-bit results.

Part of a stacked set of PRs (#204 → #221) from the color-kit assessment. Validated per branch with pnpm build and pnpm test; lint, format, typecheck, size, artifact, preprod and agents checks pass at the top of the stack.

🤖 Generated with Claude Code

[pbroom]

• refactor(react)!: remove UI components; ship state and plane hooks only #249
• docs(react): rebuild the React docs around the plane hooks #248
• feat(react): add plane renderer, geometry and adaptive-quality hooks #247
• feat(docs): syntax highlighting palette for code samples #246
• feat(docs): native API map at /api/map #245
• docs: add concept pages, guides and entry-point narratives #244
• feat(docs): live plane hero on home and the example system #243
• feat(docs): generated API reference from TypeDoc JSON #242
• refactor(core)!: derive plane query budgets from step counts and drop PlaneWithSense #241
• fix(react): render input aria-label once, on the inner input #240
• fix(driver): map absolute percents as range positions and keep tiny-step precision #239
• fix(core): uncap saturate chroma and resolve planeHue as an OKLCH hue #238
• fix(driver): reject unknown gamuts and align validation error types #237
• chore(docs): remove Sandpack, the shadcn kit and the legacy docs shell #236
• docs(core,driver,react): document the public API #235
• chore: pin control-kit to a commit with its own pnpm workspace #234
• docs: update implementation backlog for the contrast solver replacement #232
• refactor(react)!: remove ColorPlane's deprecated edge option and webgl renderer alias #231
• fix(driver): validate the gamut in getColorAreaFallbackPoint #230
• feat(driver,react): keep the previous hue when an achromatic color is entered #229
• refactor(core)!: replace the hybrid contrast solver with chroma-column tracing #228
• perf(core): trim what contrastRatio pulls in #227
• perf(core): pack the named-color table #226
• feat(core): point 'p3' users to 'display-p3' #225
• chore: ignore local .claude files in prettier #224
• refactor(core)!: retire the legacy contrast-region engine #223
• fix(core): keep hybrid contrast contours connected through folds #222
• chore: add changesets for the assessment stack #221
• docs: generated API reference and repo polish #220
• chore: adopt changesets for versioning and release notes #219
• chore: fix facade peers, licensing, and lint ignores #218
• refactor(react): reuse core contrast and transfer functions #217
• refactor(core)!: make the contrast-region engine explicit #216
• refactor(core)!: remove pre-launch aliases and trim the root export surface #215
• refactor!: name the Display P3 space 'display-p3' everywhere #214
• refactor(driver): share pointer drag throttling between slider and area #213
• feat(react): keyboard, ARIA, disabled, and focus support for sliders and areas #212
• fix(react): stable refs, WebGL context loss, and handler composition #211
• fix(react): apply controlled updates against the latest snapshot #210
• feat(core): selectable gamut mapping method #209
• fix(core): compute contrast from unrounded values #208 👈 (View in Graphite)
• fix(core): consistent achromatic hue handling and single transfer function #207
• feat(core): parse CSS Color 4 syntax #206
• fix(core): reject malformed color input instead of producing NaN #205
• chore: add fast-check and axe-core test dependencies #204
• docs: graphics and performance APIs demo page #203
• feat(core): tuple and typed-array interop for GPU and three.js #202
• feat(core): allocation-free *Into variants for hot paths #201
• feat(core): interpolation space option for mix/interpolate/generateScale #200
• main

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[greptile-apps]

Confidence Score: 3/5

[Medium risk] Refactors contrast measurement to use unrounded channel values.

The PR should not merge until region boundaries and Display P3 fill selection use the same measurements as their corresponding public contrast checks.

Findings

1.  Out-of-gamut references disagree ▶
2.  P3 fill uses sRGB check ▶

Summary

This PR moves scalar contrast and HSL/HSV/HCT conversion off rounded 8-bit RGB values, adds gamut and precision options for contrast, and switches contrast-region criteria to the public metrics.

• The region solvers’ existing chroma mapping still prevents public-check parity for out-of-gamut references.
• React’s Display P3 region-fill scoring still uses the default sRGB APCA measurement.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  C[OKLCH color] --> D[Unrounded linear sRGB]
  D --> G{Display gamut}
  G --> S[Clip sRGB channels]
  G --> P[Convert and clip P3 channels]
  S --> M[WCAG or APCA metric]
  P --> M
  R[Region request] --> RM[Map reference chroma to gamut]
  RM --> M
  M --> B[Contrast boundary]
  B --> F[React fill selection]
  F --> V[React validity check defaults to sRGB APCA]

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Project	Deployment	Actions	Updated
color-kit	Ready	Preview	Oct 10, 2026 7:09pm UTC

[pbroom]

Merge activity

• Oct 10, 6:47 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Oct 10, 7:04 PM UTC: Graphite rebased this pull request as part of a merge.
• Oct 10, 7:04 PM UTC: @pbroom merged this pull request with Graphite.