CSS & Design FXA11y

Image Color Palette & WCAG Contrast Matrix

Analyze UI graphics and photos with k-means color quantization. Extracts dominant Hex, RGB, and HSL swatches with nearest Tailwind CSS color tokens, while computing an interactive relative luminance contrast matrix for legal WCAG compliance.

1. Select ImageUpload

Extracted Dominant Swatches (0)

Foreground (Text) vs Background Surface MatrixAA: >= 4.5:1 | AAA: >= 7:1

FG Color	BG Color	Ratio	WCAG AA	WCAG AAA

K-Means Color Quantization & WCAG Relative Luminance

Technical guide on Euclidean clustering, sRGB gamma expansion, and legal accessibility compliance.

Architecture Overview

Design system engineering requires rigorous adherence to accessibility standards. The Web Content Accessibility Guidelines (WCAG 2.1 / 3.0) define strict contrast thresholds between foreground text and background surfaces. This utility extracts dominant image colors using k-means vector clustering and computes a complete relative luminance contrast matrix to guarantee ADA and Section 508 accessibility compliance.

Core Mechanics & Implementation Details

K-Means Vector Quantization in 3D RGB Space

To extract the most visually prominent palette from millions of pixels, the canvas pixel buffer is sampled into 3D RGB vectors. K-Means clustering iteratively groups neighboring colors around k centroids, minimizing intra-cluster variance until convergence. The resulting centroid coordinates represent the mathematically dominant swatches.

W3C Relative Luminance Formula

Human eyes perceive green, red, and blue with vastly different sensitivity. Relative luminance L is defined in WCAG 2.1 by first converting nonlinear sRGB color components into linear values: C_lin = C_srgb <= 0.04045 ? C_srgb / 12.92 : ((C_srgb + 0.055) / 1.055)^2.4. Luminance is computed as L = 0.2126 * R_lin + 0.7152 * G_lin + 0.0722 * B_lin.

WCAG 2.1 Contrast Ratio Thresholds

Contrast ratio between two surfaces is calculated as (L1 + 0.05) / (L2 + 0.05), where L1 is the lighter surface. Level AA compliance mandates a minimum 4.5:1 ratio for normal body text and 3:1 for large text (>= 18pt or bold 14pt). Level AAA mandates a strict 7:1 ratio for body text.

Engineering Specifications

Parameter	Standard / Value	Engineering Scope
Clustering Algorithm	K-Means Color Quantization	Iterative 3D vector centroid convergence
Compliance Standard	W3C WCAG 2.1 Level AA & AAA	Relative luminance contrast verification
Gamma Decoding	sRGB IEC 61966-2-1 Transfer Curve	Nonlinear to linear photometric conversion
Export Tokens	Tailwind CSS v3/v4 Theme & CSS Vars	Ready-to-use design system tokens

Recommended Best Practices

•Always verify brand colors pass at least WCAG AA (4.5:1) for standard body copy before deployment.
•Map extracted hex colors to standard design token names (e.g. primary, surface, muted) to enforce design consistency.
•Check contrast against both light and dark surface modes to support accessibility preferences.

Common Pitfalls & Anti-Patterns

•Calculating contrast using raw RGB arithmetic averages instead of photometric luminance produces mathematically invalid ratios.
•Assuming vibrant accent colors (like yellow or cyan) pass on white backgrounds without measuring contrast.

W3C WCAG 2.1 Luminance & Contrast Ratio Implementation

function getLuminance(r: number, g: number, b: number): number {
  const [rs, gs, bs] = [r, g, b].map((c) => {
    const s = c / 255;
    return s <= 0.04045 ? s / 12.92 : Math.pow((s + 0.055) / 1.055, 2.4);
  });
  return 0.2126 * rs + 0.7152 * gs + 0.0722 * bs;
}

export function getContrastRatio(rgb1: [number, number, number], rgb2: [number, number, number]): number {
  const l1 = getLuminance(...rgb1);
  const l2 = getLuminance(...rgb2);
  const lighter = Math.max(l1, l2);
  const darker = Math.min(l1, l2);
  return (lighter + 0.05) / (darker + 0.05);
}