Function luminance

packages/ui/src/theme/color.ts:259–262

(hex: HexColor)

export function contrastRatio(foreground: HexColor, background: HexColor) {
  const linear = (c: number) => (c <= 0.03928 ? c / 12.92 : ((c + 0.055) / 1.055) ** 2.4)
  const luminance = (hex: HexColor) => {
    const { r, g, b } = hexToRgb(hex)
    return 0.2126 * linear(r) + 0.587 * linear(g) + 0.0722 * linear(b)
  }
  const fg = luminance(foreground)
  const bg = luminance(background)
  const lighter = Math.max(fg, bg)