所有颜色均使用数组表示，如 RGBA(0, 0, 0, 0.87) 表示为 [0, 0, 0, 0.87] ，RGB(33, 33, 33) 表示为 [33, 33, 33]， #212121 表示为 [21, 21, 21]。

RGBA to RGB

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function RGBAtoRGB(background, foreground) {
  const [backRed, backGreen, backBlue] = background
  const [foreRed, foreGreen, foreBlue, foreAlpha] = foreground

  return [
    Math.round((1 - foreAlpha) * backRed + foreAlpha * foreRed),
    Math.round((1 - foreAlpha) * backGreen + foreAlpha * foreGreen),
    Math.round((1 - foreAlpha) * backBlue + foreAlpha * foreBlue)
  ]
}

// RGBA(0, 0, 0, 0.87) => RGB(33, 33, 33)
RGBAtoRGB([255, 255, 255], [0, 0, 0, 0.87])

RGB to HEX

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function RGBtoHEX([red, green, blue]) {
  return [red, green, blue].map(value => value.toString(16))
}

// RGB(33, 33, 33) => #212121
RGBtoHEX([33, 33, 33])

HEX to RGB

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function HEXtoRGB([red, green, blue]) {
  return [red, green, blue].map(value => parseInt(value, 16))
}

// #212121 => RGB(33, 33, 33)
HEXtoRGB([21, 21, 21])