Function parseCssColorToRgba

src/utils/snapCapture.ts:27–53

(color: string)

const colorProbeCtx = colorProbeCanvas.getContext('2d', { willReadFrequently: true })

function parseCssColorToRgba(color: string): RgbaColor | null {
  if (!colorProbeCtx) return null

  const ctx = colorProbeCtx
  ctx.clearRect(0, 0, 1, 1)

  // 先写入完全透明作为基底，确保读取到 alpha 信息。
  ctx.fillStyle = 'rgba(0, 0, 0, 0)'
  ctx.fillRect(0, 0, 1, 1)

  try {
    ctx.fillStyle = color
  } catch {
    return null
  }

  ctx.fillRect(0, 0, 1, 1)
  const [r, g, b, a] = ctx.getImageData(0, 0, 1, 1).data
  if (a === 0) return null

  return {
    r,
    g,
    b,
    a: a / 255,
  }
}

function compositeOver(top: RgbaColor, bottom: RgbaColor): RgbaColor {
  const alpha = top.a + bottom.a * (1 - top.a)