_blending.scss

@function blend-transparent($foreground, $background) {
  // https://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending

  $fg-opacity: opacity($foreground);
  $bg-opacity: opacity($background);

  $opacity: $fg-opacity + $bg-opacity * (1.0 - $fg-opacity);
  $factor: $bg-opacity * (1.0 - $fg-opacity);

  $red:   (red($foreground)   * $fg-opacity + red($background)   * $factor) / $opacity;
  $green: (green($foreground) * $fg-opacity + green($background) * $factor) / $opacity;
  $blue:  (blue($foreground)  * $fg-opacity + blue($background)  * $factor) / $opacity;

  @return rgba($red, $green, $blue, $opacity);
}