How do I convert a hexadecimal color to rgba with the Less compiler?

Question

Actually, the Less language comes with an embedded function called fade. You pass a color object and the absolute opacity % (higher value means less transparent):

fade(@color, 50%);   // Return @color with 50% opacity in rgba

If you don't need an alpha key, you can simply use the hexadecimal representation of the color. An rgba color with a alpha of '1' is the same as the hexadecimal value.

Here are some examples to demonstrate that:

@baseColor: #d14836;

html {
    color: @baseColor;
    /* color:#d14836; */
}

body {
    color: rgba(red(@baseColor), green(@baseColor), blue(@baseColor), 1);
    /* color:#d14836; */
}

div {
    color: rgba(red(@baseColor), green(@baseColor), blue(@baseColor), 0.5);
    /* rgba(209, 72, 54, 0.5); */
}

span {
    color: fade(@baseColor, 50%);
    /* rgba(209, 72, 54, 0.5); */
}

h3 {
    color: fade(@baseColor, 100%)
    /* color:#d14836; */
}

Test this code online: http://lesstester.com/

My Less mixin:

.background-opacity(@color, @opacity) {
    @rgba-color: rgba(red(@color), green(@color), blue(@color), @opacity);

    background-color: @rgba-color;

    // Hack for IE8:
    background: none\9; // Only Internet Explorer 8
    filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d')", argb(@rgba-color),argb(@rgba-color))); // Internet Explorer 9 and down
    // Problem: Filter gets applied twice in Internet Explorer 9.
    // Solution:
    &:not([dummy]) {
      filter: progid:DXImageTransform.Microsoft.gradient(enabled='false'); // Only IE9
    }
}

Try it.

EDITED: As seen on rgba background with IE filter alternative: IE9 renders both!, I added some lines to the mixin.

It seems that with the recent Less update 3.81, you can use just two arguments in the rgba() function.

rgba(white, 0.3) or rgba(white, 30%) => rgba(255, 255, 255, 0.3)

It works for me, but I can't find it in the official documentation.