javascript - 在白色上将RGB转换为RGBA(Convert RGB to RGBA over white)

Question

I have a hex color, eg #F4F8FB (or rgb(244, 248, 251) ) that I want converted into an as-transparent-as-possible rgba color (when displayed over white).(我有一个十六进制颜色，例如#F4F8FB （或rgb(244, 248, 251) #F4F8FB rgb(244, 248, 251) ），我想将其转换为尽可能透明的 rgba颜色（当显示为白色时）。)

Make sense?(说得通？) I'm looking for an algorithm, or at least idea of an algorithm for how to do so.(我正在寻找一种算法，或者至少是关于一种算法的构想。)

For Example:(例如：)

rgb( 128, 128, 255 ) --> rgba(   0,   0, 255,  .5 )
rgb( 152, 177, 202 ) --> rgba(  50, 100, 150,  .5 ) // can be better(lower alpha)

Ideas?(有想法吗？)

---

FYI solution based on Guffa's answer:(基于Guffa答案的FYI解决方案：)

function RGBtoRGBA(r, g, b){
    if((g == null) && (typeof r === 'string')){
        var hex = r.replace(/^s*#|s*$/g, '');
        if(hex.length === 3){
            hex = hex.replace(/(.)/g, '$1$1');
        }
        r = parseInt(hex.substr(0, 2), 16);
        g = parseInt(hex.substr(2, 2), 16);
        b = parseInt(hex.substr(4, 2), 16);
    }

    var min, a = (255 - (min = Math.min(r, g, b))) / 255;

    return {
        r    : r = 0|(r - min) / a,
        g    : g = 0|(g - min) / a,
        b    : b = 0|(b - min) / a,
        a    : a = (0|1000*a)/1000,
        rgba : 'rgba(' + r + ', ' + g + ', ' + b + ', ' + a + ')'
    };
}

RGBtoRGBA(204, 153, 102) == RGBtoRGBA('#CC9966') == RGBtoRGBA('C96') == 
    {
       r    : 170,
       g    : 85 ,
       b    : 0  ,
       a    : 0.6,
       rgba : 'rgba(170, 85, 0, 0.6)' 
    }

  ask by Mark Kahn translate from so

Answer 1

Take the lowest color component, and convert that to an alpha value.(取最低的颜色分量，并将其转换为alpha值。)

Then scale the color components by subtracting the lowest, and dividing by the alpha value.(然后，通过减去最低值并除以alpha值来缩放颜色分量。)

Example:(例：)

152 converts to an alpha value of (255 - 152) / 255 ~ 0.404

152 scales using (152 - 152) / 0.404 = 0
177 scales using (177 - 152) / 0.404 ~ 62
202 scales using (202 - 152) / 0.404 ~ 123

So, rgb(152, 177, 202) displays as rgba(0, 62, 123, .404) .(因此， rgb(152, 177, 202) rgba(0, 62, 123, .404) rgb(152, 177, 202)显示为rgba(0, 62, 123, .404) 。)

I have verified in Photoshop that the colors actually match perfectly.(我已经在Photoshop中验证颜色实际上完全匹配。)