Convert RGBA to RGB

rgbatorgb.js

function RGBAtoRGB(r, g, b, a, r2,g2,b2){
    var r3 = Math.round(((1 - a) * r2) + (a * r))
    var g3 = Math.round(((1 - a) * g2) + (a * g))
    var b3 = Math.round(((1 - a) * b2) + (a * b))
    return "rgb("+r3+","+g3+","+b3+")";
    } 

$("#result").html(RGBAtoRGB(225,110,0,0.5,255,255,255));
​

How to get color opacity from origin color and resulted(based on r canal)

var a = (r3 - r2) / (r - r2);