Calculate alpha from background color, source color and final color

calculateAlpha.js

function calculateAlpha(bgColor, srcColor, finalColor) {
    // Convert a hex color to its rgb components
    function hexToRgb(hex) {
        const bigint = parseInt(hex.replace("#", ""), 16);
        return {
            r: (bigint >> 16) & 255,
            g: (bigint >> 8) & 255,
            b: bigint & 255,
        };
    }

    const rgbBg = hexToRgb(bgColor);
    const rgbSrc = hexToRgb(srcColor);
    const rgbFinal = hexToRgb(finalColor);

    // Calculate alpha for each channel
    const alphaR = (rgbFinal.r - rgbBg.r) / (rgbSrc.r - rgbBg.r);
    const alphaG = (rgbFinal.g - rgbBg.g) / (rgbSrc.g - rgbBg.g);
    const alphaB = (rgbFinal.b - rgbBg.b) / (rgbSrc.b - rgbBg.b);

    // Check if the alpha values are consistent across all channels
    if (Math.abs(alphaR - alphaG) < 0.01 && Math.abs(alphaG - alphaB) < 0.01) {
        return (alphaR + alphaG + alphaB) / 3; // Return average alpha
    } else {
        throw new Error("Inconsistent alphas across channels. Cannot compute a single alpha value.");
    }
}

// Test the function
const bgColor = "#ffffff";
const srcColor = "#404040";
const finalColor = "#737373";

console.log(calculateAlpha(bgColor, srcColor, finalColor));