Algorithm to modify brightness for RGB image?

Question

I know there is formula for going RGB -> Luminance, but I need given a brightness parameter to modify the RGB values of an image. How do I do that?

Thanks

Answer 1

My recommendation would be the same as ChrisA.'s answer, with one difference:

Use the HSP color space instead, as it is an approximation of Photoshop's algorithm and has better results.

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For the sake of not just linking to HSP's site (which frankly should be more than enough; it's just that I don't like to answer without examples), here is my C# implementation, which follows the site's:

#region Definitions
//Perceived brightness to Red ratio.
private const double Pr = .299;
//Perceived brightness to Green ratio.
private const double Pg = .587;
//Perceived brightness to Blue ratio.
private const double Pb = .114;
#endregion

//Expected ranges: Hue = 0-359... Other values = 0-1
public static ColorRGB ToRGB(double hue, double saturation, double perceivedBrightness, double alpha) {
    //Check values within expected range
    hue = hue < 0 ? 0 : hue > 359 ? 359 : hue;
    saturation = saturation < 0 ? 0 : saturation > 1 ? 1 : saturation;
    perceivedBrightness = perceivedBrightness < 0 ? 0 : perceivedBrightness > 1 ? 1 : perceivedBrightness;
    alpha = alpha < 0 ? 0 : alpha > 1 ? 1 : alpha;
    //Conversion
    var minOverMax = 1 - saturation;
    double r, g, b;
    if (minOverMax > 0) {
        double part;
        if (hue < 0.166666666666667D) { //R>G>B
            hue = 6 * (hue - 0); part = 1 + hue * (1 / minOverMax - 1);
            b = perceivedBrightness / Math.Sqrt(Pr / minOverMax / minOverMax + Pg * part * part + Pb);
            r = b / minOverMax; g = b + hue * (r - b);
        }
        else if (hue < 0.333333333333333D) { //G>R>B
            hue = 6 * (-hue + 0.333333333333333D); part = 1 + hue * (1 / minOverMax - 1);
            b = perceivedBrightness / Math.Sqrt(Pg / minOverMax / minOverMax + Pr * part * part + Pb);
            g = b / minOverMax; r = b + hue * (g - b);
        }
        else if (hue < 0.5D) {   //  G>B>R
            hue = 6 * (hue - 0.333333333333333D); part = 1 + hue * (1 / minOverMax - 1);
            r = perceivedBrightness / Math.Sqrt(Pg / minOverMax / minOverMax + Pb * part * part + Pr);
            g = r / minOverMax; b = r + hue * (g - r);
        }
        else if (hue < 0.666666666666667D) { //B>G>R
            hue = 6 * (-hue + 0.666666666666667D); part = 1 + hue * (1 / minOverMax - 1);
            r = perceivedBrightness / Math.Sqrt(Pb / minOverMax / minOverMax + Pg * part * part + Pr);
            b = r / minOverMax; g = r + hue * (b - r);
        }
        else if (hue < 0.833333333333333D) { //B>R>G
            hue = 6 * (hue - 0.666666666666667D); part = 1 + hue * (1 / minOverMax - 1);
            g = perceivedBrightness / Math.Sqrt(Pb / minOverMax / minOverMax + Pr * part * part + Pg);
            b = g / minOverMax; r = g + hue * (b - g);
        }
        else { //R>B>G
            hue = 6 * (-hue + 1D); part = 1 + hue * (1 / minOverMax - 1);
            g = perceivedBrightness / Math.Sqrt(Pr / minOverMax / minOverMax + Pb * part * part + Pg);
            r = g / minOverMax; b = g + hue * (r - g);
        }
    }
    else {
        if (hue < 0.166666666666667D) { //R>G>B
            hue = 6 * (hue - 0D); r = Math.Sqrt(perceivedBrightness * perceivedBrightness / (Pr + Pg * hue * hue)); g = r * hue; b = 0;
        }
        else if (hue < 0.333333333333333D) { //G>R>B
            hue = 6 * (-hue + 0.333333333333333D); g = Math.Sqrt(perceivedBrightness * perceivedBrightness / (Pg + Pr * hue * hue)); r = g * hue; b = 0;
        }
        else if (hue < 0.5D) { //G>B>R
            hue = 6 * (hue - 0.333333333333333D); g = Math.Sqrt(perceivedBrightness * perceivedBrightness / (Pg + Pb * hue * hue)); b = g * hue; r = 0;
        }
        else if (hue < 0.666666666666667D) { //B>G>R
            hue = 6 * (-hue + 0.666666666666667D); b = Math.Sqrt(perceivedBrightness * perceivedBrightness / (Pb + Pg * hue * hue)); g = b * hue; r = 0;
        }
        else if (hue < 0.833333333333333D) { //B>R>G
            hue = 6 * (hue - 0.666666666666667D); b = Math.Sqrt(perceivedBrightness * perceivedBrightness / (Pb + Pr * hue * hue)); r = b * hue; g = 0;
        }
        else { //R>B>G
            hue = 6 * (-hue + 1D); r = Math.Sqrt(perceivedBrightness * perceivedBrightness / (Pr + Pb * hue * hue)); b = r * hue; g = 0;
        }
    }
    return new ColorRGB(r, g, b, alpha);
}

//Expected ranges: 0-1 on all components
public static ColorHSP FromRGB(double red, double green, double blue, double alpha) {
    //Guarantee RGB values are in the correct ranges
    red = red < 0 ? 0 : red > 1 ? 1 : red;
    green = green < 0 ? 0 : green > 1 ? 1 : green;
    blue = blue < 0 ? 0 : blue > 1 ? 1 : blue;
    alpha = alpha < 0 ? 0 : alpha > 1 ? 1 : alpha;
    //Prepare & cache values for conversion
    var max = MathExtensions.Max(red, green, blue);
    var min = MathExtensions.Min(red, green, blue);
    var delta = max - min;
    double h, s, p = Math.Sqrt(0.299 * red + 0.587 * green + 0.114 * blue);
    //Conversion
    if (delta.Equals(0)) h = 0;
    else if (max.Equals(red)) {
        h = (green - blue) / delta % 6;
    }
    else if (max.Equals(green)) h = (blue - red) / delta + 2;
    else h = (red - green) / delta + 4;
    h *= 60;
    if (h < 0) h += 360;
    if (p.Equals(0))
        s = 0;
    else
        s = delta / p;
    //Result
    return new ColorHSP(h, s, p, alpha);
}