Increase or decrease color saturation

Question

I would like to know the algorithm to increase or decrease one RGB color saturation

for example if I have the color rgb(200, 30, 40) (red) the function stub would be

function Saturation(color, factor)
where color.r = 200, color.g= 30 and color.b=40

Anyone knows a library or has a code snippet that does that?

Answer 1

Following Bali Balo suggestion I came up with:

RGBtoHSV= function(color) {
        var r,g,b,h,s,v;
        r= color[0];
        g= color[1];
        b= color[2];
        min = Math.min( r, g, b );
        max = Math.max( r, g, b );


        v = max;
        delta = max - min;
        if( max != 0 )
            s = delta / max;        // s
        else {
            // r = g = b = 0        // s = 0, v is undefined
            s = 0;
            h = -1;
            return [h, s, undefined];
        }
        if( r === max )
            h = ( g - b ) / delta;      // between yellow & magenta
        else if( g === max )
            h = 2 + ( b - r ) / delta;  // between cyan & yellow
        else
            h = 4 + ( r - g ) / delta;  // between magenta & cyan
        h *= 60;                // degrees
        if( h < 0 )
            h += 360;
        if ( isNaN(h) )
            h = 0;
        return [h,s,v];
    };

HSVtoRGB= function(color) {
        var i;
        var h,s,v,r,g,b;
        h= color[0];
        s= color[1];
        v= color[2];
        if(s === 0 ) {
            // achromatic (grey)
            r = g = b = v;
            return [r,g,b];
        }
        h /= 60;            // sector 0 to 5
        i = Math.floor( h );
        f = h - i;          // factorial part of h
        p = v * ( 1 - s );
        q = v * ( 1 - s * f );
        t = v * ( 1 - s * ( 1 - f ) );
        switch( i ) {
            case 0:
                r = v;
                g = t;
                b = p;
                break;
            case 1:
                r = q;
                g = v;
                b = p;
                break;
            case 2:
                r = p;
                g = v;
                b = t;
                break;
            case 3:
                r = p;
                g = q;
                b = v;
                break;
            case 4:
                r = t;
                g = p;
                b = v;
                break;
            default:        // case 5:
                r = v;
                g = p;
                b = q;
                break;
        }
        return [r,g,b];
    }

by converting to the HSV (hue, saturation and value) format you can manually change the S component in this manner:

var hsv= RGBtoHSV ([200,100,100]);
alert(hsv)
hsv[1] *= 1.5;
alert(hsv)
var rgb= HSVtoRGB(hsv);
alert(rgb); //new color

Answer 2

Here's a quick and dirty way that probably isn't correct in any technical way, but involves less computation than converting to HSV and back (so renders quicker if that matters):

Grayscale is the equivalent of calculating luminosity averaged over the RGB parts of the pixel. We can mix grayness by applying a value weighting to the gray part versus the colored part:

var pixels = context.getImageData(0, 0, canvas.width, canvas.height);
grayscale = function (pixels, value) {
    var d = pixels.data;
    for (var i = 0; i < d.length; i += 4) {
        var r = d[i];
        var g = d[i + 1];
        var b = d[i + 2];
        var gray = 0.2989*r + 0.5870*g + 0.1140*b; //weights from CCIR 601 spec
        d[i] = gray * value + d[i] * (1-value);
        d[i+1] = gray * value + d[i+1] * (1-value);
        d[i+2] = gray * value + d[i+2] * (1-value);
    }
    return pixels;
};

So instead of adding "grayness", we can take it away and add the relevant color back in to "saturate":

saturate = function (pixels, value) {
    var d = pixels.data;
    for (var i = 0; i < d.length; i += 4) {
        var r = d[i]; 
        var g = d[i + 1];
        var b = d[i + 2];
        var gray = 0.2989*r + 0.5870*g + 0.1140*b; //weights from CCIR 601 spec
        d[i] = -gray * value + d[i] * (1+value);
        d[i+1] = -gray * value + d[i+1] * (1+value);
        d[i+2] = -gray * value + d[i+2] * (1+value);
        //normalize over- and under-saturated values
        if(d[i] > 255) d[i] = 255;
        if(d[i+1] > 255) d[i] = 255;
        if(d[i+2] > 255) d[i] = 255;
        if(d[i] < 0) d[i] = 0;
        if(d[i+1] < 0) d[i] = 0;
        if(d[i+2] < 0) d[i] = 0;
    }
    return pixels;
};

Again, disclaimer that this "looks" saturated but probably in no way adheres to the technical definition of "saturation" (whatever that is); I've simply posted this in the hope that it's helpful to passers-by.