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

Answer 1

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.