I spent a couple of hours working through Mark Khan’s article on creating a colour picker widget in javascript. It’s a nice walk through, providing a good basis and a nice explanation of the technique used.

I thought I’d highlight a couple of useful generic colour functions in the code, as well as a couple I have written myself. The RGB to HSV conversion functions may be of particular interest to those who wish to extend the functionality of the colour picker in the original article.

Converting RGB to Hexcadecimal

I’d never really bothered to learn the relationship between RGB values and the hex code so often seen in CSS files. With a little investigation it appeared that it’s not that difficult a concept to grasp

In computing truecolour graphics describe 256 shades of Red, Green and Blue. This equates to somewhere in the region of 16 million different colours (256 x 256 x 256). Truecolor can be represent as three separate RGB values, such as 255, 255, 255 for white, but can also be described by a hexadecimal code such as #FFFFFF.

Hexadecimal is a base-16 numeral system. The symbols 0-9 and A,B,C,D,E,F represent values 0 - 15 respectively. Each truecolor RGB value maps very nicely to a two digit hexadecimal code as 16 x 16 = 256. Therefore in the white example #FFFFFF, each two digit code (FF) represents 255 for each of the of the RGB colour channels.

Converting an integer to a hex value is therefore very simple using javascript’s built in Integer.toString() method

/**
* Converts integer to a hexidecimal code, prepad's single digit hex codes with 0 to
* always return a two digit code.
*
* @param {Integer} i Integer to convert
* @returns {String} The hexidecimal code
*/
function intToHex(i) {
    var hex = parseInt(i).toString(16);
    return (hex.length < 2) ? "0" + hex : hex;
}

Converting in the other direction is unsuprisingly just as simple, this time using javascript’s parseInt() method. We need to precede our hex string with “0x” to tell the parseInt() method that the number is a hexadecimal. See the Mozilla javascript docs for more details.

/**
* Converts a hex number back to an integer, uses the javascript parseInt method
* with a base number (or radix) of 16. EG: parseInt("0x" + hex);
*
* @param {String} hex The hex code to convert
* @returns {Integer} The integer conversion of the hex number
*/
function hexToInt(hex) {
    return parseInt("0x" + hex);
}

Converting RGB to HSV

Hue, Saturation and Value (or Hue, Saturation and Brightness) is a common way of representing points within the RGB truecolor spectrum. These conversion functions were not present in the original colour picker article but their addition will give us some useful functionality.

Not being particularly gifted in mathematics I won’t attempt to explain in detail what’s going on here. I based these equations on the Wikipedia article for HSV, I suggest anybody wanting a deeper understanding of the concept start there.

/**
* Converts HSV to RGB value.
*
* @param {Integer} h Hue as a value between 0 - 360 degrees
* @param {Integer} s Saturation as a value between 0 - 100 %
* @param {Integer} v Value as a value between 0 - 100 %
* @returns {Array} The RGB values  EG: [r,g,b], [255,255,255]
*/
function hsvToRgb(h,s,v) {

    var s = s / 100,
         v = v / 100;

    var hi = Math.floor((h/60) % 6);
    var f = (h / 60) - hi;
    var p = v * (1 - s);
    var q = v * (1 - f * s);
    var t = v * (1 - (1 - f) * s);

    var rgb = [];

    switch (hi) {
        case 0: rgb = [v,t,p];break;
        case 1: rgb = [q,v,p];break;
        case 2: rgb = [p,v,t];break;
        case 3: rgb = [p,q,v];break;
        case 4: rgb = [t,p,v];break;
        case 5: rgb = [v,p,q];break;
    }

    var r = Math.min(255, Math.round(rgb[0]*256)),
        g = Math.min(255, Math.round(rgb[1]*256)),
        b = Math.min(255, Math.round(rgb[2]*256));

    return [r,g,b];

}	

/**
* Converts RGB to HSV value.
*
* @param {Integer} r Red value, 0-255
* @param {Integer} g Green value, 0-255
* @param {Integer} b Blue value, 0-255
* @returns {Array} The HSV values EG: [h,s,v], [0-360 degrees, 0-100%, 0-100%]
*/
function rgbToHsv(r, g, b) {

    var r = (r / 255),
         g = (g / 255),
  	 b = (b / 255);	

    var min = Math.min(Math.min(r, g), b),
        max = Math.max(Math.max(r, g), b),
        delta = max - min;

    var value = max,
        saturation,
        hue;

    // Hue
    if (max == min) {
        hue = 0;
    } else if (max == r) {
        hue = (60 * ((g-b) / (max-min))) % 360;
    } else if (max == g) {
        hue = 60 * ((b-r) / (max-min)) + 120;
    } else if (max == b) {
        hue = 60 * ((r-g) / (max-min)) + 240;
    }

    if (hue < 0) {
        hue += 360;
    }

    // Saturation
    if (max == 0) {
        saturation = 0;
    } else {
        saturation = 1 - (min/max);
    }

    return [Math.round(hue), Math.round(saturation * 100), Math.round(value * 100)];
}