convert to 3-digit hex color code

Question

I've been using 3-digit hex color values in CSS for a long time: #fff, #999, #069, etc. I can see how the repeating letters/numbers are merged to create a 3-digit hex color code, but I don't fully understand the pattern to be able to write a converter in PHP. Is there documentation for this?

Edit: Oh, perhaps my question wasn't clear. I need to know how some of the 6-digit hex color values are converted to 3-digits. xxxxxx (ffffff) and xxyyzz (006699) – these are the only two patterns, correct?

Answer 1

function hexfix(str) {
  var v, w;
  v = parseInt(str, 16);	// in rrggbb
  if (str.length == 3) {
    // nybble colors - fix to hex colors
	//  0x00000rgb              -> 0x000r0g0b
	//  0x000r0g0b | 0x00r0g0b0 -> 0x00rrggbb
	w = ((v & 0xF00) << 8) | ((v & 0x0F0) << 4) | (v & 0x00F);
	v = w | (w << 4);
  }
  return v.toString(16).toUpperCase();
 }

var hex1 = 'AABBCC',
    hex2 = 'ABC';

document.body.appendChild(document.createTextNode(hex1+" becomes "+hexfix(hex1)+'.  '));
document.body.appendChild(document.createTextNode(hex2+" becomes "+hexfix(hex2)+'.  '));

Something like this.

Answer 2

To convert a 3-character hex code into a 6 character one, you need to repeat each character:

$hex = '#fff';
$hex6 = '#' . $hex[1] . $hex[1] . $hex[2] . $hex[2] . $hex[3] . $hex[3];

If you want to convert it to decimal you can use the hexdec function