I have written a function which uses the deltaE-function of the CMC.
It returns much better results than imagecolorclosestHWB() or imageColorClosest().
A test-script is online on http://www.fuhrpark-software.de/spielwiese/color.php?color=ffff00
<?php
/*
* Returns the index of the palette-color which is most similar
* to $givenColor.
*
* $givenColor and the colors in $palette should be either
* formatted as (#)rrggbb
* (e. g. "ff0000", "4da4f3" or "#b5d7f3")
* or arrays with values for red, green and blue
* (e. g. $givenColor = array( 0xff, 0x00, 0x00 ) )
*
* References:
* function rgb2lab
* - http://www.f4.fhtw-berlin.de/~barthel/ImageJ/ImageJ.htm
*
* function rgb2lab & function deltaE
* - http://www.brucelindbloom.com
*/
function getNearestColor(
$givenColor,
$palette = array( 'blue' => '43aafd','red' => 'fe6256','green' => '64b949','yellow' => 'fcf357',
'black' => '656565','white' => 'fdfdfd','orange' => 'fea800','purple' => '9773fe')
) {
if(!function_exists('rgb2lab')) {
function rgb2lab($rgb) {
$eps = 216/24389; $k = 24389/27;
// reference white D50
$xr = 0.964221; $yr = 1.0; $zr = 0.825211;
$rgb[0] = $rgb[0]/255; //R 0..1
$rgb[1] = $rgb[1]/255; //G 0..1
$rgb[2] = $rgb[2]/255; //B 0..1
// assuming sRGB (D65)
$rgb[0] = ($rgb[0] <= 0.04045)?($rgb[0]/12.92):pow(($rgb[0]+0.055)/1.055,2.4);
$rgb[1] = ($rgb[1] <= 0.04045)?($rgb[1]/12.92):pow(($rgb[1]+0.055)/1.055,2.4);
$rgb[2] = ($rgb[2] <= 0.04045)?($rgb[2]/12.92):pow(($rgb[2]+0.055)/1.055,2.4);
// sRGB D50
$x = 0.4360747*$rgb[0] + 0.3850649*$rgb[1] + 0.1430804 *$rgb[2];
$y = 0.2225045*$rgb[0] + 0.7168786*$rgb[1] + 0.0606169 *$rgb[2];
$z = 0.0139322*$rgb[0] + 0.0971045*$rgb[1] + 0.7141733 *$rgb[2];
$xr = $x/$xr; $yr = $y/$yr; $zr = $z/$zr;
$fx = ($xr > $eps)?pow($xr, 1/3):($fx = ($k * $xr + 16) / 116);
$fy = ($yr > $eps)?pow($yr, 1/3):($fy = ($k * $yr + 16) / 116);
$fz = ($zr > $eps)?pow($zr, 1/3):($fz = ($k * $zr + 16) / 116);
$lab = array();
$lab[] = round(( 116 * $fy ) - 16);
$lab[] = round(500*($fx-$fy));
$lab[] = round(200*($fy-$fz));
return $lab;
}
}
if(!function_exists('deltaE')) {
function deltaE($lab1, $lab2) {
// CMC 1:1
$l = 1; $c = 1;
$c1 = sqrt($lab1[1]*$lab1[1]+$lab1[2]*$lab1[2]);
$c2 = sqrt($lab2[1]*$lab2[1]+$lab2[2]*$lab2[2]);
$h1 = (((180000000/M_PI) * atan2($lab1[1],$lab1[2]) + 360000000) % 360000000)/1000000;
$t = (164 <= $h1 AND $h1 <= 345)?(0.56 + abs(0.2 * cos($h1+168))):(0.36 + abs(0.4 * cos($h1+35)));
$f = sqrt(pow($c1,4)/(pow($c1,4) + 1900));
$sl = ($lab1[0] < 16)?(0.511):((0.040975*$lab1[0])/(1 + 0.01765*$lab1[0]));
$sc = (0.0638 * $c1)/(1 + 0.0131 * $c1) + 0.638;
$sh = $sc * ($f * $t + 1 -$f);
return sqrt(
pow(($lab1[0]-$lab2[0])/($l * $sl),2) +
pow(($c1-$c2)/($c * $sc),2) +
pow(sqrt(
($lab1[1]-$lab2[1])*($lab1[1]-$lab2[1]) +
($lab1[2]-$lab2[2])*($lab1[2]-$lab2[2]) +
($c1-$c2)*($c1-$c2)
)/$sh,2)
);
}
}
if(!function_exists('str2rgb')) {
function str2rgb($str)
{
$str = preg_replace('~[^0-9a-f]~','',$str);
$rgb = str_split($str,2);
for($i=0;$i<3;$i++)
$rgb[$i] = intval($rgb[$i],16);
return $rgb;
}
}
$givenColorRGB = is_array($givenColor?$givenColor:str2rgb($givenColor);
$min = 0xffff;
$return = NULL;
foreach($palette as $key => $color) {
$color = is_array($color)?$color:str2rgb($color);
if($min >= ($deltaE = deltaE(rgb2lab($color),rgb2lab($givenColorRGB))))
{
$min = $deltaE;
$return = $key;
}
}
return $return;
}
?>