Talking about Php and Tutorials.
It’s time to see how to calculate difference between two colors.
I am using the CIE1994 formula because it’s quite accurate and not that complex to calculate.
Here it is, assuming I have two colors in Lab format
where
(thanx to Bruce Lindbloom)
First, I am turning the scripts explained at Color difference algorithm into functions, this way:
function rgb_to_xyz($rgb){
$red = $rgb[0];
$green = $rgb[1];
$blue = $rgb[2];
$_red = $red/255;
$_green = $green/255;
$_blue = $blue/255;
if($_red>0.04045){
$_red = ($_red+0.055)/1.055;
$_red = pow($_red,2.4);
}
else{
$_red = $_red/12.92;
}
if($_green>0.04045){
$_green = ($_green+0.055)/1.055;
$_green = pow($_green,2.4);
}
else{
$_green = $_green/12.92;
}
if($_blue>0.04045){
$_blue = ($_blue+0.055)/1.055;
$_blue = pow($_blue,2.4);
}
else{
$_blue = $_blue/12.92;
}
$_red *= 100;
$_green *= 100;
$_blue *= 100;
$x = $_red * 0.4124 + $_green * 0.3576 + $_blue * 0.1805;
$y = $_red * 0.2126 + $_green * 0.7152 + $_blue * 0.0722;
$z = $_red * 0.0193 + $_green * 0.1192 + $_blue * 0.9505;
return(array($x,$y,$z));
}
and…
function xyz_to_lab($xyz){
$x = $xyz[0];
$y = $xyz[1];
$z = $xyz[2];
$_x = $x/95.047;
$_y = $y/100;
$_z = $z/108.883;
if($_x>0.008856){
$_x = pow($_x,1/3);
}
else{
$_x = 7.787*$_x + 16/116;
}
if($_y>0.008856){
$_y = pow($_y,1/3);
}
else{
$_y = (7.787*$_y) + (16/116);
}
if($_z>0.008856){
$_z = pow($_z,1/3);
}
else{
$_z = 7.787*$_z + 16/116;
}
$l= 116*$_y -16;
$a= 500*($_x-$_y);
$b= 200*($_y-$_z);
return(array($l,$a,$b));
}
Now, it’s time to determine color difference, using the above formula:
function de_1994($lab1,$lab2){
$c1 = sqrt($lab1[1]*$lab1[1]+$lab1[2]*$lab1[2]);
$c2 = sqrt($lab2[1]*$lab2[1]+$lab2[2]*$lab2[2]);
$dc = $c1-$c2;
$dl = $lab1[0]-$lab2[0];
$da = $lab1[1]-$lab2[1];
$db = $lab1[2]-$lab2[2];
$dh = sqrt(($da*$da)+($db*$db)-($dc*$dc));
$first = $dl;
$second = $dc/(1+0.045*$c1);
$third = $dh/(1+0.015*$c1);
return(sqrt($first*$first+$second*$second+$third*$third));
}
And you’ll find the difference between colors.
Next time, a real application using all this boring theory… (you will be surprised…)
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