RGB to closest predefined color

Question

Edit:

With the answer given I made this function

function grabclosestcolor($r, $g, $b){
    $colors = array(array(124,12,12),array(7,7,11),array(110,224,219),array(123,123,123),array(124,177,74),array(130,86,53),array(77,77,77),array(164,124,68),array(204,196,132),array(164,148,147),array(163,123,67),array(26,122,26), array(195,195,50),array(193,193,193),array(255,248,73),array(243,243,243));
    $differencearray = array();
    foreach ($colors as $value) {
        $difference = sqrt(pow($r-$value[0],2)+pow($g-$value[1],2)+pow($b-$value[2],2));
        array_push($differencearray, $difference);
        $smallest = min($differencearray);
        $key = array_search($smallest, $differencearray);
        return $colors[$key];
        }
    }

---

My goal is this. I grab a picture and loop through each pixel and grab its x,y, and rgb.

Instead of just grabbing the rgb, I have a predefined array and I'm looking for the closest match from the color I grabbed to the predefined array. The goal here is to only use colors from the predefined array. Here is my array of colors.

$colors = array(array(124,12,12),array(7,7,11),array(110,224,219),array(123,123,123),array(124,177,74),array(130,86,53),array(77,77,77),array(164,124,68),array(204,196,132),array(164,148,147),array(163,123,67),array(26,122,26), array(195,195,50),array(193,193,193),array(255,248,73),array(243,243,243));

and here is my existing code that loops through it all.

$int = imagesx($im) - 1;
$int2 = imagesy($im) - 1;
$start2 = 0;
do{
    $start = 0;
    do{
        $rgb = imagecolorat($im, $start, $start2);
        $r = ($rgb >> 16) & 0xFF;
        $g = ($rgb >> 8) & 0xFF;
        $b = $rgb & 0xFF;
        $value = rgb2hex($r,$g,$b).":$start:$start2";
        array_push($colorsofimage, $value);
    } while($int > $start++);
} while($int2 > $start2++);

rgb2hex is a User Defined Function, but what I want to accomplish is to change that function with the function to grab the closest color.

$colorsofimage contains an array of each pixels info with hex:x:y what i want it to be is rgb2hex(NEWFUNCTION($r,$g,$b)); So that the new hex is the 1 out of the predefined array.

I hope you understood, because I have no clue how to do it because I don't know the algorithm of a color.

Answer 1

You have to calculate the distance to each color, and pick the smallest.

There are a few ways to do this. A simple method would be to calculate the distance would be:

sqrt((r-r1)^2+(g-g1)^2+(b-b1)^2)

A better method might be to incorporate the weighted values to calculate a distance, for instance the values used when converting RGB->YUV:

Y = 0.299 * R + 0.587 * G + 0.114 * B

in that case you would use

sqrt(((r - r1) * .299)^2 + ((g - g1) * .587)^2 + ((b - b1) * .114)^2)

Of course, since you don't need the exact distances, just a comparison, you can and probably should just skip the square root, making the last calculation:

((r - r1) * .299)^2 + ((g - g1) * .587)^2 + ((b - b1) * .114)^2