Map value to color scale

Question

I have a list of values which should be plotted to a map with a certain color. The plotting to the map is already done, but I need to figure out a way to map the value

Answer 1

Colors values are representations. Numeric colors as well as hexadecimal colors. A "not grayscale" color contains at least 2 different informations: Red value, green value or blue values may be different. Performing operation on its representation gives wrong result. The 'Mapping" must then be performed on each pieces of information. You need to extract red, green and blue values, perform the mapping seperatly, then build the representation of the result color. Here is a quick helper that use a "min color" and "max color", performs mapping on red, green and blue values, according to the "n" value you need to work with, then return the result color in hexadecimal string. It works for any colors or gray scale color as well.

function linear_color($from, $to, $ratio) {
        // normalize ralio
        $ratio = $ratio<0?0:($ratio>1?1:$ratio);
        // unsure colors are numeric values
        if(!is_numeric($from))$from=hexdec($from);
        if(!is_numeric($to))$to=hexdec($to);

        $rf = 0xFF & ($from >> 0x10);
        $gf = 0xFF & ($from >> 0x8);
        $bf = 0xFF & $from;
        $rt = 0xFF & ($to >> 0x10);
        $gt = 0xFF & ($to >> 0x8);
        $bt = 0xFF & $to;
        return str_pad( dechex(($bf + (($bt-$bf)*$ratio)) + ($gf + (($gt-$gf)*$ratio) << 0x8) + ($rf + (($rt-$rf)*$ratio) << 0x10)), 6,'0',STR_PAD_LEFT);
}

Just specify 2 colors as numeric value or hexadecimal string (without hash!) like this :

$color_from = hexdec('c2c2c2');
$color_to = hexdec('1eb02b');
for($i=-0.2; $i<=1.3; $i+=0.04){
        echo '
';
        echo 'Result color when n = '.$i.'';
        echo '
';
    }