How to alter brightness of a single rgb color simply and easily via php?

Question

A quesion about RGB color and finding the simplest, tiniest, php conversion code for manipulating the lightness/darkness of a given RGB hue.

Imagine a variable $colorA containning a valid six char RGB color, like F7A100 which we want to make a bit lighter and/or darker:

$color  = B1B100;                     // original RGB color manually set.

Then, at any page have that color bit darker/lighter on the fly:

$colorX = someFunction($color, +10);  // original color 10 steps lighter
$colorY = someFunction($color, -25);  // original color 25 steps darker

What would be YOUR way of solving this? Keep the RGB as is or first change it to HSL? Hints and suggestions are welcome. Your sample/code is welcome too.

This really focuses to the TINIES / SIMPLES / SHORTEST possible code to just make the same hue bit darker/lighter.

I deliberately do not suggest my code, as I want to keep possibilities open in here.

Answer 1

For slight alteration of brightness you can convert the hexadecimal values to decimal, manipulate them and convert back to hexadecimal like this:

function alterBrightness($color, $amount) {
    $rgb = hexdec($color); // convert color to decimal value

    //extract color values:
    $red = $rgb >> 16;
    $green = ($rgb >> 8) & 0xFF;
    $blue = $rgb & 0xFF;

    //manipulate and convert back to hexadecimal
    return dechex(($red + $amount) << 16 | ($green + $amount) << 8 | ($blue + $amount));
}

echo alterColor('eeeeee', -10); //outputs e4e4e4

Beware that this code does not handle overflow for one color - if one color value becomes less than 0 or more than 255 you will get an invalid color value. This should be easy enough to add.

For drastic changes in brightness, convert to HSL and manipulate the lightness.

Using the functions from the Drupal code, this can be done like this:

$hsl = _color_rgb2hsl(_color_unpack('eeeeee'));
$hsl[2] -= 10;
$rgb = _color_pack(_color_hsl2rgb($hsl));
echo $rgb;  //outputs e4e4e4

Answer 2

The absolutely simplest solution is to add some constant (like 1) to each part of the color representation: [R, G, B]. This is due to the fact that max values of all [R, G, B] represent white, while min values - black. In pseudo-code (assuming 255 is max, sorry, I don't know PHP):

lighter(R, G, B) = [
    min(255, R + 1), 
    min(255, G + 1), 
    min(255, B + 1)
]

You must keep in mind though that this transformation is way too simplistic and the proper implementation would be to convert to HSL/HSB, increase H and transform back to RGB.