Lighten or darken a given colour

color_luminance.php

<?php
/**
 * Lightens/darkens a given colour (hex format), returning the altered colour in hex format.7
 * @param str $hex Colour as hexadecimal (with or without hash);
 * @percent float $percent Decimal ( 0.2 = lighten by 20%(), -0.4 = darken by 40%() )
 * @return str Lightened/Darkend colour as hexadecimal (with hash);
 */
function color_luminance( $hex, $percent ) {
	
	// validate hex string
	
	$hex = preg_replace( '/[^0-9a-f]/i', '', $hex );
	$new_hex = '#';
	
	if ( strlen( $hex ) < 6 ) {
		$hex = $hex[0] + $hex[0] + $hex[1] + $hex[1] + $hex[2] + $hex[2];
	}
	
	// convert to decimal and change luminosity
	for ($i = 0; $i < 3; $i++) {
		$dec = hexdec( substr( $hex, $i*2, 2 ) );
		$dec = min( max( 0, $dec + $dec * $percent ), 255 ); 
		$new_hex .= str_pad( dechex( $dec ) , 2, 0, STR_PAD_LEFT );
	}		
	
	return $new_hex;
}

@bjorn2404: Nice, it doesn't seem to work for straight black though #000000.

This is working as intended. Adjusting the luminescence of a color value lightens or darkens it by a percentage factor of itself. Any percentage increase of solid black ( RGB 0, 0, 0) will result in zero.

I suspect what you're trying to to is change the tint of the black and bring it into a gray/white space? This would require a different function altogether.

This is a different approach.
It expects and returns an array of the rbg color values, like array(0, 100, 255).
But you can of course create your own simple decimal to hex converter in there.

	// 0% =  black
	// 50% = no change
	// 100% = white
	function luminate($color, $percent)
	{
		foreach ($color as &$value)
		{
			$value = min(max(0, self::luminateValue($value, $percent)), 255);
		}

		return $color;
	}

	function luminateValue($value, $percent)
	{
		// no change
		if ($percent == 50)
		{
			return $value;
		}

		// ratio = value from 0 to 2
		$ratio = $percent * 2 / 100;

		// darken color
		if ($percent < 50)
		{
			return $value * $ratio;
		}

		// lighten color

		// reverse ratio
		$ratio = 2 - $ratio;

		$diff  = (255 - $value) * $ratio;

		return 255 - $diff;
	}

This works for me:

function luminance($hexcolor, $percent)
{
  if ( strlen( $hexcolor ) < 6 ) {
    $hexcolor = $hexcolor[0] . $hexcolor[0] . $hexcolor[1] . $hexcolor[1] . $hexcolor[2] . $hexcolor[2];
  }
  $hexcolor = array_map('hexdec', str_split( str_pad( str_replace('#', '', $hexcolor), 6, '0' ), 2 ) );

  foreach ($hexcolor as $i => $color) {
    $from = $percent < 0 ? 0 : $color;
    $to = $percent < 0 ? $color : 255;
    $pvalue = ceil( ($to - $from) * $percent );
    $hexcolor[$i] = str_pad( dechex($color + $pvalue), 2, '0', STR_PAD_LEFT);
  }

  return '#' . implode($hexcolor);
}

Oh, great. Thank you.

Thank you