Desaturate an image by a factor

desaturate.php

<?php
$image = imagecreatefrompng( 'trees.png' );
$width = imagesx( $image );
$height = imagesy( $image );

// Create
$output = imagecreatetruecolor($width, $height);

for( $y = 0; $y < $height; $y++ ){
    for( $x = 0; $x < $width; $x++ ){
        // Get pixel
        $rgb = imagecolorat( $image, $x, $y );
        $r = ($rgb >> 16) & 0xFF;
        $g = ($rgb >> 8) & 0xFF;
        $b = $rgb & 0xFF;

        $f = 1; // desaturate by 100%
        $L = ( 0.3 * $r ) + ( 0.6 * $g ) + ( 0.1 * $b );

        $r = $r + $f * ($L - $r);
        $g = $g + $f * ($L - $g);
        $b = $b + $f * ($L - $b);

        imagesetpixel( $output, $x, $y, imagecolorallocate( $output, $r, $g, $b ));
    }
}

header('Content-Type: image/png');
imagepng($output);

Explanation:
$r = ($rgb >> 16) & 0xFF;
$g = ($rgb >> 8) & 0xFF;
$b = $rgb & 0xFF;

($rgb >> 16) means to convert $rgb to binary and shift 16 bits to the right, discarding the bits.
($rgb >> 16) & 0xFF means to do a bitwise AND operation on the shifted binary with the binary of 0xFF. 0xFF may as well be 255 and the results will be the same: ($rgb >> 16) & 255

What ($rgb >> 16) & 0xFF does is get the first 8 bits of the shifted $rgb. The rest are discarded.

See: http://code.tutsplus.com/articles/understanding-bitwise-operators--active-11301

Note alpha can be fetched with just imagecolorat without using imagecolorindex:

$transparency = ($color >> 24) & 0x7F;