Compare image

compare-image.php

<?php

function diff_images($a, $b)
{
	$max_x = max(imagesx($a), imagesx($b));
	$max_y = max(imagesy($a), imagesy($b));

	$diff = imagecreatetruecolor($max_x, $max_y);

	for ($x = 0; $x < $max_x; ++$x)
		for ($y = 0; $y < $max_y; ++$y)
			imagesetpixel($diff, $x, $y,
				create_color($diff, diff_color(
					imagecolorat($a, $x, $y),
					imagecolorat($b, $x, $y))));
	
	return $diff;
}

function create_color($img, $color)
{
	list($red, $green, $blue) = $color;
	return imagecolorallocate($img, $red, $green, $blue);
}

function diff_color($a, $b)
{
	$rgb_a = rgb($a);
	$rgb_b = rgb($b);

	$diff = array();

	for ($i = 0; $i < 3; ++$i)
		$diff[$i] = abs($rgb_a[$i] - $rgb_b[$i]);
	
	return $diff;
}

function rgb($color)
{
	$r = ($color >> 16) & 0xFF;
	$g = ($color >> 8) & 0xFF;
	$b = $color & 0xFF;

	return array($r, $g, $b);
}

if ($_SERVER['REQUEST_METHOD'] == 'POST')
{
	$img_a = imagecreatefrompng($_FILES['a']['tmp_name']);
	$img_b = imagecreatefrompng($_FILES['b']['tmp_name']);

	$diff = diff_images($img_a, $img_b);

	header('Content-Type: image/png');
	imagepng($diff);	
}
else
{
	echo '<form method="post" enctype="multipart/form-data">';
	echo '<input type="file" name="a"> vs <input type="file" name="b">';
	echo '<button type="submit">Compare</button>';
	echo '</form>';
}