water.php

<?php

$target = $_GET['target'];
$watermark = $_GET['watermark'];

if ((strlen($target) == 0) || (strlen($watermark) == 0)) {
  exit;
}

if (!file_exists($target) || !file_exists($watermark)) {
  exit;
}

// I don't understand why there isn't a GD utility function for this
function imagecreatefrom($filename) {
  $ext = strrchr($filename, '.');

  if (!$ext) {
    $ext = '';
  }  

  switch ($ext) {
  
  case '.jpg':
  case '.jpeg':
    return imagecreatefromjpeg($filename);

  case '.png':
    return imagecreatefrompng($filename);

  case '.gif':
    return imagecreatefromgif($filename);

  default:
    // Might as well assume it's a jpeg
    return imagecreatefromjpeg($filename);
  }
}

// Get original file dimensions
list($t_width, $t_height) = getimagesize($target);
list($w_width, $w_height) = getimagesize($watermark);

// The two options for scaling depend on the relative aspect-ratios of the target and watermark (e.g., portrait vs. lanscape)
// In other words: if you scale the watermark image, which dimension hits first: width or height?

// If target image is "taller" than the watermark image, then we want the full width, and scale the height
if (($t_height / $t_width) > ($w_height / $w_width)) {
  // We want the full width in the output
  $out_width = $t_width;
  // And scale the height accordingly
  $out_height = intval(($t_width / $w_width) * $w_height);
  // We also need to adjust the y-offset; which is (half-of-the-target - half-of-the-scaled-watermark)
  $out_x = 0;
  $out_y = intval(($t_height / 2) - ($out_height / 2));
} else {
  // Otherwise we want full-height, and scale the width
  $out_height = $t_height;
  $out_width = intval(($t_height / $w_height) * $w_width);
  // Similar to above, we need to offset the image horizontally
  $out_y = 0;
  $out_x = intval(($t_width / 2) - ($out_width / 2));
}

// The output image is always the same size as the target
$out_image = imagecreatetruecolor($t_width, $t_height);

$t_image = imagecreatefrom($target);
$w_image = imagecreatefrom($watermark);


// The target image is copied as-is
imagecopyresampled($out_image, $t_image, 0, 0, 0, 0, $t_width, $t_height, $t_width, $t_height);

// Then we overlay it with the appropriately-scaled watermark
imagecopyresampled($out_image, $w_image, $out_x, $out_y, 0, 0, $out_width, $out_height, $w_width, $w_height);


// Content type
header('Content-type: image/png');
// Output
imagepng($out_image, null, 9);

?>