I’ve started a website where I intend to sell thousands of products from a number of distributors through drop-shipping (the products go directly to the customer).

For reasons that I don’t understand, the distributors have watermarked their images, and don’t provide unwatermarked versions unless you’re an already well-established customer of theirs.

For the purpose of this demo, a watermark is a constant-colour “stamp” which is given opacity and pasted into the original image.

As I intend to be a good customer, I figured it would be okay for me to simply “unwatermark” the images.

There are a number of instructions online which show how to /fake/ an unwatermaking – by basically smudging the area where the watermark is.

However, as most watermarking appears to follow a single method, it is actually possible to simply reverse the process and remove the watermark, after a little trial and error.

Let’s consider an example. Here is an image, a stamp, and the merge of the two:

(original is here)

To reverse this, you need to know what algorithm was used to create the watermark, and what the original watermark was.

Most people use a fairly simple method to watermark their images:

The stamp is one single colour, usually gray (#808080 in RGB) which will be visible on images which are both light and dark.

The stamp is then given an opacity (30% in my case above), and pasted directly over the original image.

The formula for any particular colour channel (R, G and B) on any pixel is: C₃=(1-p)C₁+pC₂, where p is opacity (0 to 1), C₁ is the colour value for the original image, C₂ is the stamp’s colour value, and C₃ is the resulting image’s colour value.

To reverse the watermarking, you need to convert the formula to see what it is in respect to C₁: C₁=(C₃-pC₂)/(1-p).

As most stamps will be using a middle gray (#808080), you just have to guess at the opacity. .3 is a good start.

For some reason I’m not yet sure of, the code I came up with did unwatermark the image, but too much… the points where the watermark were, ended up being too bright. So I needed to add a darkening aspect, reducing the brightness of the result of the above calculation.

I’m not going to hold your hand if you can’t make this work, but here’s the code I ended up with (assumes the images are exactly 400×400 in size). The original should be ‘original.jpg’, and the stamp should be ‘stamp.png’ (with white where transparent pixels should be).

$p=.3; // opacity

$f1=imagecreatefrompng('stamp.png');
imagepalettetotruecolor($f1);
$f2=imagecreatetruecolor(400, 400);
$f3=imagecreatefromjpeg('original.jpg');
imagepalettetotruecolor($f3);

for ($x=0;$x<400; ++$x) {
  for ($y=0; $y<400; ++$y) {
    $rgb1=imagecolorat($f1, $x, $y);
    $rgb3=imagecolorat($f3, $x, $y);
    $r3 = ($rgb3 >> 16) & 0xFF;
    $g3 = ($rgb3 >> 8) & 0xFF;
    $b3 = $rgb3 & 0xFF;
    if ($rgb1==16777215) { // white. just copy
      $c=imagecolorallocate($f2, $r3, $g3, $b3);
      imagesetpixel($f2, $x, $y, $c);
      continue;
    }
    $r1 = ($rgb1 >> 16) & 0xFF;
    $g1 = ($rgb1 >> 8) & 0xFF;
    $b1 = $rgb1 & 0xFF;
    $r2=c($r1, $r3, $p);
    $g2=c($g1, $g3, $p);
    $b2=c($b1, $b3, $p);
    $c=imagecolorallocate($f2, $r2, $g2, $b2);
    imagesetpixel($f2, $x, $y, $c);
  }
}
imagejpeg($f2, 'unwatermarked.jpg');

function c($c1, $c2, $p) {
  $c=c1($c1, $c2, $p);
  $c3=$c-(255-$c)*.2;
  return $c3<0?0:(int)$c3; 
} 
function c1($c2, $c3, $p) {
  $c=($c3-$c2*$p)/(1-$p);
  return $c>255?255:(int)$c;
}