Hidden Watermarks in GIMP

In this tutorial, I'll show you how to add a hidden watermark to your images using GIMP and have the ability to "unlock the secret" to prove that the work is really yours. The steps provided herein will also work in Photoshop, Paint Shop Pro or other photo imaging software that have similar blend modes; however, the steps discussed herein are mine. I would respectfully ask that if you convert these steps to another software that you give me credit for them, as well as send me a note that you have done so, including the URL where the steps are posted. It would be very embarrassing for you to be identified as ripping a tutorial specifically designed to stop ripping!

You may use this technique to protect your commercial works; however, using or describing this technique in a way that brings you financial gain is NOT authorized.

I will be covering most steps in detail, but this tutorial does assume some basic knowledge of GIMP. If you need any clarification, please feel free to ask.

First of all, I would like to thank droz for his immense help in testing/refining this technique. We've tried it out on various images of light and dark colors, as well as image types (png, jpg, gif, etc.) and have found that it works in MOST cases. He contacted me several months ago about some ripped artwork on deviantart and a new group called that was being formed there called the Ripped Art Task Force He knew that I had been working on some concepts for watermarking and wanted an update of the status. Well to be quite honest, I had put it off. But, because of his encouragement to get my butt in gear and the fact that he told me about this new group....I felt it necessary to start work on it immediately. So, Big thanks, droz!

I would also like to point out that this doesn't work in ALL cases. Nothing is foolproof and if someone wants to get around this method, I'm sure they can. However, if this saves one person a headache from dealing with their work being stolen, the technique is more than worth it. Please re-read what I've just stated. It's not completely foolproof.

Let's begin.....The image below is a compilation of two images: one watermarked, the other without a watermark. Some images have been automatically resized by the forum. In firefox, you can right-click on the image and choose View Image to view a full-size, full-quality version.

So, which one is which? Frankly, I haven't a clue! Grin

I can't remember which one I placed where. So, what's the point? Glad you asked! There are actually two points:

We will create an image with a hidden watermark. No unsitely stamp to distract from the beautiful artwork.
We will also be able to "unlock" the hidden watermark so that you can prove the image is really yours.

Both of these tasks will be done using GIMP...no special scripts or plugins. I'm using GIMP version 2.2.17. Those of you using the newer beta versions may note differences in the layout of your menus, GUI look, etc., so you may have to improvise a little to find everything, but I'm pretty sure all the options will be there.

Step 1 - Creating the Watermark

This step will only have to be done once. From here on out, you'll be able to rapidly watermark an image you so choose.

Create a new transparent image....I'm using one approximately 100X35 pixels in size.
Grab your text tool and with any font style and appropriate size (you could even use a personal logo if you have that available), type what ever information you want for your watermark. I suggest using a white font color as that appears to work best in most situations, but you can experiment with other colors as you want.

Here's my result:

In the layer dialog window, you'll notice there are 2 layers as shown below:

Highlight the upper layer, as shown in blue, by clicking on it. Then, go to: Layer Menu > Merge Down to combine the layers in to one.

Step 2

Now, let's convert our image to a pattern. Go to Select Menu > All (or Ctrl+A)

Then, go to Script-FU Menu > Selection > To Pattern...

and assign names in the locations highlighted:

Click the OK button and GIMP will automatically save the selection as a pattern in the appropriate location and refresh your patterns for immediate use! Click on your patterns dialog tab to see your pattern. Mine is here:

Now, the hard part is done! Let's make some watermarks!

Step 3

Open the image you want to watermark and create a duplicate of it. Do this immediately to avoid messing up your original! I repeat: Do this immediately to avoid messing up your original!

Go to Image Menu > Duplicate (or Ctrl+D)

Step 4

Close your original image. This is your original artwork and will be used later as the Key to "unlock the hidden watermark."

On the duplicate image, save it under a new name. This will be the watermarked image that you will post online. I suggest you save it as a png as pngs appear to have less compression artifacts (more about this later), but you save it in the format you like.

Here is our duplicate image we will be watermarking....

Let's add a new layer above the original image layer....

Go to the bottom of the layer dialog window and click this button:

or go to Layer Menu > New Layer to add a new layer.

Your layer dialog window should look something like this:

Step 5

With the new layer highlighted, open the Pattern Dialog window and click on your pattern and, while clicking, drag it over to your image like so:

The result should resemble this:

Step 6

Now, let's lower the opacity of the pattern layer. What you want to do is reduce the opacity so that the pattern is no longer visible. Zoom in on the image and experiment with various opacity settings, but DON'T set it to absolute zero. I'm using 2.0 as shown below, but yours might be as high as 10. EXPERIMENT.

Here's my result:

When you're happy with the results, save your image. THIS IS THE IMAGE YOU WILL BE POSTING TO THE WEB! Remember, I suggest you use a png, but you let your experience be your guide. Obviously pngs won't work for animations, but I'm not sure this technique will cover animations. I'm testing that out.

Step 7 - Unlocking the "Hidden Clue"

Alright, you've posted your watermarked image to the web and the unknowing thief has swiped the image and claimed it as his (or her's - yes, sad, but true, there are female thiefs! Shocker

) The matter has been brought to your attention and you've pointed it out the webmaster hosting the stolen image. The webmaster feels your pain, but wants proof.

Here's where the original comes in handy and this is how you use it.....

Open up the watermarked image or the original, doesn't really matter which one. Then, go to File Menu > Open as Layer....(Ctrl+Alt+O)

and choose the the other image. For example, if you opened the watermarked image first, choose the original image in the Open as Layer menu option.

Highlight the Upper Layer as shown in the attached and set its Blend Mode to "Grain Extract"

Here's my result:

You can also try setting the upper layer's blend mode to "Divide":

Result:

You should now be able to see the hidden watermark clearly within the image. Merging both those layers and sharpening the image (Filters Menu > Enhance > Sharpen) a few times may also enhance your results.

And there you have it. A simple, yet effective way to add a hidden watermark to your image and unlock the key for proof.

Is this system perfect? Nope. Far from it. But, hopefully it will be helpful to you in providing some level of protection to your valuables.

Things to consider:

I recommended saving your images as pngs. In my tests, saving them a jpgs really introduces a lot of compression artifacts. The artifacts may destroy your hidden watermark to a considerable level. Also, a thief may take your watermarked image that you saved as a png and covert it to a jpg, possibly destroying your proof. Keep that in mind.
Ways to improve the ability of these steps: try different fonts, font sizes, colors in your patterns. Make up several styles and test them as pngs and jpgs to see how they fair against your different artwork. Dark-colored art may work better with a different pattern than light-colored work, etc.
Other blend modes may work, but these 2 seem to be the best.

As far as the Which is Which image I posted earlier....well, here's the key....

I think from both examples, it's clear that the image on the right was the watermarked image!

Well, I hope this tutorial has proved valuable to you. Let me know if you have any questions or need additional information.

Happy GIMPing!

Art