Estimated reading time: 3 minutes

In the previous article we mentioned the technique of quickly '”invisible watermark“, a technique that would allow us to prove paternity of a digital photograph. The operation (as already described) is simple. The underlying principle of all is to have a picture “x” (my original photo I just have to keep) and create a picture “and” (the photo that contains the watermark invisible to circulate in internet). To prove to be the author of the photo just to compare a particular procedure by superimposing the two graphics immagni. Magically comparirà il watermark with our name and copyright information.

For our tutorial we will use Gimp and we will take a cue from the page Plugin Registry dedicata al Photo Watermark, but with a difference that will… the difference.

First, open the photo you want to mark. I did some testing with any type of photo, for the tutorial I decided to use a photo with good contrast.

Let us go into menu “File” – “Create” – “Photo Watermark”. I used the following parameters.

Gimp will create an image that contains our watermark.

Now back to the initial photo and click on “Filters” – “Render” – “Photo” – “Watermark”.

It seems that nothing has happened, but if we pay attention to the levels we see that tab was added a level “watermark” with an opacity of 3%.

If you pay attention to the image, however,, you will realize that especially in dark areas (but also in those clear) is visible against the watermark, provate and zoommare. If one of us wants to steal the image it can also remove. At this point I suggest a variation from the short tutorial of the Plugin Registry, we try to lower the opacity to “1”. Even zooming will not notice differences. This last image that we save will be our image “and”. Save and publish your photos on Flickr.

Now we can do the litmus test and see if we have all the tools to prove that they have the authorship of photos.

We open with Gimp image “and”, click on “File” – “Open as Layers” and select the photo “x”, the original. If the two images have not been retouched further should be perfectly superposable.

We select the level “x” (the original) and change the mode of picture “Usual” a “Grain Extract”.

The image will appear as all gray.

We merge the two layers into one and click on “Colors” – “Levels”.

And click on the button “Auto”. The result is immediate.

The “counter” of this procedure is that if a derivative work is made from our photo (variations in color or image degradation), this entire process is no longer useful.