Home : Webmonkey : Graphics : Twins Illusion


I posted the partially covered picture to the right on various internet profiles and immediately began receiving question about whether I had a twin. In case you haven't already guessed: no, it's just an illusion.

The illusion involves a few tools and tricks that can be achieved with any decent image editor. For this tutorial, I've used Gimp because it's a pretty good editor and it also happens to be free


You'll need to start with a couple of images that are very similar. I've used two pictures that were taken within minutes of eachother, using a web camera. It's best if the pictures were created at the same time because there is likely to be less lighting discrepancies between the two pictures. If you use pictures taken at different times or different places, you'll soon notice that the colours from the lighting and the position of shadows very quickly give the image away.

Next, you'll need to decide which picture will form the base. I've chosen to use the picture on the left as the base.

You'll need to use the free-form select tool (sometimes know as the free-form marquee tool) to copy the person from the other picture so that they can be pasted into the base picture. When doing this, it is a very good idea to zoom right in so that little shakes of the mouse don't have a huge impact. Another tip is that image editors will often allow you to use another key (usually shift, ctrl or alt) in order to add to or remove from the selection. This means that you don't have to do it all in one long mouse click. It also means that, if you make a mistake, you can go back and subtract it from the selection.

A good idea (especially if you're using very similar images) is to grab a bit of the background as well as the person. That way, you'll have something to guide you when lining everything up in the next step. There is also a trick I do later which relies you having a bit of background.

Now that you've selected the person and some background, if you cut them out of the photo, you'll end up with a ver sharp, jagged line. To smooth the line out a bit, use the "feather" option to blur the edge of the selection a little. How much you feather will depend on the resolution of the picture. My web cam pics are pretty low resolution, so I only feathered by about 3 pixels.

Now we copy the person and paste him into the base picture. I've set the transparency of the new layer to 66% so that I can use the background to line things up properly.

Once everything is lined up properly, it's time to break out the eraser tool. Leaving the pasted image partially transparent will help. In this illusion, the person from behind is reaching in front of the person in front, so we need to erase the person in front's shoulder. As you get closer to the edge, it's a good idea to adjust the transparency of your eraser tool. This will give you a softer more subtle edge.

Things are starting to come together. I'll use the "levels" tool to adjust the colours to match as much as possible. There is a difference because the web cam automatically adjusts its contrast and brightness. I'll then use the eraser to even out that line in the background. Once again, using transparency to make it subtley blend.

There's still a bit of blue on the guy in front's shoulder, so I'll attack that with the eraser (with transparency). There is also a blue glow on the arm that's reaching forward, which is a little bit trickier.

Finally, the final product!