A strange picture whose expression changes when you see it apart "Hybrid Image"

The image above, the face on the left is angry and the face on the right looks normal, but let's leave the computer while watching this picture for a moment. It is unexpected that the left and right faces should be reversed. This seems to be called "Hybrid Images" (hybrid image).

What kind of reason is this way and examples of other photos are as follows.
The basic principle is as follows. It is an example by Flash.

Dr. Angry and Mr. Smile

When it becomes blurred or sharp, the things that can be seen by that will be different, so the image will be reversed. About this, MIT announced research results at SIGGRAPH 2006, and you can see various example images on the following page.

Hybrid Images @ MIT

There are many images like this


Below is an example.

David & Adriana on Flickr - Photo Sharing!

Also, even if you are a near-sighted person you can not see it as if you do not leave far away, removing the glasses. It might be interesting to make an advertisement poster applying this.