Artwork by: Stephan Pabst
Anamorphosis is a perspective technique in visual arts. When seen from the usual point of view, it looks like a distorted image. But the image will become clear from a specific and unconventional angle.
The most recognizable examples, that are often used to explain anamorphosis, are road markings.
image source: Metalbac & Farbe
However, when looking at road markings from up close, you can see that the image is stretched along the length of the road, to make it appear normal-shaped from the usual vantage point. This is why road markings are technically not anamorphic illusions, but use an effect called foreshortening.
Oblique anamorphosis (perspective)
Images with oblique anamorphosis need to be viewed from a very unusual angle. A famous example in art is the painting The Ambassadors by Hans Holbein.
See large-scale detailed photo of this painting at the Google Art Project
The distorted image on the floor in front of the ambassadors is an anamorphic optical illusion. If you are reading this on your phone, go ahead, close one eye and view this image with your other eye just above the left bottom corner of your phone, almost holding the phone parallel with your eyes (it is specific and unconventional).
You will see something like this:
Catoptric Anamorphic illusions (mirror)
To create a catoptric anamorphic optical illusion, you need a cylindrical mirror in the middle of an image. The angles of reflection on a curved surface create the correct image in the mirror. An entertaining item is this mirror cup and saucer set:
