KONDO's Stereogram Workshop

Why do they look 3D?

1. The wonderful picture which you can see invisible objects

A shape and a word are hidden in fig.1. Can't you see them? Don't give up, and watch the picture carefully. Imagining that a shape and a word are hidden, see far away vacantly. Then 3D image will appear... Hum, you can't see them yet? OK, read this text and you will be able to see them.
Pictures like fig.1 are called "stereograms". In this text, I would like to explain why stereograms look 3D.


fig.1 Stereogram "Eat it!"

2. The mechanism that human beings get a 3D effect

How do human beings get a 3D effect? Main factors are given on table.1.

factor explanation
focal distance It is identical to focus mechanism of a camera. A lens of an eye is thickened to see a nearer object.
both eyes parallax Triangular surveying from both eyes. Both eyes are let closer to see a nearer object.
size of an object A big object feels nearer.
moving speed of an object The object which crosses faster feels nearer.

table.1 factor of 3D effect

Here the focal distance and both eyes parallax are absolute values which was measured by highly efficient sensor in one's eyes. On the other hand, since object size and moving speed confirm to relative comparison with an other, a 3D effect can be deceived easily like fig.2. But the 3D effect got by fig.2 is not so real.


fig.2 the picture which the left man looks far

Since stereograms deceive both eyes parallax which is not usually mistaken, the 3D effect on stereograms is very real and even the illusion as if you peep in the world of a strange dimension is given.

3. Theory of stereograms

When you see an object by both eyes, you make your eyes closer to see the near object like fig.3(a). Then, like fig.3(b), two same objects are placed to be seen by only a one-sided eye respectively, you will recognize only one object and it looks more far than a real position. Some kind of stereograms which require spectacles have some mechanism in spectacles and so on to let a left and a right eye see different objects.


fig.3 position recognition by both eyes parallax

Then, what mechanism do the stereograms like fig.1 which can be seen without spectacles have? Please look hard at fig.1 once more, and you can find similar patterned repetitions in the horizontal direction. This is a key point.


fig.4 a very simple stereogram

Fig.4 is a very simple example of stereogram. The circles in an upper row look more far (or nearer, in another point of view) than screen position and have same depth. The circles in a middle row have a depth different from an upper row. The circles in a lower row have different depths which are gradually changed. Even such a simple picture looks 3D! Though right and left eyes commonly see the same circle, when you see them by a distant eye or cross-eye, right and left eyes misunderstand and slip in one piece of circle in the horizontal direction. At this time they look 3D.


fig.5 theory

Fig.5 explains why the circles in a lower row of fig.4 look 3D. By repeating similar pattern in the horizontal direction in this way, both eyes are deceived and a 3D effect is produced. And by changing interval of pattern, depths are changed.
Note that fig.5 explains the case you see the circles by a distant eye and the circles look more far. In this case, in order to parallel a glance from ordinariness, it is called with "parallel method". When you see the circles by cross-eye on the contrary, the circles look nearer and it is called with "crossing method".

4. Let's try

Now you understand the theory of stereogram, let's try to see them! Imagine that there is an object in more far than a picture, and see it by a distant eye. Even though then a focus becomes dim, it is all right. If a 3D image can be seen, it is in focus naturally. Moreover, when your face is reflected in the screen, it is also a good way to focus on it.
KONDO's Stereogram Workshop exhibits my original works of stereograms, so please see them. In addition, all stereograms of mine are made in "parallel method", but they are enjoyable enough in "crossing method".
Now, can you see them?

This text was written by Kazuhiko Kondo.
copyright (c) 1994-95 Kazuhiko Kondo

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Kazuhiko Kondo
Kariya, Aichi, JAPAN

Last Modified: 14-May-99