Imre, just take a look at this easy to learn 3d view image:
http://www.ustr.net/pictures/learn3d.gif
Try to change your eyes point of view and see the 2 red squares as 3
squares, perfectly aligned, then move your eyes slowly down. You will
see all the black squares as usual, but the center black square has
moved deep into the screen.

After you learn how to do that, take a look at this one:
http://www.ustr.net/pictures/image3d.gif (640x480 image)
http://www.ustr.net/pictures/image3d8.gif (800x600 image)
Same technique, try to see the two "WAGNER"s as 3 words one aside the
other, then slowly move your eyes up and you will have a nice surprise.

Observing the black squares disposition at the first image, you will
understand how it is done. The most complex images just use any noisy
signal that repeats as vertical bars and part is shifted to give you the
distance perception.  It is not difficult to produce a program (even in
basic) that can convert *any* low detail image to 3d.  Different color
objects can generate different shifts, so colors at the original image
objects will end up in different distance to the viewer at the shifted
image. This is just a simple trick to the brain "distance meter".  As
much fine grain of the image noise, better resolution at the image, but
very fine grain turns to be a problem to the brain to do the matching of
stereo images, and it looses effect.

Wagner.

You wrote:
Hi,
excuse for trying to connect to this interessant topic, I'd have a
question:

there are a lot of so-called "magic images" - I even saw a book filled
with them - which are actually grey spots. It is said that looking this
pictures with somehow relaxed eyes true images occur. I did never manage
to see such hidden image. How does it work? Does it require some
personal
skill or training (or maybe inborn gifts)?