At one stage there was a program which ran on a PC to produce these pictures.
There was also an article in a Dr Dobbs journal about 5 years ago on the theory
behind them. It may have been that article that had a link to the program.

the pictures can also be in colour, and the article had a picture produced by
Sony to illustrate the effect.

> 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)?