[pluijzer]

I had a book with these patterns once. The last few pages explained how you could make them yourself. Basically you make grid of lines with repeating patterns, like XXYYZZXXYYZZXXYYZZ. Then at certain places you can either make the pattern longer or shorter, like XXYYZZXYYZZXXYYZZ, or XXYYZZXXXYYZZXXYYZZ. One of them makes it appear to come forward, the other backward (forgot which ones). You can remove/add more elements to make them appear further or closer away. I used to make this as a child on graphic paper with colored pencils. You cannot do that anymore with a job and children ;)