Since Wilhelm had password protected his stack I reverse engineered the effect. While coding I tried to make it as fast as possible. Wilhelm at the time did amazing things regarding image manipulation.
There is a selection "tool" which lets you choose the area of interest and a live preview of the resulting mirror. Use it also to generate smaller selections and even one pixel high and a couple of pixels wide etc to get interesting effects. Spoiler: a 1 by 1 pixel selection gives you a monochrome mirror.

It can be quite addictive to generate those mirrors and marvel at the resulting image.
For the stack I used just a very small image to see what happens but it is advised to import a larger image/s to experiment. You can also drag-drop images on the central image to import them. Additionally there is an option to export the resulting pattern in case you want to use it as wallpaper...
Kind regards
Bernd