In order to downsize an 8×8 pixel picture to a 4×4 pixel picture,
Starting with an 8×8 pixel picture,
we build a new one, using four downsized pictures.
In the downsized pictures, we may use the same pixels of the original
- but then we lose information!
Better: we use for the four smaller pictures different pixels.
In this way, we will not lose information!
- all pixels reappear (but in different order): they are permuted.