An autostereogram gives the optical illusion of a three-dimensional image, based on a normal two-dimensional image. Please google on 'autostereogram' or 'magic eye' for more info.

I wondered whether it was possible to make a 3D printed autostereogram. This thingy proofs it can be made and that it works. :-)
If you print the attached thingy in two different colors (by pausing at 1 mm height and changing filament), you'll get a usable sterogram. It is optimized to viewed from 25 - 30 cm.

Note: not everyone is capable of seeing the hidden object in an autostereogram. When in doubt about that, download an arbitrary autostereogram first and try to see it on your monitor.

Anycubic I3 Mega

Yes

Doesn't Matter

0.2

20%

I started with a white filament and paused the printer at layer 6 to change the filament to black.
(Cura offers 'post processing scripts' to help you with that).

I made this as large as possible for my printer. If your printer has a smaller bed, you can rescale this thing. But i wouldn't make it much smaller than, say, 90%

The 'pixels' in my autosterogram are represented as blocks with squared tops and bottoms. Some of these blocks are only touching another block on just one vertical edge. Strictly spoken, this can not be represented decently in an STL file. I understand that it is an ambigious situation when two objects just 'touch' each other on a common point or a common edge. Since they don't share any volume, it's hard to say whether it is one object or two.
If you open this STL file for analysis (for example in meshmixer), it will complain about non-manifold blahblah. Since Cura and my design application had no problem with reading and manipulating this STL file, I didn't put extra effort in removing this non-manifoldinish.
But, maybe your slicer reacts differently.