an stereogram is a 3d perception out of a flat surface
make an stereogram with a volume then your perciven a fourth D
for example i looked double at two pencils overimposing them with my eyes by seeing double
in my 3D world the point of the pencils were aiming away
but in my fourth D perception the pencil was aiming at me
the pencils went away in the 3rd D but come towards in the 4th
