bathing architecture in f78v

[Koen G]

If the walls are supposed to encircle the green water then it's not just a lack of perspective - it's a different perspective. Like Picasso

JKP's example shows well that some visual tricks were used to show bodies of water inside a structure, and while this isn't exactly photorealistic, we understand quite well what is meant.

So I would say that either

1. perspective is used in a way we are not used to
   
2. the image is meant literally, but we do not understand exactly what kind of structure is depicted
   
3. the image is symbolic in some way
   

One possible explanation is that the perspective shows various points of view at once. The green body of water is shown as if we are looking down on it, while the walls are shown from the perspective of someone standing on the ground and seeing the blue water in the "lake" through the windows.

On the other hand, the top edge of the wall seems to form the top edge of the pool as well.
And to complicate matters even further, some of the nymphs have blue streaks on them, perhaps indicating that either the blue or the green was added later, or someone changed his mind halfway through.