Image depth


Can anyone offer a technical explanation of how a stereo system recreates image depth? Why are some center images behind the speakers, and others in front of the speakers, for example.
Should there be any depth to a mono recording, or should the image be directly in line with the speakers?
cakids
Looking at that picture in Soundstage the speakers are too close to the back wall or window as it may be. That's a very reflective surface to have a speaker in front of.

This seems to be equivalent to moving the speakers farther out into the room. This is usually enhanced with a diffuser panel between the speakers on the front wall.
Interesting read on the topic:

https://ohmspeaker.com/news/are-we-out-of-depth/

https://en.m.wikipedia.org/wiki/Hafler_circuit

I have heard highly customized dealer showrooms set up to deliver realistic imaging depth with appropriate-orchestral recordings, meaning specific player locations in a large 3-d area could be identified exactly, with a 2 channel system, but few if any rooms in people’s homes are designed for that.

Rectangular rooms or anything like that are the limitation. 

Same setup by same dealer at a show in a more conventional room: Not so much....more like all the rest.
I just completed most of my mods/upgrades in my system. The list is too long to describe. 
New high tech  mods will affect sound fq resolution = depth. 
Ck all my YT uploads , espec my latest , as now my sound has depth.  That is fq resolution/instrumental separations.

Depth is real, 
What before was FLAT,,now has depth. 
Yeah cost mea  bunch, but well worth it. 
Most stock components  need mods to acheive depth. 
A amp lab is not going to put in high price/high tech parts, Just not, Never did , never will. 
I find that soundstage size is a direct function of the placement, but imo the equipment/electronics dominate the matter of depth of soundstage. 

One of the variables that influence depth of soundstage is the sense of width. I use the PureAudioProject Trio15 Horn 1 Speakers (see my reviews at Dagogo.com on the variants of this speaker) both Portrait (vertical) and Landscape (horizontal). The soundstage changes dramatically with these two options. But, still, the biggest impact on depth is the electronics and cables. 
I'm not sure this is relevant to your question but it is interesting in how people differ in perception of music and speech. 

http://deutsch.ucsd.edu/psychology/pages.php?i=201#Technical.php