From: John Wann <john1@HMS01.HMS.UQ.OZ.AU>
Subject: Re: SCI: Visual Cues to Depth Perception

I don't believe that the important issue is what cues can specify 
depth perception, there are a large number, as previously listed, 
and only one or two are necessary in a display to yield a suitable 
percept of some depth.  What is more relevant is the difference 
between cues that yield "absolute" and relative estimates of 
depth.  By absolute I mean a body-scaled indication of where an 
object is placed:

Stereo-cues for instance are "absolute" (scaled by IOD), so that if 
your virtual body is in scale, they specify whether you can reach 
out and pick something up, or whether you have to locomote to 
get it.

Interposition, however, specifies that one (occluded) object lies 
behind another but gives no indication of the absolute depth of 
either**. 

To be useful, relative depth cues may need scaling or calibrating.  
The important issue is whether a VR, which implements a number 
of "standard" cues to object depth, includes enough information 
for the world to be perceived in natural body-scaled terms 
("affordances" for the James Gibson followers).  In systems that 
don't use stereo displays, this may be a particularly important 
consideration.

John Wann
Edinburgh Visual Perception Lab.
_______________________
** Tasks can often be performed using relative cues like  
interposition by comparing the apparant position of a virtual 
hand with the object (is it in front or behind ?), but this is a 
pretty clumsy way to interact.