From: sweberg@garfield.wes.army.mil (Scott D. Weberg)
Subject: Re: TECH: HMD stereopsis?
Date: Wed, 14 Apr 1993 13:52:08 GMT
Organization: USACE Waterways Experiment Station


In article <1qgflvINN4ct@shelley.u.washington.edu> battlec@cs.rpi.edu  
(Clark Battle) writes:
> 
> 
> Someone Who knows a HELL of a lot more than me about this field than
> me (my masters project advisor) told me that most HMDs do not feed
> seperate images to each eye to truely simulate stereopsis. He said
> that most of them only present the same image to each eye.  Is this
> true?  If so what's the point?  Which ones do present two images (from
> two prps in the vrc system) and which ones dont?  Is this question
> ignorant?

Well, this sounds a little bit wrong to me.  First of all the HMDs do not  
"feed" the images at all (at least not the ones I know about).  It is the  
rendering software that draws the images, and then those images are fed by  
the *software* to the HMD.  The process is basically the same for the VPL  
HMD and Virtual Research's HMD.  So, can the software feed the *same*  
image to both eyes?  Yes, if it is programmed to run that way, but then  
you are not really experiencing a 3-D stereo image.

What the VPL software does, and our software for the BOOM, is to draw the  
same scene from 2 slightly different viewpoints.  The 2 viewpoints are  
maybe an inch or so apart, to represent the distance between your eyes.   
So the actual images being sent to the HMD are slightly different, so that  
they produce a "realistic" stereo image.  As I said, the distance between  
the 2 viewpoints has to represent the distance between the person's eyes  
(the interocular distance), and this is different for every person.  So  
you have to have a calibration routine that will customize it for each  
user.  There are other anomalies that can occur.  For instance, I have  
lazy eye; my left eye is weaker than my right.  This has a definite affect  
on the interocular setting that works for me.  

I don't really understand all of it, and I don't understand this talk  
about your eyes crossing to focus on the image.  When the interocular  
distance is not set correctly, I can cross my eyes to force the 2 images  
to become 1.  But when the distance is set correctly, my eyes relax and I  
see one image - it doesn't feel like they are crossing at all.  That's the  
whole point of being able to calibrate this setting, isn't it?

Anyway, we do send different images to each eye on the systems I have  
worked with.  Now I think there are some cheaper HMDs that only accept one  
image - in other words they don't let you send 2 images into the HMD.  In  
that case, you wouldn't have a true stereo image.

Scott D. Weberg
Computer Scientist
Army Corps of Engineers
Waterways Experiment Station

Opinions are my own, not necessarily those of the ACE.