maximum tolerable positive parallax

[Bruce Keefe]

Hi Guys,

I'm currently trying to work out what the maximum tolerable positive parallax is
for our display.

I should mention that we are running a Psychology experiment and intend to display the footage only to one person in order to achieve an orthostereoscopic projection. We will not be displaying any negative parallax, only 0 and positive parallax. The observer will be viewing the screen from a distance of 4 metres. The stereoscopic window will be approximately 2.44 m by 3.5 m.

So far my understanding of how to compute the maximum positive parallax is as follows:

1.The maximum on screen positive parallax should not exceed the human inter ocular
distance to avoid divergence.

2.Excessive vergence/accommodation conflicts may result in fatigue and/or the
images becoming difficult to fuse. This is probably less of a problem in our set
up because accommodation will be at optical infinity and vergence will be near
to it.

My problem is that there does not seem to be a hard and fast rule for computing
the maximum positive parallax – different people use different rules. Lipton,
for example, gives a rule for negative (max parallax 3% of screen width), but
not positive parallax, which other people then go on to apply for positive
parallax.

I'd like to try and attack this from a first principles direction, i.e. how do
we avoid fatigue/eye strain/diplopia considering the limits of the visual
system?

For example:
If we take the two degree rule (max 2 degrees separation between accommodation and vergence) and do some quick calculations (for an IPD/IAD of 63 mm) we see that for viewing distances greater than 1.8 meters we should not encounter any problems for objects viewed with positive parallax. That is to say beyond 1.8 meters any accomodation/vergence conflicts should not result in fatigue or images that are hard to fuse because they are less than 2 degrees. That's great for us, however I'd be surprised if this were truly the case given that both sources (accommodation/vergence) are still informative from 2 to 3 meters and perhaps beyond, meaning that conflicts may still have an effect. I guess I'd really like to see the reasoning behind the 2 degree rule laid out, perhaps it's a good rule of thumb but doesn't hold for all situations?

The only reference I've found that looks at this is a SPIE paper by Woods et al. (1993) 'Image Distortions in Stereoscopic Video Systems' which shows data for a viewing distance of .8m. It would be great to see if there is any empirical data for viewing distances greater than this.

If anyone can point me in the direction of any references that might help me
answer this question, I'd be very grateful :)


Thanks

Bruce