Re: Freeviewing

[P3D Jeremy Hinton <Jeremy.Hinton@xxxxxxxxxxxxx>]

A very late contribution to the thread on Freeviewing: why some folks can 
do it, others cant, why cross or parallel, and why some see size reduction 
and others don't.....


When we view objects normally, the eyes do two things (ok, they do others, 
but only these two are relevant to why we can or cannot freeview, and how 
we can achieve it)to cope with differences in distance: 

1)      they converge (to look at close objects), and 

2)      they focus (accommodate). 

Normally these two functions are more or less linked, because real objects 
require that both eyes be directed at them, and 
focussed on them. Very generally speaking, if you are long-sighted, you 
focus beyond the point where your eyes converge, and if you 
are short sighted you focus a little nearer than that point.

In freeviewing, you need to seperate convergence and accommodation - for 
parallel freeviewing you 'converge' more or less to infinity, but need to 
focus on the real distance of the stereo pair. 
ie. you focus closer than your convergence would normally expect. For cross 
eyed freeviewing, you converge to approximately half the 
real distance of the stereo pair, but still need to focus at the real 
distance.
ie. you focus further than your convergence would normally expect.

SO... near-sighted folks generally find it easier to parallel free-view.
far-sighted folks generally find it easier to cross-eye view.
Even more generally, we become more far-sighted with age, so older folks 
may prefer cross-eyed, but as our ability to accommodate is also declining 
this may not be a good guide.

Some folks find it difficult to divorce accommodation from convergence 
(it's not natural?). Practice. 

Oh, convergence and accomodation are also cue to distance, so whether you 
freeview cross-eyed or parallel affects the apperent size of the stereo 
pair you are looking at. (looks bigger in parallel viewing, minified in 
cross-eyed viewing)

If the centres of the image pair are seperated by a distance significantly 
greater than that of your eyes, cross eyed viewing is of course the only 
way of freeviewing.


(please note the 'generally','most', etc. The world is full of exceptions, 
Except me, I'm normal.)

I could go on, but perhaps I'd better quit.


Cheers


Jeremy


------------------------------