P3D Re: Dark Vergence & Cross-viewing

[Andrew Woods <andrew@xxxxxxxxxxxxxxxxxxxxx>]

George Themelis wrote:
>Thanks Linda and Andrew for pointing this out... I have noticed it with
>my eyes too.  I have a litle bit "exophoria" I think...
>
>>This is certainly true in my personal experience. I have "exophoria"
>>meaning my eyes tend to wander "out" if one of my eyes is blocked, for
>>example if I am sitting behind someone at a theatre and I can only see
>>with one eye, the other eye loses its "fixation". It feels weird.
>>Anyway, I find that I can easily parallel view but I can't cross view at
>>all because my eyes just won't do it! 
>
>I don't understand what you mean by your eyes won't do it Linda.

I think I understand...  I experienced something probably akin to what you 
describe when I watched a 3D movie a while back which had quite a lot of
vertical parallax.  The 3D scene was a room, with a gas lamp set on a table
in the middle of the room.  The room was initially fully lit but the
room lighting gradually reduced until all you could see was just the
gas lamp.  At the beginning of the scene I could fuse the 3D image just
fine - even though there was a lot of vertical parallax.  As the room
lighting dropped I found it harder and harder to fuse the image - until
the point at which I could not fuse the image at all when the scene 
was almost totally black except for the single gas lamp in the middle of 
the scene.

Prof. Ian Howard of Kent University in Ontario, Canada performed some
experiments which revealed that with larger 3D images (that filled more
of your field-of-view) a person's eyes would be more willing to correct
for alignment errors in the image.  In his experiments the alignment error
was rotation.

I see this as akin to a "fusion magnet".  If the viewed image is large, the
fusion magnet will be stronger and therefore be more able to excercise those
eye muscles to obtain image fusion.

In this case I think Linda is only talking about a 2D movie.

>Your eyes will converge, right?  If they converge to a close-by
>point where their extention at the place of the image will be at
>the left image for the right image and the right image for the left
>eye, what is the obstacle in cross viewing?  

What?  I think oranges and apples are possibly being confused here.

Everyone's eyes converge - otherwise they wouldn't be able to fuse
anything in the real world closer than infinity.  (Converge: where the 
optical axes of the eyes incline towards each other - my definition)
On a similar line of thought - in real world viewing a person's eyes
will never diverge (because there is nothing past infinity). (Diverge: where
the optical axes of the eyes branch out in different directions - my 
definition based on the Maquarie Dictionary).

Restating your point to how I think I understand it "When viewing a 
stereoscopic image (e.g. on a PC with shutter glasses), fusing an object 
which projects out of the screen will require the eyes to converge on a 
point out of the screen while the eyes must remain focused on the screen.  
How is this different from cross-viewing?"

It is different from the point that some points in the 3D scene on a
computer screen (viewed with shutter glasses) will be coincident with
the screen surface (and therefore the focus distance will equal the
convergence distance).  This does not happen with free-viewed images.

Also, some people find it hard to view stereoscopic images where the
images come too far out of the screen.  To cross-view a stereo-pair
it may require the person's eyes to converge closer than their eyes can 
comfortably converge.

>Does your conscentration
>break when you try to make the jump in attention from the close-by
>object to the sereo image?  Don't your eyes "lock" or fuse the image?
>Have you practiced the recommended excercises to cross viewing?
>Can you corss your eyes at will?

Linda?


Andrew Woods              http://info.curtin.edu.au/~iwoodsa


24 days to go to the Stereoscopic Displays and Applications conference
      http://info.curtin.edu.au/~iwoodsa/stereoscopic
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