# The Naked Scientists Forum

### Author Topic: Why don't objects appear doubled when viewing in 3D?  (Read 1949 times)

#### martinnorth

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##### Why don't objects appear doubled when viewing in 3D?
« on: 14/01/2010 00:15:31 »
Hi,

I have a question that relates to human vision and perception of images in 3d using stereoscopic techniques.

When I (and I assume everyone else) shift my depth of focus, objects that are now not being focussed on diverge, and I see them double. E.g if I hold out my thumb, and then focus on something across the room, I see 2 thumbs.

Does this effect have a name?

I've also observed that this effect doesn't seem to occur when watching a 3d movie. Objects behind or in front of the object I'm focusing on don't appear doubled.

Has anyone else observed this to be the case? If so, could anyone theorise as to why? I've been contemplating this for some time and I can't think of a reason.

Martin
« Last Edit: 14/01/2010 04:04:17 by martinnorth »

#### martinnorth

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• Posts: 3
##### Why don't objects appear doubled when viewing in 3D?
« Reply #1 on: 18/01/2010 00:17:33 »
I saw Avatar again on the weekend, and as far as I was able to observe (not easy to perform detailed observations in a cinema) parallax definitely doesn’t occur when watching stereoscopic images.

This has me intrigued.

As an example, imagine 3 objects, A, B and C, with A closest to the viewer, and C furthest away. The viewer focuses on object B. Due to the slight difference in viewing angle of each eye, both objects A and C will be “doubled”. If the viewer shifts focus to object A, both B and C will “doubled” with the apparent distance being greater for object C, as it is further away from the focal point.

Now imagine representing this scene stereoscopically. Two images are taken from slightly different positions, converging on object B. In the resulting images, the image of object C will be diverged, in that in the left hand image, the object will be further to the left, in the right hand image further to the right. The opposite will be true for object A, as it is in front of the object in focus.

These images are then presented to a viewer, one to each eye, to produce a 3D effect. The question is, will all 3 objects be represented clearly or will A and C be “doubled”? When the viewer changes focus to object A as above, will B and C be “doubled”?

From my experience watching Avatar, the answer is no to both. Then question then becomes, why?

My guess is that it comes down to the way the brain is interpreting the images, but why are they being interpreted differently at all? In terms of the brain using parallax to judge distance, the stereoscopic images should be the same as what the viewer would see in the same situation.

My theory is that there is another visual queue involved that tells the brain that it is not looking at real objects. One idea is that it relates to the process of adjusting the focus of the lens of each eye. This doesn’t occur in stereoscopic viewing, and maybe this difference is sufficient to tell the brain to interpret what it is seeing differently.

With all of the attention on 3D at the moment, I’d be surprised if this wasn’t an area of research, but I haven’t been able to find any additional info.

Does anyone have any alternative theories or observations? Does anyone have a 3D platform they could use to verify what I’ve observed?

Martin

#### martinnorth

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• Posts: 3
##### Why don't objects appear doubled when viewing in 3D?
« Reply #2 on: 01/02/2010 01:20:30 »
No one else has any thoughts on this?

Martin

#### The Naked Scientists Forum

##### Why don't objects appear doubled when viewing in 3D?
« Reply #2 on: 01/02/2010 01:20:30 »