3D – Present A Different Image To Each Eye – Simple, Right? Wrong!

Those of us with “normal” vision see the world with our two eyes in 3D all of the time. We see the space between things and perceive distance, size, texture, etc.

It’s different when you go to a 3D movie or look at one of my autostereoscopic photographs because you are looking at a flat surface and perceive 3D by way of an optical illusion. There are many subtle differences (and many not so subtle) between normal “seeing” and watching a 3D movie that are important to understand and consider. The biggest difference is that for normal viewing the 3D is “real”. You see an object in front of another object and you perceive the distance. When you reach out to touch the object, that perception is verified. A 3D movie on the other hand is created with an optical illusion. It is made possible because our eyes/brain has an amazing capability to decouple focus from convergence and see and perceive the illusion as if it were real.

Some of you are asking, “What does decouple focus from convergence mean?“.

For normal viewing our eyes focus and converge on the same point in space (the thing we are looking at). Just like separate camera lenses, each eye focuses on an object independently. Then our brain processes those two retinal images into a single image with depth. Because the focusing and merging is done separately, we can perform a trick whereby we fool the brain into processing retinal images that converge at a different point from focus. The only thing the brain cares about is the alignment, size and similarity of the images on each retina. It doesn’t matter what the focusing distance is. Our brain just totally disregards this disparity… or does it? Since most people seem to be able to perceive 3D looking at 3D movies, we just make that assumption. My guess is that our brain does enable our perception of focus distance but that we suppress the conflict that happens with a 3D movie in the same way we suppress other perception conflicts (think flying in an airplane for example where our inner ear conflicts with what we see). Perhaps some of us aren’t able to suppress this conflict as easily and manifest some sort of discomfort with the experience; like getting a headache or feeling nausea.

However, it is generally thought of as a “given” that this isn’t a big deal and that perception conflicts occur in nature and the brain just “handles it”. I think that is probably true – but I’m not a scientist or doctor and it would be nice to read that my opinion has some basis in true science.  Perhaps it does and someone will comment?

You still might be wondering how a 3D movie decouples focus from convergence. The fact is that it has to do that. The motion picture screen has to be the point of focus at all times. That is the source of the reflected light and the focus point of the projector in the back of the theater. The left eye and right eye images will be offset from each other based upon whether the objects depicted are in front of the screen or going into the screen. The eyes converge or diverge to align the objects on screen but the point of focus is always the screen surface. At this point, I just have to jump up and down and say THAT IS PRETTY AMAZING!!!  Our brain is really incredible and adaptable in a way that we don’t even have to think about.

Stay tuned for more… coming soon ;^)



Filed under 3D, 3D Motion Picture, S3D, stereovision

3 responses to “3D – Present A Different Image To Each Eye – Simple, Right? Wrong!

  1. joel kollin


    I just started reading this blog so I apologize if I am discussing something that’s been covered before.

    I think it is true that accommodation (focus) can be an important cue for near objects and/or close viewing of a display. But in a typical 3D movie where the viewer is great than 3m/10ft from the screen I think its pretty negligible. At least for movies that aren’t overdoing the “coming at ya” gag.

    The eye has a fairly small pupil, so if you figure there is at least 0.5 diopter of depth-of-field – that would mean everything between 2m and infinity is pretty much in focus.

    More amazing to me is that people close to your autostereo pics can suppress the cue conflict. I look forward to seeing your work in person, but I suspect its easier to do with autostereo given correct motion parallax.

    I am doing research on this now, and will let you and the rest of the community know when I have some concrete results to share.

    • Joel,
      I think the more interesting piece of the story is how the brain adapts and ignores perception conflicts and gives priority to cues that the brain can more easily interpret – even if they “aren’t real” as in the case of 3D imagery. Scientists more and more are recognizing and appreciating the brain’s plasticity and ever adapting capabilities to changing stimulus at any age. With regards to vision, I find it amazing that people with strabismus “adapt” and are able to function WITHOUT stereopsis vision. Many even disregard the notion that anything is wrong with them and cling to how they perceive the world as “normal” for them. It is the same for people with a fear of heights and other perception issues. Even though they might understand that their perceptions aren’t “normal” they accept them as “normal” for them and resist “treatment”. I find that fascinating – the reluctance to change perceptions. And of course, in some people the opposite is true. They constantly seek to modify their perception of the world.

      At any rate, I believe it is the brain’s adaptation and plasticity that makes viewing stereoscopic and autostereoscopic imagery possible. The nice thing about autostereoscopic imagery is that there are more complementary 3D cues than with stereoscopic imagery. You don’t need “artificial” eyewear and as you move your angle of view looking at the image the perspective changes. I believe that’s why several people have told me that they “see” more 3D in my photographys than they “see” at a 3D movie or when viewing anaglyphs.

      Finally, it is interesting to note that Dr. Barry told me that motion parallax wasn’t something she experienced as a strabismic to any great extent. It was only after she achieved stereopsis that she “discovered” motion parallax and became greatly aware of it.

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