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How do 3D movies work?

  1. Nov 2, 2009 #1
    Just some mount ago I saw 3D movies first time and I was pretty amazed. By googling I could not fully understand how such effect is done, why do I have this feeling that things are closer or further away and I could look around them.
  2. jcsd
  3. Nov 2, 2009 #2


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    You have two eyes, each one sees the world from a different perspective and your brain constructs a depth model by comparing the images that go to each eye.

    What 3D movies do is project using two projectors and give you polarized lenses so each eye is only watching one of the projectors. Because they are projecting a different image onto each eye instead of a single image on both eyes they are able to control your depth perception.
  4. Nov 2, 2009 #3
  5. Nov 3, 2009 #4


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    I learned about the cross-eyed view method from Starlog magazine some 20-30 years ago. It's my secret weapon for solving "what's different" puzzles. Those are the ones where 2 almost identical pictures are presented, with a dozen or so subtle differences, such as a flower missing from a window box or whatnot. If you go cross-eyed while looking at them side-by-side, you see a composite image between them. Once your focus is locked in, you can look around the new image just as if it was real. Anything that's different, such as the missing flower, 'glows'. The particular object is semi-transparent because you see it with only one eye. Don't mention this to anyone; as I said, it's a secret.

  6. Nov 3, 2009 #5


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    A few more details, the quality of the effect is dependent upon the filtering method. Usually this is done by polarization, like how sunglasses work. The light is linearly polarized, one eye is horizontally polarized and the other is vertically. Then the lens of the 3D glasses are polarizing filters. However, this has two disadvantages. The first is that linear polarized filters remove 50% of the light incident on the filter. This means that a movie theater needs to have a much brighter lamp in its projectors to compensate for the filtering. The second disadvantage is that the effectiveness of the filter is dependent on the orientation of the filter with respect to the screen. If you tilt your head sideways, the polarization of the filters is now different with respect to the light reflected by the screen and now the left image bleeds into the right eye and vice-versa. This creates a ghosting effect.

    The better 3D theaters now use circularly polarized light. This polarization does not experience the reduction in light intensity for light passing through the filters nor is it affected by the rotation of the filters. So you do not get the ghosting effects or the loss in light intensity.

    Another form of filtering that I have experienced uses refresh rates. For example, the image on the screen is updated at 60 Hz, but the image refreshes with the image meant for opposing eyes. That is for 1/60 of a second we see the left image, then the right image is displayed for 1/60 of a second, and so on. The glasses are synched to the refresh rate and turn on and off the appropriate eye to match with the screen. The problem with this is that the glasses need to be an active device, making it more complex, bulky and requiring a power source. It was a neat demonstration and I think this was before they figured out the circularly polarized 3D theaters.
  7. Nov 3, 2009 #6


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    I wonder if anyone else has encountered this problem. I'm severely acrophobic (which might be a bit strange for a pilot), and my first encounter with both 3-D and I-Max was in Las Vegas when I was down for a pool tournament. The tiers were extremely steep, and I had to ask for seating in the handicap section since there was a relatively wide space in front of the seats. Then the whole fence (steel pipe framework with aircraft cable stringers) dropped down into our laps like the safety bars on a roller coaster. I'm pretty sure that my fingers left dents in that damned pipe.
    I figured that I would be fine, since the film was about life on a space colony. My acrophobia just doesn't exist in an aircraft, and by extrapolation it shouldn't in an orbital station either. So... the first friggin' scene was a gantry shot looking down on a shuttle orbiter on the launch pad. That's when the dents occurred. Luckily, I held short of having to change my pants.
    Anyhow, to the point. It was a great movie, and I loved it, but I had one bitching headache during the whole thing. Long after it was over, I figured out why. I was trying to focus my vision on things that the camera didn't have in focus. For instance, the actress in the foreground tending to her plants was the focal item of the camera, but I was trying to see what was behind her. I went six kinds of cross-eyed during the attempt, and it didn't happen.
    Luckily, they now have a couple of I-Max theatres in my area, which have regular seating.
    Anyhoo... I was just wondering if anyone else has had the focus problem.
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