#1412 How does a 3D movie work?

How does a 3D movie work? A 3D movie works by having two very slightly different versions of the movie that are filmed a tiny distance apart.

We, like most predators, have our eyes at the front of our faces, pointing forwards. Most prey animals have their eyes at the side of the head. The advantage of having side facing eyes is that it gives a much wider field of view and these animals can see if a predator is creeping up on them. Some animals have an almost 360°field of view, which is a great advantage if you spend your life eating plants and trying not to be eaten. However, having your eyes at the side of the head doesn’t allow you to calculate distance or speed. Most predators have forward facing eyes because they have sacrificed the need to see what is behind them to being able to tell how far away their prey is and how fast it is moving. However, it must be said that not all herbivores have their eyes at the sides of their heads. Primates are generally herbivores, but they have forward facing eyes. This is primarily because they need to be able to just the distance between trees and branches so that they don’t miss. There is also another theory that primates have forward facing eyes so that they can look “through” the leaves. If you hold up your finger and focus on it, you cannot see behind. If you shift your focus to what is behind your finger, the finger becomes almost transparent and you can see “through” it. This is called “X-ray vision” and it might help primates to see through leaf clutter.

We can sense depth and distance because of the gap between our eyes. There is roughly 5 cm between your left eye and your right eye, which means you see two very slightly different images. You can obviously test this by picking up a pen and looking at it through your left eye and then through your right eye. The pen moves. This is what lets the brain work out how far away something is. The brain uses four things to calculate the distance: the retinal disparity, parallax, convergence, and everything you have learned about the world. Retinal disparity is the slight difference in the two images the eyes see. If there is a big gap between the two images, they are close to you and if there is a small gap, they are far away. Parallax is how the objects move in relation to the things behind them. If you hold your pen up and look through your left and your right eye, the pen will jump from side to side in relation to the shelf on the wall. If you put the pen in front of the shelf and do the same thing, the pen won’t move as much. Convergence is how much your eyes have to move inwards to look at something. The closer it is, the more your eyes will move in. And the most important thing is a lifetime of experience judging distances. Your brain has a pretty good idea of how big certain things should be. That is why optical illusions using perspective fool us.

A 3D movie makes use of this to trick us into sensing depth in a 2D screen. The movie is filmed with a camera that has two lenses, each about 5 cm apart, filming separately. This makes two versions of the movie from a slightly different perspective. The idea is that when the two movies are played back, we will see the one filmed with the right camera in our right eye and the one filmed with the left camera in our left eye. The problem is how to make sure the one eye only sees the film intended for it?

There are three ways to do this. The first, and probably the most famous, is to use different colored lenses in glasses. They usually have a green lens and a red lens. The two movie streams are colored either red or green. When they are played, the green movie is blocked by the green lens and the red movie is blocked by the red lens, so each eye only sees the one movie. The problem with this is the quality is low and the movie will not have perfect colors. The second way of seeing a 3D movie is with polarized glasses. The two separate movies are played onto the same screen from two movie projectors. Each projector has a polarizing filter over its lens. A polarizing filter can change the direction the light travels in. One movie is made to go clockwise and the other anticlockwise. Customers then wear glasses that have two polarized lenses in them. Each lens is polarized so as to only let in the light that is traveling in the right direction. Amazing. And the third way is to use active shutter glasses. These glasses have a shutter over each eye. The two streams of the 3D movie are played interlaced. Every frame is for a different eye. Left eye, right eye, left eye, right eye, and so on. The shutters in the glasses are timed to block the opposite eye, so you only see the one viewpoint with each eye. They alternate so fast that you cannot tell. These glasses are expensive and they have moving parts.

Other than in theme parks, where they use polarizing lenses, 3D movies are probably on the way out. They are losing popularity. One of the problems is that they make a lot of people sick. Just like motion sickness, where the signals from your eyes and your ears don’t match, with 3D glasses, the signals between your eyes and brain don’t match. The image tells your brain that something is moving closer towards you, but your eyeballs don’t need to turn to look at it, as they would if the object actually was close to you. This difference can make people nauseous and cause headaches. And this is what I learned today.

Sources

https://en.wikipedia.org/wiki/3D_film

https://www.seavieweyecare.com/blog/how-do-3d-movies-work.html

https://en.wikipedia.org/wiki/Polarization_(waves)

https://panoptes.livejournal.com/88860.html

https://en.wikipedia.org/wiki/Depth_perception

https://www.bbc.com/future/article/20141013-why-do-your-eyes-face-forwards

https://pmc.ncbi.nlm.nih.gov/articles/PMC4901450

Photo by cottonbro studio: https://www.pexels.com/photo/group-of-friends-wearing-3d-glasses-while-watching-a-movie-8263349/