22 Dec 2017

Star Wars: The Last Jedi, and 3D

The Last Jedi is a fun movie, packed with science fiction, lots of it more fiction than science. (I kept wondering how people were able to open hatches into deep space with no oxygen tanks and no protective gear, and still survive). Suspend belief and enjoy the story.

And also enjoy the 3D picture. It's very cool. It's also only the tip of the iceberg for 3D perception.

How do 3D movies work? 

One way of perceiving depth is using our stereoscopic vision. Our eyes are about five centimeters apart, so the view from each is slightly different. Our brains are smart enough to integrate those two views and figure out, using triangulation, how far away objects are.

Our brilliant brains automatically do the math to figure out that the orange is closer than the apple

So to get stereoscopic vision in a movie, the movie has to be shot using two cameras, side by side. Because our eyes are about 6 cm (2.5 inches) apart, that's the optimal distance that the two camera lenses should be apart, to make the movie look realistic for humans. If the lenses are too far apart, the world will look very small; if the lenses are too close, objects will look gigantic. Some more subtlety about the cameras: they won't normally be parallel to each other, but will be turned slightly towards each other. The "plane of convergence" is where the two cameras are focused to meet. Objects closer to that plane will "pop" off the screen.
Now the trick is to get the left hand camera image to the viewer's left eye and the right hand image to the viewer's right eye. Both movies are projected onto the screen, one with a vertical and one with a horizontal polarizing filter. You can think of these polarizing filters as only allowing horizontally or vertically vibrating light to pass through. The movie goer wears glasses with lenses which are also polarized - one horizontal and one vertical. So each lens only admits light from one of the two projectors. And Bingo! The left eye only sees the images from the left hand camera and the right eye only the images from the right hand camera.

Earlier versions of the technology used red and green filters to do the job, but that messed up the colours a little, so the polarizing filters work much better.

Now what if you want to watch The Last Jedi with your pet squirrel? Will he be able to enjoy the movie if you make him a very small pair of polarized glasses. Sadly, it turns out not.
Eastern Grey Squirrel. Photo by BirdPhotos.com via Wikimedia Commons.
Not all animals have the gift of stereoscopic vision. If your eyes don't both face front, you have to make other arrangements for depth perception. So 3D movies work for us apes, and probably for wookiees and porgs, but not for squirrels, chameleons or most birds. One strategy that birds and squirrels use is to bob their heads up and down. By moving your eye, objects close to your eye seem to move and objects far away seem to be static. (Think of looking sideways out of a car or train window). So based on how much objects seem to move, a bobbing bird brain will perceive depth.

There are other cues for human distance perception, beyond stereoscopic vision. Moving objects coming toward you appear bigger. This may not seem like a hugely accurate way to measure distance, but it can be, with some practice. Mansoor Ali Khan (also known as the Nawab of Pataudi) was a brilliant cricket batsman. At the age of 20, when he was already a star player, he had a car accident which essentially destroyed the vision in his right eye. Six months later he had learned to play with only one eye and represented India in international matches against England. To understand the depth perception problem faced by a cricketer, you should know that the size of ball, the speed and distance to the bat is almost exactly the same as for baseball. That means less than half a second from ball release to bat. You need awfully good depth perception to make contact! Khan could only have been using the apparent size of the ball to figure out the distance.

Not useful for baseball or cricket, but another depth perception clue is "distance haze". Because light from distant objects has to travel through a lot of atmosphere, it gets scattered and less light reaches the eye. So distant objects appear less sharp than close objects. Photographers sometimes sharpen their pictures by increasing the contrast of distant objects.

The future of 3D movies

Scientists are working on systems to show 3D movies without glasses. The technique is to project two pictures in slices, so that the viewer can position themselves to see an appropriate picture in each eye. This is a huge challenge for practical viewing, when you have hundreds of viewers, each in a different position in the theatre. But teams are working with systems of lenses and mirrors in front of the screen, and have already had some limited success with low resolution pictures and a small number of viewers.

Will they be successful? Perhaps. If the tiny Resistance led by Leia and Rey can survive against the powerful First Order, anything's possible.

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