Bringing stereoscopic 3D to everyone sitting in the cinema
A recent trip to the movie theater has sparked a small obsession with 3D images in my second grader. Or, more accurately, an obsession with the polarized glasses the theater handed out. She’s been wearing them around the house, I think searching for images that would magically pop-out at her thanks to… well, the glasses? She did find that my LCD monitor’s colors would shift depending on the angle of the lenses, but eventually realized that there was more to simulating 3D than simply wearing ineffective sunglasses.
Two views for three dimensions
To get what the glasses did in the movie theater, it helps to know what our eyes do to see actual 3D space. Technically, science can’t put the full story together yet. Many ideas have been offered about exactly what your brain does with the information from your eyes, and confusingly they all seem to have a grain of truth to them, even if it conflicts with other explanations. Basically, our eyes take in two distinct but similar images that get processed by our brain into a single “image” with three dimensional depth to it. How that gets put together is where things get a little squishy, since tests have shown that our brain weighs edges of objects it can see, edges it can’t see but can imagine, and then maybe fully 2D images of the world for other clues as well. It seems that making the most of our binocular vision has been enough of an evolutionary advantage that we have more than one system at work parsing all that visual data.
Fortunately for movie-goers, glasses manufacturers don’t need to know the exact neurological workflow to trick our brain into perceiving depth. As long as as two slightly different images can be served to our eyes we can get the desired effect. Objects far away should look more or less the same to both eyes, but as you’ll see if holding something next to your face, objects close to you are seen from different angles. The first stereoscopes, Viewmaster toys and virtual reality goggles all create this effect by literally showing a new image directly to each eye. But that only works if the images are right on your face, so movie theaters have to employ some other tactics.
The trick of 3D glasses is to somehow show both angles of an image in the same space. The old red-cyan glasses that first defined 3D movies did this by drawing part of an image in red, part in cyan, and then using colored lenses to hide the opposite color from each eye. So one eye sees the image drawn in red at one angle, and the opposite eye sees how it was drawn in cyan. The effect works, but only if you’re happy to basically omit realistic color.
More modern glasses swap filtering with color for filtering with polarization. Polarized lenses block light that’s not in alignment with the tiny “blinds” embedded in the lens. So for example, your right eye will receive the image from light moving on a horizontal alignment, while the left will see light waves on a vertical alignment. If you take your glasses off during a 3D movie, you’ll see both versions of the image in both eyes at once, reducing the depth to a flat, fuzzy picture. If you’ve used polarized sunglasses, you’ve probably noticed how tilting your head changes what light gets through the lens, and so many movie theaters actually use a circular polarization to allow viewers to wiggle around a bit more.
There are other ways to see 3D. Nintendo had a fair amount of success with the 3DS, which offered two images on the same screen, although that was limited to a specific viewing angle for a single user to work. If none of these options seem fun, wait a bit longer and we might be able to be treated to 3D images created by lasers aimed directly at our eyeballs. For now I hope my daughter will just stick to playing with polarized lenses and maybe some Google Cardboard.
Source: Two Eyes, Two Views: Your Brain and Depth Perception by Vilayanur S. Ramachandran and Diane Rogers-Ramachandran, Scientific American