Three Dimensions: What Does That Really Mean?

The holy grail of display technology is to replicate what you see in the real world. This means video playback in 3D — but when it comes to displays, what is 3D anyway?

You don’t need me to tell you how far away we are from succeeding in replicating real life in a video display. Despite all the hype, there are only a couple of different approaches to faking those three-dimensions. Let’s take a look at what they are, and why they can call it 3D, but they’re not fooling us into believing we’re seeing real life… yet.

My Superpower: I See Things Different (But So Do Many Others)

When you share a secret with the world you lose all element of surprise, but this one’s too good to keep under wraps. I have a genuine bona-fide superpower, one that all those comic-book authors would have no doubt incorporated into their characters had they only thought of it.

A strabismus is no barrier to writing, 3D vision not required!

It’s pretty mundane really, no radioactive flies or anything were involved, instead I have a thing called a strabismus. It’s a congenital condition, my left eye looks about 20 degrees to the left of centre, and the superpower it conveys is that I can see over my shoulder to something of what is happening behind me. I should have a cape or something, but sadly our Hackaday overlords’ budgets don’t stretch that far.

In reality having peripheral vision that stretches at its leftward extreme much further than everyone else’s isn’t much use beyond a party trick, but having a strabismus has another more significant effect. Since the two views on the world that my brain could see while it was developing in my very early childhood were not aligned with each other, I don’t have stereoscopic vision. A quick read around the subject reveals that I share this to some extent  with as many of 12% of people, so perhaps quite a few readers will understand. That’s not to say that I and people like me inhabit a 2-dimensional world as if viewing it through a TV screen though. Instead we see in 3D in a different way to people with stereoscopic vision, and as a result the subject of 3D imaging has become something of a personal fascination.

Types of 3D Displays

Stereoscopic Displays | If you think of a 3D display, the chances are you will first imagine a stereoscopic one. Watching Avatar in a cinema with a set of polarised glasses perhaps, or even the vomit-inducing Nintendo Virtual Boy console. Your brain is being tricked into believing it has a 3D object in front of it through the means of each of your eyes being individually fed a view of the same scene but from slightly different viewpoints. It’s easy to make a stereoscopic imaging system for the relatively trifling expense of an extra camera and a bit of display trickery, and delivering the correct image to each eye is a long-ago-solved problem.

A 3D object on a spinning-helix volumetric display we featured back in 2015.

Parallax Displays | There is of course another type of 3D imaging, that which uses parallax to give an impression of 3D. With these images there is an impression of 3D gained as you move around an object. The movement reveals sides of it that were previously hidden, and your brain uses that store of information to infer a 3D shape. This can be as simple as one of those novelty printed images that breakfast cereals used to give away for children with a plastic lenticular lens, it can be a holographic image, or an advanced electronic display system such as a rotating volumetric display or a flat lenticular display as Alex Hornstein demonstrated at our recent Superconference. It’s true to say that if such a display has enough resolution it could also be a stereoscopic one in that a different point of view could be presented to each eye, but since this approach works even when they lack that resolution it’s safe to say that parallax alone conveys enough for the brain to be fooled into seeing in 3D.

Hybrid Approaches | Finally there are displays that combine both stereoscopic and parallax effects to generate a 3D image. Immersive VR using a set of goggles such as one of the Oculus products provide the viewer with a primary stereoscopic image whose parallax changes as the point of view moves. The parallax effect is however not there with the primary aim of providing a 3D image, instead it is part of the effect of moving around the immersive world.

Perhaps All We Need is Parallax

As somebody without stereoscopic vision I have no use for 3D movies or headsets. I do however find myself in a unique position to evaluate the effectiveness of the parallax effect delivered by those display technologies that support it, because I can do so without the distraction of stereoscopy. My perceived 3D is perhaps best imagined as being done in software, I know the mug of tea next to my keyboard is a 3D object because my brain knows from experience that mugs usually are 3-dimensional, and it has learned to interpolate dimensions and distances from that experience supplemented by parallax information delivered as I move my point of view. It’s not perfect, for example I have a problem with fast moving objects such as a thrown ball, and for some reason I often grasp an inch short of drawer handles, but it’s otherwise pretty good. I am told that some people who grow up with stereoscopic vision but who then lose sight in one eye as adults do not successfully evolve this software 3D, it would be interesting to hear from readers with experience in the comments.

Festival Of Britain visitors watch a 3D presentation in 1951. The National Archives UK [OGL]However for “in-brain software 3D vision” to work there has to be real parallax rather than just a semblance. The Oculus presents me with a very confusing image of two jarringly different stereoscopic views, and when I cover one eye to see only one picture I enter a very bizarre world of two dimensions with a few objects popping out in full 3D. These stereoscopic views are a one-size-fits-all viewing experience. If your visual cortex is accustomed to seeing from a different perspective from what you’re presented (and of course everyone’s wetware is slightly different so this will affect everyone) the perfect effect can never quite be reached.

A hackspace acquaintance had a 1st generation Oculus Rift dev kit a few years ago and we all had a try on an early demo set in an office. There was a house of cards on the desk, and for me it was as if I was seeing the room in 2D as if on a TV screen, with an unexpected full-3D house of cards jutting out of the scene.  The parallax effect was probably slightly distorted compared to the real world or maybe it lacked sufficient resolution for me to be able to interpolate 3D from the whole scene, instead for other users the device is almost wholly reliant on the stereoscopic effect. By comparison the parallax-only technologies present a good impression of 3D to my perception. Laser holograms and volumetric displays produce the effect of a real almost graspable object within their space, as do lenticular displays with the strength of the effect being dependent on their resolution.

What this tells me with my unusual experience on the matter is that all the information required for complete 3D display perception is present as much in a parallax based view as it is in a stereoscopic one. And what several generations of the 3D cinema fad since the 1950s should also tell us is that while stereoscopic 3D unquestionably works for those with the ability to perceive it, it consistently fails to become popular while it requires the viewer to wear glasses or headsets. Plus, once you move your head, the lack of parallax data breaks any illusion of depth that had been created.

3D is So Close, Yet So Far Away

We are now several years into the easy availability of VR headsets and headset attachments for mobile phones, yet it’s still extremely rare to encounter someone actually using one. Sure we’ve all given it a try, but even with a significant amount of content now available in stereoscopic form it’s questionable whether they’ve moved beyond the same level of novelty that 3D glasses in the cinema had. To steal a phrase from The X files, I want to believe when it comes to 3D displays, but my eye condition notwithstanding, I think we still have a way to go.

Header image: Matthew Henry [CC0].