Microsoft just announced HoloLens, which “brings high-definition holograms to life in your world.” A little while ago, Google invested heavily in Magic Leap, who, in their own words, “bring magic back into the world.” A bit longer ago, CastAR promised “a magical experience of a 3D, holographic world.” Earlier than that, zSpace started selling displays they used to call “virtual holographic 3D.” Then there is the current trailblazer in mainstream virtual reality, the Oculus Rift, and other, older, VR systems such as CAVEs.
While these things are quite different from a technical point of view, from a user’s point of view, they have a large number of things in common. Wouldn’t it be nice to have a short, handy term that covers them all, has a well-matching connotation in the minds of the “person on the street,” and distinguishes these things from other things that might be similar technically, but have a very different user experience?
How about the term “holographic?”
First off, the words “holography,” “hologram,” and “holographic image” have very precisely defined meanings. According to the dictionary (or Wikipedia or the Center for the Holographic Arts), none of the above things are remotely holographic.
- Holography is a technique, based on coherent (laser) light and interference, to record and view images, that, when presented properly, precisely recreate the three-dimensional visual appearance of recorded real objects that are no longer there.
- A hologram is the imprint on a recording medium (such as photographic film) that is created during holographic recording. The hologram itself is not an image (it doesn’t look anything like the recorded objects), and it is not three-dimensional, either.
- A holographic image is the three-dimensional image that is reconstructed by shining (coherent) light onto a hologram. Some sources, and pretty much everyone who is not a holography expert, refers to the holographic images as “holograms” as well, somewhat confusingly. Based on the strict definition, the hologram is not the floaty 3D thing, but the flat transparent plate behind it (or in front of it).
That’s all nice and precise, but when non-experts think about holograms, they don’t think about lasers and interference, i.e., about the technology that was used to record and view them, but about the experience, i.e., the fact that holograms, or, more precisely, holographic images, are apparently solid three-dimensional objects floating in thin air. In other words, the most remarkable thing about holograms is not how they’re made, but the illusion they create.
And how do holographic images create the illusion of solid three-dimensional objects? That has nothing to do with lasers and interference patterns, but with how the visual system in our brains sees the physical world. When viewing close-by objects, there are six major depth cues that help us perceive three dimensions:
- Perspective foreshortening: farther away objects appear smaller
- Occlusion: nearer objects hide farther objects
- Binocular parallax / stereopsis: left and right eyes see different views of the same objects
- Monocular (motion) parallax: objects shift depending on how far away they are when head is moved
- Convergence: eyes cross when focusing on close objects
- Accommodation: eyes’ lenses change focus depending on objects’ distances
As it turns out, holographic images recreate all six of these cues (yes, even accommodation), perfectly fooling our brains into seeing things that aren’t really there. Based on this and the duck test, wouldn’t it make sense to call display systems that are not based on lasers and interference, but create the same illusion, “holographic” as well, given that they quack like real holographic images?
Let me be bold and make two further allowances for practical reasons: let’s ignore depth cue 6 for the time being, and limit the illusion of solid 3D objects to at least one viewer. Then let me propose the following definition:
A holographic display is a system that creates the visual illusion of solid three-dimensional objects by recreating depth cues 1 through 5 for at least one viewer at a time.
I argue that this definition is useful in the sense outlined above: it differentiates a large class of specific devices that have very similar visual capabilities from other devices that may be based on very similar technologies but look and feel different, while aligning well with the general public’s notion of what a hologram is. Let’s apply the test to a list of existing or proposed devices:
- Reality: covers all possible depth cues for unlimited numbers of viewers at the same time. Clearly super-holographic.
- Real holographic viewing system (hologram plus proper illumination): covers all six depth cues for any number of viewers. Holographic+.
- Volumetric displays: depth cues 1-6 for any number of viewers, but small display volume, and some have problems with cue 2 (occlusion). Still, holographic+.
- Project Vermeer: depth cues 1-6 for any number of viewers, but small display volume/viewing angle. Nonetheless, holographic+.
- CAVE: depth cues 1-5 for a single viewer. Requires eye wear, but the definition (intentionally) says nothing about eye wear. By the way, here’s a video demonstrating depth cues 1, 2, and 4 in a CAVE (cues 3 and 5 are lost in recording with a 2D video camera):
- Head-tracked 3D TV: same as CAVE, but smaller field of view (definition, again intentionally, doesn’t mention that).
- zSpace and HP’s Zr Virtual Reality Display (which appears functionally identical): same as head-tracked 3D TV, but smaller field of view. Still holographic.
- Head-tracked auto-stereoscopic display: covers depth cues 1-5 for a single viewer, does not require head gear. Holographic.
- Oculus Rift DK2: depth cues 1-5 for a single viewer. Obviously requires pretty involved head gear, but holographic.
- CastAR: depth cues 1-5 for a single viewer, and multiple viewers can potentially see the same virtual objects in the same place when wearing their own headsets. Holographic.
- Magic Leap and HoloLens: if these really work as implied by marketing, they will cover at least depth cues 1-5, maybe even 6. Either way, if true then holographic.
- Near-eye light field display: depth cues 1-3 and 5-6, and cue 4 if combined with some positional head tracking mechanism. In that case, holographic; otherwise, not.
- Holovision: covers depth cues 1-6(!) for any number of viewers, but the only virtual object it can show is a flat display screen plus whatever 2D image happens to be on that screen. Squeaks by on a technicality (a flat screen is a three-dimensional object).
- Multi-zone auto-stereoscopic display: provides only a crude approximation of depth cue 4, but can be viewed by more than one person at a time. It’s a borderline case, but I would argue it’s not holographic because severe parallax artifacts break the illusion of solid objects.
- Oculus Rift DK1: misses depth cue 4 due to lack of positional head tracking. Not a holographic display.
- GearVR: same as Oculus Rift DK1.
- 3D TV: no depth cue 4 either; not holographic.
- Desktop 3D monitor: same as 3D TV.
- Head-tracked desktop 2D monitor (TrackIR or similar): has depth cue 4, but misses 3 and 5. Not holographic.
- Two-zone auto-stereoscopic display: misses depth cue 4; not holographic.
- Tupac Shakur at Coachella: just 2D video back-projected onto a transparent screen. Misses depth cues 3, 4, and 5, looks completely flat from the side. Definitely not holographic.
- Will.I.Am on CNN: just green-screen compositing between synchronized cameras. Was completely invisible to Anderson Cooper on-stage, and missed depth cues 3, 4, and 5 when viewed via regular TV. Not even in the same ballpark.
After this list, let me reiterate my reasoning. I have shown many types of holographic displays to a lot of people, and there are two common reactions: “it’s like a hologram!,” or “it’s so much better than X,” where X is some non-holographic display system. The first reaction tells me that the term “holographic” is a good match, and the second tells me that the dividing line between holographic/non-holographic displays is a relevant one. That’s good enough for me.
That said, there are important differences between holographic displays and real holograms. Most currently-existing holographic displays don’t provide for proper accommodation, leading to accommodation/vergence conflict and decoupling, only work for a single viewer, and require some headgear (though, granted, for a large percentage of the population, even reality requires glasses to view).
But, in closing, there’s one difference that’s often brought up that’s actually not a difference: the misconception that holographic images are free-standing. A CAVE, for example, requires several large display screens to prop up the illusion of virtual objects, and I often hear “well, once we have holograms figured out, you won’t need those screens anymore” — but that’s wrong. Even real holographic images need something behind them, namely holograms (in the sense of hologram plates). In a CAVE or head-tracked 3D TV, virtual objects are cut off the moment they leave the pyramid-shaped volume between the viewer’s eyes and the screen(s), and the exact same thing is true for real holograms. If you want a life-size holographic image, you need an at least life-size hologram plate. Holographic projectors, like those familiar from science fiction (“Help me, Obi-Wan Kenobi”), are just that — science fiction. As long as we get that into everyone’s heads, we’ll be fine.