Holo-this, holo-that. Holograms are so bamboozling that the term often gets used colloquially to mean ‘fancy-looking 3D image’, but holograms are actually a very specific and interesting method for capturing light field scenes which have some real advantages over other methods of displaying 3D imagery. RealView claims to be using real holography to solve a major problem inherent to AR and VR headsets of today, the vergence-accommodation conflict. Our favorite holo-skeptic, Oliver Kreylos, examines what we know about the company’s approach so far.
Guest Article by Dr. Oliver Kreylos
Oliver is a researcher with the UC Davis W.M. Keck Center for Active Visualization in the Earth Sciences (KeckCAVES). He has been developing virtual reality as a tool for scientific discovery since 1998, and is the creator of the open-source Vrui VR toolkit. He frequents reddit as /u/Doc_Ok, tweets as @okreylos, and blogs about VR-related topics at Doc-Ok.org.
RealView recently announced plans to turn their previous desktop holographic display tech into the HOLOSCOPE augmented reality headset. This new headset is similar to Magic Leap‘s AR efforts in two big ways: one, it aims to address the issue of vergence-accommodation conflict inherent in current VR headsets such as Oculus Rift or Vive, and AR headsets such as Microsoft’s HoloLens; and two, we know almost no details about it. Here they explain vergence-accommodation conflict:
Note that there is a mistake around the 1:00 minute mark: while it is true that the image will be blurry, it will only split if the headset is not configured correctly. Specifically, that will not happen with HoloLens when the viewer’s inter-pupillary distance is dialed in correctly.
Unlike pretty much everybody else using the holo- prefix or throwing the term “hologram” around, RealView vehemently claims their display is based on honest-to-goodness real interference-pattern based holograms, of the computer-generated variety. To get this out of the way: yes, that stuff actually exists. Here is a Nature article about the HoloVideo system created at MIT Media Lab.
The remaining questions are how exactly RealView creates these holograms, and how well a display based on holograms will work in practice. Unfortunately, due to the lack of known details, we can only speculate. And speculate I will. As a starting point, here is a demo video, allegedly shot through the display and without any special effects:
I say allegedly, but I do believe this to be true. The resolution is surprisingly high and quality is surprisingly good, but the degree of transparency in the virtual object (note the fingers shining through) is consistent with real holograms (which only add to the light from the real environment shining through the display’s visor).
There is one peculiar thing I noticed on RealView’s web site and videos: the phrase “multiple or dynamic focal planes.” This seems odd in the context of real holograms, which, being real three-dimensional images, don’t really have focal planes. Digging a little deeper, there is a possible explanation. According to the Wikipedia entry for computer-generated holography, one of the simpler algorithms to generate the required interference patterns, Fourier transform, is only able to create holograms of 2D images. Another method, point source holograms, can create holograms of arbitrary 3D objects, but has much higher computational complexity. Maybe RealView does not directly create 3D holograms, but instead projects slices of virtual 3D objects onto a set of image planes at different depths, creates interference patterns for the resulting 2D images using Fourier transform, and then composes the partial holograms into a multi-plane hologram. I want to reiterate that this is mere speculation.
This would literally create multiple focal planes, and allow the creation of dynamic focal planes depending on application or interaction needs, and could potentially explain the odd language and the high quality of holograms in above video. The primary downside of slice-based holograms would be motion parallax: in a desktop system, the illusion of a solid object would break down as the viewer moves laterally to the holographic screen. Fortunately, in head-mounted displays the screen is bolted to the viewer’s head, solving the problem.
So while RealView’s underlying technology appears legit, it is unknown how close they are to a real product. The device used to shoot above video is never shown or seen, and a picture from the web site’s medical section shows a large apparatus that is decidedly not head-mounted. I believe all other product pictures on the web site to be concept renders, some of them appearing to be (poorly) ‘shopped stock photos. There are no details on resolution, frame rate, brightness or other image specs, and any mention of head tracking is suspiciously absent. Even real holograms need head tracking to work if the holographic screen is moving in space by virtue of being attached to a person’s head. Also, the web site provides no details on the special scanners that are required for real-time direct in-your-hand interaction.
Finally, there is no mention of field of view. As HoloLens demonstrates, field of view is important for AR, and difficult to achieve. Maybe this photo from RealView’s web site is a veiled indication of FoV:
In conclusion, while we know next to nothing definitive about this potential product, computer-generated holography is a thing that really exists, and AR displays based on it could be contenders. Details remain to be seen, but any advancements to computer-generated holography would be highly welcome.