VR Research Archives

Meta says its ultimate goal with its VR hardware is to make a comfortable, compact headset with visual finality that’s ‘indistinguishable from reality’. Today the company revealed its latest VR headset prototypes which it says represent steps toward that goal.

Meta has made it no secret that it’s dumping tens of billions of dollars in its XR efforts, much of which is going to long-term R&D through its Reality Labs Research division. Apparently in an effort to shine a bit of light onto what that money is actually accomplishing, the company invited a group of press to sit down for a look at its latest accomplishments in VR hardware R&D.

Reaching the Bar

To start, Meta CEO Mark Zuckerberg spoke alongside Reality Labs Chief Scientist Michael Abrash to explain that the company’s ultimate goal is to build VR hardware that meets all the visual requirements to be accepted as “real” by your visual system.

VR headsets today are impressively immersive, but there’s still no question that what you’re looking at is, well… virtual.

Inside of Meta’s Reality Labs Research division, the company uses the term ‘visual Turing Test’ to represent the bar that needs to be met to convince your visual system that what’s inside the headset is actually real. The concept is borrowed from a similar concept which denotes the point at which a human can tell the difference between another human and an artificial intelligence.

For a headset to completely convince your visual system that what’s inside the headset is actually real, Meta says you need a headset that can pass that “visual Turing Test.”

Four Challenges

Zuckerberg and Abrash outlined what they see as four key visual challenges that VR headsets need to solve before the visual Turing Test can be passed: varifocal, distortion, retina resolution, and HDR.

Briefly, here’s what those mean:

Varifocal: the ability to focus on arbitrary depths of the virtual scene, with both essential focus functions of the eyes (vergence and accommodation)
Distortion: lenses inherently distort the light that passes through them, often creating artifacts like color separation and pupil swim that make the existence of the lens obvious.
Retina resolution: having enough resolution in the display to meet or exceed the resolving power of the human eye, such that there’s no evidence of underlying pixels
HDR: also known as high dynamic range, which describes the range of darkness and brightness that we experience in the real world (which almost no display today can properly emulate).

The Display Systems Research team at Reality Labs has built prototypes that function as proof-of-concepts for potential solutions to these challenges.

Varifocal

To address varifocal, the team developed a series of prototypes which it called ‘Half Dome’. In that series the company first explored a varifocal design which used a mechanically moving display to change the distance between the display and the lens, thus changing the focal depth of the image. Later the team moved to a solid-state electronic system which resulted in varifocal optics that were significantly more compact, reliable, and silent. We’ve covered the Half Dome prototypes in greater detail here if you want to know more.

Virtual Reality… For Lenses

As for distortion, Abrash explained that experimenting with lens designs and distortion-correction algorithms that are specific to those lens designs is a cumbersome process. Novel lenses can’t be made quickly, he said, and once they are made they still need to be carefully integrated into a headset.

To allow the Display Systems Research team to work more quickly on the issue, the team built a ‘distortion simulator’, which actually emulates a VR headset using a 3DTV, and simulates lenses (and their corresponding distortion-correction algorithms) in-software.

Doing so has allowed the team to iterate on the problem more quickly, wherein the key challenge is to dynamically correct lens distortions as the eye moves, rather than merely correcting for what is seen when the eye is looking in the immediate center of the lens.

Retina Resolution

On the retina resolution front, Meta revealed a previously unseen headset prototype called Butterscotch, which the company says achieves a retina resolution of 60 pixels per degree, allowing for 20/20 vision. To do so, they used extremely pixel-dense displays and reduced the field-of-view—in order to concentrate the pixels over a smaller area—to about half the size of Quest 2. The company says it also developed a “hybrid lens” that would “fully resolve” the increased resolution, and it shared through-the-lens comparisons between the original Rift, Quest 2, and the Butterscotch prototype.

While there are already headsets out there today that offer retina resolution—like Varjo’s VR-3 headset—only a small area in the middle of the view (27° × 27°) hits the 60 PPD mark… anything outside of that area drops to 30 PPD or lower. Ostensibly Meta’s Butterscotch prototype has 60 PPD across its entirely of the field-of-view, though the company didn’t explain to what extent resolution is reduced toward the edges of the lens.

Continue on Page 2: High Dynamic Range, Downsizing »

A team of researchers from NVIDIA Research and Stanford published a new paper demonstrating a pair of thin holographic VR glasses. The displays can show true holographic content, solving for the vergence-accommodation issue. Though the research prototypes demonstrating the principles were much smaller in field-of-view, the researchers claim it would be straightforward to achieve a 120° diagonal field-of-view.

Published ahead of this year’s upcoming SIGGRAPH 2022 conference, a team of researchers from NVIDIA Research and Stanford demonstrated a near-eye VR display that can be used to display flat images or holograms in a compact form-factor. The paper also explores the interconnected variables in the system that impact key display factors like field-of-view, eye-box, and eye-relief. Further, the researchers explore different algorithms for optimally rendering the image for the best visual quality.

Commercially available VR headsets haven’t improved in size much over the years largely because of an optical constraint. Most VR headsets use a single display and a simple lens. In order to focus the light from the display into your eye, the lens must be a certain distance from the display; any closer and the image will be out of focus.

Eliminating that gap between the lens and the display would unlock previously impossible form-factors for VR headsets; understandably there’s been a lot of R&D exploring how this can be done.

In NVIDIA-Stanford’s newly published paper, Holographic Glasses for Virtual Reality, the team shows that it built a holographic display using a spatial light modulator combined with a waveguide rather than a traditional lens.

The team built both a large benchtop model—to demonstrate core methods and experiment with different algorithms for rending the image for optimal display quality—and a compact wearable model to demonstrate the form-factor. The images you see of the compact glasses-like form-factor don’t include the electronics to drive the display (as the size of that part of the system is out of scope for the research).

You may recall a little while back that Meta Reality Labs published its own work on a compact glasses-size VR headset. Although that work involves holograms (to form the system’s lenses), it is not a ‘holographic display’, which means it doesn’t solve the vergence-accommodation issue that’s common in many VR displays.

On the other hand, the Nvidia-Stanford researchers write that their Holographic Glasses system is in fact a holographic display (thanks to the use of a spatial light modulator), which they tout as a unique advantage of their approach. However, the team also writes that it’s possible to display typical flat images on the display as well (which, like contemporary VR headsets, can converge for a stereoscopic view).

Not only that, but the Holographic Glasses project touts a mere 2.5mm thickness for the entire display, significantly thinner than the 9mm thickness of the Reality Labs project (which was already impressively thin!).

As with any good paper though, the Nvidia-Stanford team is quick to point out the limitations of their work.

For one, their wearable system has a tiny 22.8° diagonal field-of-view with an equally tiny 2.3mm eye-box. Both of which are way too small to be viable for a practical VR headset.

However, the researchers write that the limited field-of-view is largely due to their experimental combination of novel components that aren’t optimized to work together. Drastically expanding the field-of-view, they explain, is largely a matter of choosing complementary components.

“[…] the [system’s field-of-view] was mainly limited by the size of the available [spatial light modulator] and the focal length of the GP lens, both of which could be improved with different components. For example, the focal length can be halved without significantly increasing the total thickness by stacking two identical GP lenses and a circular polarizer [Moon et al. 2020]. With a 2-inch SLM and a 15mm focal length GP lens, we could achieve a monocular FOV of up to 120°”

As for the 2.3mm eye-box (the volume in which the rendered image can be seen), it’s way too small for practical use. However, the researchers write that they experimented with a straightforward way to expand it.

With the addition of eye-tracking, they show, the eye-box could be dynamically expanded up to 8mm by changing the angle of the light that’s sent into the waveguide. Granted, 8mm is still a very tight eye-box, and might be too small for practical use due to variations in eye-relief distance and how the glasses rest on the head, from one user to the next.

But, there’s variables in the system that can be adjusted to change key display factors, like the eye-box. Through their work, the researchers established the relationship between these variables, giving a clear look at what tradeoffs would need to be made to achieve different outcomes.

As they show, eye-box size is directly related to the pixel pitch (distance between pixels) of the spatial light modulator, while field-of-view is related to the overall size of the spatial light modulator. Limitations on eye-relief and converging angle are also shown, relative to a sub-20mm eye-relief (which the researchers consider the upper limit of a true ‘glasses’ form-factor).

An analysis of this “design trade space,” as they call it, was a key part of the paper.

“With our design and experimental prototypes, we hope to stimulate new research and engineering directions toward ultra-thin all-day-wearable VR displays with form-factors comparable to conventional eyeglasses,” they write.

The paper is credited to researchers Jonghyun Kim, Manu Gopakumar, Suyeon Choi, Yifan Peng, Ward Lopes, and Gordon Wetzstein.

Meta Reveals VR Headset Prototypes Designed to Make VR ‘Indistinguishable From Reality’