Projection and Distortion in Wide-FoV HMDs

There is an on-going, but already highly successful, Kickstarter campaign for a new VR head-mounted display with a wide (200°) field of view (FoV): Pimax 8k. As I have not personally tried this headset — only its little brother, Pimax 4k, at the 2017 SVVR Expo — I cannot discuss and evaluate all the campaign’s promises. Instead, I want to focus on one particular issue that’s causing a bit of confusion and controversy at the moment.

Early reviewers of Pimax 8k prototypes noticed geometric distortion, such as virtual objects not appearing in the correct places and shifting under head movement, and the campaign responded by claiming that these distortions “could be fixed by improved software or algorithms” (paraphrased). The ensuing speculation about the causes of, and potential fixes for, this distortion has mostly been based on wrong assumptions and misunderstandings of how geometric projection for wide-FoV VR headsets is supposed to work. Adding fuel to the fire, the campaign released a frame showing “what is actually rendered to the screen” (see Figure 1), causing further confusion. The problem is that the frame looks obviously distorted, but that this obvious distortion is not what the reviewers were complaining about. On the contrary, this is what a frame rendered to a high-FoV VR headset should look like. At least, if one ignores lenses and lens distortion, which is what I will continue to do for now.

Figure 1: Frame as rendered to one of the Pimax 8k’s screens, according to the Kickstarter campaign. (Probably not 100% true, as this appears to be a frame submitted to SteamVR’s compositor, which subsequently applies lens distortion correction.)

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3D Camera Calibration for Mixed-Reality Recording

Mixed-reality recording, i.e., capturing a user inside of and interacting with a virtual 3D environment by embedding their real body into that virtual environment, has finally become the accepted method of demonstrating virtual reality applications through standard 2D video footage (see Figure 1 for a mixed-reality recording made in VR’s stone age). The fundamental method behind this recording technique is to create a virtual camera whose intrinsic parameters (focal length, lens distortion, …) and extrinsic parameters (position and orientation in space) exactly match those of the real camera used to film the user; to capture a virtual video stream from that virtual camera; and then to composite the virtual and real streams into a final video.

Figure 1: Ancient mixed-reality recording from inside a CAVE, captured directly on a standard video camera without any post-processing.

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Accommodation and Vergence in Head-mounted Displays

Why do virtual objects close to my face appear blurry when wearing a VR headset? My vision is fine!

And why does the real world look strange immediately after a long VR session?

These are another two (relates ones) of those frequently-asked questions about VR and head-mounted displays (HMDs) that I promised to address a while back.

Here’s the short answer: In all currently-available HMDs, the screens creating the virtual imagery are at a fixed optical distance from the user. But our eyes have evolved to automatically adjust their optical focus based on the perceived distance to objects, virtual or real, that they are looking at. So when a virtual object appears to be mere inches in front of the user’s face, but the screens showing images of that object are — optically — several meters away, the user’s eyes will focus on the wrong distance, and as a result, the virtual object will appear blurry (the same happens, albeit less pronounced, when a virtual object appears to be very far away). This effect is called accommodation-vergence conflict, and besides being a nuisance, it can also cause eye strain or headaches during prolonged VR sessions, and can cause vision problems for a short while after such sessions.

Now for the long answer.

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Boom, Headshot

There was an article on Medium yesterday: My First Virtual Reality Groping. In it, a first-time VR/HTC Vive user describes how she was virtually groped by another player inside an online multi-player VR game, within the first three minutes of her first such endeavor, and how it ruined her experience and deeply disturbed her.

I do not know what to call player “BigBro442’s” behavior, but I do know that it is highly inappropriate, and toxic for VR as a whole. This, people, is why we can’t have nice things. This is by far not the first instance of virtual harassment or VR griefing that I’ve heard of, but it’s the one that got me thinking because of this comment on the article:

This is reality. The best we can do is educate, starting with articles like this.

No. That is not true. We can do better than that. Unlike reality, where someone might be assaulted inside their own home, or in some dark back alley, with no witnesses around or evidence left behind, this is virtual reality, which only exists as a sequence of ones and zeros on some Internet server. That server has absolute knowledge of anything that goes on anywhere inside the virtual world it maintains, like an omniscient Big Brother. If virtual harassment happens in virtual reality, maybe virtual reality needs virtual courts.

Here is a not-so-modest proposal, off the top of my head, using SteamVR/Steam as example platforms:

  • Any server maintaining a virtual world potentially used by more than one person at the same time keeps a ring buffer of each connected user’s avatar state for the last, say, five minutes. That’s not overly demanding: sampling a head tracker and two hand trackers at, say, 30 Hz, over five minutes, results in approx. 750kB of data total, per user.
  • The client user interface of any shared virtual environment contains a button in some easily accessible standard place, say in SteamVR’s overlay, to file a harassment complaint.
  • If a user (“Alice”) files a complaint, several things happen. Most importantly, the server immediately dumps the avatar state ring buffers of all connected (or recently connected) players to a file. Second, Alice is immediately charged a small fee, say $5, on the credit card associated with her Steam account. This is a micro-transaction, an existing Steam feature. The fee’s purpose is to discourage another form of harassment, namely filing frivolous complaints against innocent users.
  • Files generated by complaints, with personally identifying information redacted, will be reviewed by a peer group of humans. This might be done by appointed moderators, or might even be crowd-sourced.
  • If review determines that behavior contained in the 5-minute replay violates community standards, Alice will be refunded the fee she was charged, and offending user Bob’s Steam account will be temporarily suspended, say for one day on the first offense, starting either immediately or the next time Bob attempts to log in. And I mean Bob’s entire Steam account is suspended, not just his access to one particular server or shared VR application: Bob’s on time-out and can go read a book.
  • If review determines that the complaint was without merit, nothing happens to accused user Bob, and Alice is not refunded her fee. If Alice disagrees, she can raise the stakes by re-filing the same complaint for another $5 fee, the total $10 then being refundable or not, etc.
  • If review cannot reach agreement, or review does not happen within a reasonable time frame, Alice is refunded her fee.

Okay, so this is ridiculous, right? Not from a technical feasibility point of view, which I think I laid out above, but from an organizational and cultural point of view. One might say that it is a severe regulatory overreach, a violation of the freedom and the very fundamental principles of online gaming, and that the idea of community review is ludicrous on the face of it.

Well, I might have agreed — until recently, that is, when I stumbled across this. Holy Moly! What’s that? Multi-player game servers retaining state data of all players, which can be dumped to a permanent file as evidence for later peer review by a number of appointed or self-appointed judges, with crowd-sourced verdicts and suspensions or bans handed out to cheaters, and judges being rewarded or punished for good or bad judgment? And it works?

If cheating in Counter-Strike is a big enough deal to create a system like this, would it be so outrageous to apply the same basic idea to harassment in shared virtual reality, which, due to VR’s strong sense of immersion and presence, arguably has a larger negative impact on the harassed than losing a round of CS?

Discuss.

Technology Transfer

I found out today that HTC now ships a tool to measure users’ inter-pupillary distances with new Vive VR headsets. When I say “tool,” I mean a booklet with instructions in many languages, and a ruler printed along one edge of each page:

IPD measurement chart shipped by HTC with new Vives.

Figure 1: IPD measurement chart shipped by HTC with new Vives. Image courtesy of reddit user DanielDC88, image source.

I thought this was great on multiple levels. For one, measuring the user’s IPD and entering it into the VR software, either manually or through a sensor on a physical IPD adjustment knob or slider on the headset, as in both Vive and Oculus Rift, is an important component of creating convincing VR displays. The more people get used to that, the better.

On the second level, I was proud. On April 9, 2014, I wrote an article on this here blog titled “How to Measure Your IPD,” which describes this exact method of using a mirror and a ruler. It even became one of my more popular articles (the fifth most popular article, actually, with 33,952 views as of today). I was a little less proud when I looked at my own article again just now, and realized that my diagrams were absolutely hideous compared to those in HTC’s booklet. Oh well. Continue reading

Keeping VR users from hurting themselves

Just the other day, I jumped on the wayback machine and posted an article about our work in immersive tele-collaboration, featuring research (and a video) from about four years ago. The shame! I figured it would be excusable that one time, and I would never do it again. Oh well, here we go.

Keeping VR users from hurting themselves

… or their expensive VR equipment.

It’s a pretty big deal. Virtual Reality, especially its head-mounted implementation, is quite good at overriding its users’ sense of place and space. “Presence,” or the feeling of bodily being in a place where one knows to be not, is a powerful and compelling experience, but it has a downside: users experiencing it lose touch with their real physical environments. Exhibit A: Figure 1 (granted, there are some concerns that the following video clip was staged, but let’s pretend it’s for reals).

Figure 1: When instinct takes over. Source: imgur

To prevent this kind of thing from happening — at least in most cases — Valve implemented a system called “Chaperone” into the SteamVR run-time framework that runs their and HTC’s Vive VR headset (and potentially other headsets, through Valve’s OpenVR layer). Continue reading

Remote Collaborative Immersive Visualization

I spent the last couple of days at the first annual meeting of “The Higher Education Campus Alliance for Advanced Visualization” (THE CAAV), where folks managing or affiliated with advanced visualization centers such as KeckCAVES came together to share their experiences. During the talks, I saw slides showing Vrui‘s Collaboration Infrastructure pop up here and there, and generally remote collaboration was a big topic of discussion. During breaks, I showed several people the following video on my smartphone (yes, I finally joined the 21st century), and afterwards realized that I had never written a post about this work, as most of it predates this blog. So here we go.

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Optical Properties of Current VR HMDs

With the first commercial version of the Oculus Rift (Rift CV1) now trickling out of warehouses, and Rift DK2, HTC Vive DK1, and Vive Pre already being in developers’ hands, it’s time for a more detailed comparison between these head-mounted displays (HMDs). In this article, I will look at these HMDs’ lenses and optics in the most objective way I can, using a calibrated fish-eye camera (see Figures 1, 2, and 3).

Figure 1: Picture from a fisheye camera, showing a checkerboard calibration target displayed on a 30" LCD monitor.

Figure 1: Picture from a fisheye camera, showing a checkerboard calibration target displayed on a 30″ LCD monitor.

Figure 2: Same picture as Figure 1, after rectification. The purple lines were drawn into the picture by hand to show the picture's linearity after rectification.

Figure 2: Same picture as Figure 1, after rectification. The purple lines were drawn into the picture by hand to show the picture’s linearity after rectification.

Figure 3: Rectified picture from Figure 2, re-projected into stereographic projection to simplify measuring angles.

Figure 3: Rectified picture from Figure 2, re-projected into stereographic projection to simplify measuring angles. Concentric purple circles indicate 5-degree increments away from the projection center point.

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Lasers Are Not Magic

“Can I make a full-field-of-view AR or VR display by directly shining lasers into my eyes?”

No.

Well, technically, you can, but not in the way you probably imagine if you asked that question. What you can’t do is mount some tiny laser emitter somewhere out of view, have it shine a laser directly into your pupil, and expect to get a virtual image covering your entire field of view (see Figure 1). Light, and your eyes, don’t work that way.

Figure 1: A magical retinal display using a tiny laser emitter somewhere off to the side of each eye. This doesn't work in reality.

Figure 1: A magical retinal display using a tiny laser emitter somewhere off to the side of each eye. This doesn’t work in reality. If a single beam of light entering the eye could be split up to illuminate large parts of the retina, real-world vision would not work.

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Head-mounted Displays and Lenses

“It can’t be comfortable or healthy to stare at a screen a few inches in front of your eyes.”

The popularity of Google Cardboard, and the upcoming commercial releases of the Oculus Rift, HTC Vive, and other modern head-mounted displays (HMDs) have raised interest in virtual reality and VR devices in parts of the population who have never been exposed to, or had reason to care about, VR before. Together with the fact that VR, as a medium, is fundamentally different from other media with which it often gets lumped in, such as 3D cinema or 3D TV, this leads to a number of common misunderstandings and frequently-asked questions. Therefore, I am planning to write a series of articles addressing these questions one at a time.

First up: How is it possible to see anything on a screen that is only a few inches in front of one’s face?

Short answer: In HMDs, there are lenses between the screens (or screen halves) and the viewer’s eyes to solve exactly this problem. These lenses project the screens out to a distance where they can be viewed comfortably (for example, in the Oculus Rift CV1, the screens are rumored to be projected to a distance of two meters). This also means that, if you need glasses or contact lenses to clearly see objects several meters away, you will need to wear your glasses or lenses in VR.

Now for the long answer. Continue reading