The Accommodation-Vergence conflict and how it affects your kids (and yourself)


Remember that first time you put on a VR headset? You were immersed in a virtual world and (hopefully) it was breath taking. Then, if you were in VR for a good amount of tine, when you took off the headset the real world felt… weird. Not right. Something odd about it. That feeling is very common for first-timers and usually goes away within a few minutes. That weirdness is your brain and eyes re-adjusting and switching back to their normal way of operating, after being forced to work in a very unusual manner.

So what is going on?

It’s common knowledge that depth perception stems from the fact that we have two eyes, and we can infer depth from the displacement of objects between the two images. Large displacement (parallax) means close object, small or no displacement means far away object. This is true, but there much more to it. There are in fact a whole bunch of cues that we use to infer depth. Lets talk about two of these.


Eyes are a marvel of engineering. Camera lenses have multiple optical elements, some are movable. To change focus, the elements move in relation to one another. Eyes have only a single element (the lens), but unlike cameras, our lens is soft and squishy. To change focus, special muscles pull or relax the lens, changing it’s shape.


This is called the Accommodation reflex and it’s an important depth cue for objects that are close to us (less than 10 meters / 30 feet).


Our eyes can move independently, and when looking at an object they converge on it.

Again, this is another depth cue.

The Conflict

In the real world, Accommodation and Vergence are always in sync. VR is a different story. In a VR headset the screen is in a fixed distance from your eyes, usually around 3 meters (9 feet). The actual screen is of course much closer, but there are some optics in the headset to make it appear as if it’s 3 meters away. Your eyes remain focused on the screen all the time. If a virtual object is a 20cm away, your eyes are converging on a spot that is 20cm away but they are focusing (accommodating) on the screen which is 3 meters away. This is totally wrong and never happens anywhere except VR headsets.



If you have a VR headset you can easily test this. Go into a VR environment and look at something on the horizon or far away. Now hold up the controller to your face. Note how both the controller and the horizon are in perfect focus. This is optically impossible and your brain is actually freaking out over it.

But wait! Surely 3D movies / TVs also have this problem, and they have been around for a long time. This is correct. However, the big difference is that 3D displays don’t block out the real world and are usually far away from the user. They also cause this conflict, but to a much lesser degree.


As stated, the Accommodation-Vergence conflict is the reason why the real world feels weird after being in VR. It’s also the main cause of eye-strain / headaches, and some people are more sensitive than others. I’ve seen reports of VR users losing depth perception for hours, and even days, after a long VR session. While in adults this is probably temporary, it does raise concerns in regards to children.

How does prolonged exposure to the Accommodation-Vergence conflict affect a developing visual system (ie, kids)? Nobody knows. There’s no real data or studies on this topic, which brings me to the most important conclusion of this article:

Limit children’s exposure to VR. Until we know more, it’s best to err on the side of caution. Short exposure is probably fine, but don’t let them spend hours in there, and insist on frequent breaks.

What about VR in education? Yes, this is a big problem. On one hand, VR can be an amazing tool in education, on the other, it also might cause permanent vision problems. Moderation is key, and if you are an educator I will lave this decision in your hands.

I’m aware of this study, which showed a small improvement in vision when they tested a small group of kids for a short period of time. However, they did not test depth perception, which is the main area of concern.

Don’t Panic

The good news is that the Accommodation-Vergence conflict is solvable, and the solution is lightfield technology. I won’t go into much detail here, lightfields are damn complicated, but I will say what they solve the conflict and will allow for multiple planes of focus and more natural eye behavior in VR. However, the technology is still in it’s infancy and it will take 3-5 years before we see it in consumer VR headsets.

Written by Shachar “Vice” Weis, founder and CEO of Packet39, Virtual Reality training solutions for the manufacturing and power industries.

Further Reading

I installed a TPCast, was blown away, and then went back to tethered. A very subjective review.

A few days ago I had the opportunity to grab a TPCast and install it on my home office Vive. Here are my thoughts:

  • Lots of parts, clunky. Battery+holder, receiver, transmitter, router (why?)
  • Setup was easier than expected. Pretty much plug in everything, install the TPCast software.
  • Had some issues that went away after a reboot
  • Exhilarating feeling of freedom without the tether. Mind blowing really.
  • Very quickly realized I have nowhere to keep the battery. I usually VR in my underwear
  • Transmitter makes a very annoying squeal / buzz when it’s working
  • I see a green bar at the very edge of peripheral vision on the right side. I increased my FOV on my Vive by using a thin facepad. With the stock facepad the green bar is not visible, but it’s really hard to go back to stock FOV.
  • I did not see a reduction in image quality or lag
  • I did notice some jitterness with motion-intensive games like SoundBoxing. I can’t put my finger on exactly what is the difference, but it’s not as smooth as tethered. But again, only with very active games.
  • The Vive microphone is not supported, nor is the headset camera or extra USB port.

I’m now back to good old tether. Why? It’s just.. clunky. A collection of small annoyances. The battery in my pocket. The annoying buzz. The green bar glowing just outside my view. The extra tripod for the transmitter and extra cables for the router. Having to charge my headset. None of these is a huge deal, but put together.. I decided to go back to tethered.


UPDATE – Nov 2, 2017

The great people of Opentpcast are working to solve some of these issues, I understand that the microphone is now working, and the camera very soon (for owners of the newer batches of Vives). Also they managed to improve tracking and reduce jitters. Check it out here:

In addition, I’ve seen unconfirmed reports that the US version of TPCast eliminates the annoying transmitter whine. I’ll update this post when more information becomes available.


UPDATE – Dec 21, 2017

My TPCast sat in a box for a few weeks, until I decided I want to use it in an upcoming demo. I tried on two different laptops and couldn’t get it to work. Swapped cables, used display port instead of HDMI, bypassed the linkbox, tried older versions of TPCast assistant, talked to support, nothing helped. Eventually I gave up and installed OpenTPCast. And it worked. Just like that. I setup two vives in the same space, using a single pair of basestations, and did a direct comparison between a tethered vive and a TPCast’ed vive. My thoughts:

  • Going untethered is still exhilarating
  • Tracking is noticeably affected, for the worse. Occasional visible jitters, jumps and pauses.
  • Display sporadically flickers in and out of low-resolution / high-compression mode. Visible compression artifacts will appear for a fraction of a second.
  • OpenTPCast uses VirualHere for the USB-over-wifi. VirtualHere requires a license, which is $25 USD.
  • Green and blue bars at the edge of vision are still there.
  • It made me sick! I’m not sure if it’s the jitters or some kind of lag that is visually unnoticeable and affected me over time. After walking around for a few minutes I started feeling a hint of queasy, which has not happened to me in VR for a long while.

UPDATE – Jan 2, 2017

I decided to throw a belated New Year VR party for a dozen people, and to use TPCast while I’m at it. Here are my impressions from this event:

  • It took an extra 20 minutes to setup the TPCast system, on top of the usual Vive setup. I found that the only way to get it working reliability is to enable the USB drivers manually, one by one (in VirtualHere). At some point it was almost working but then Windows Audio service crashed and I had to reboot and start over. This is probably related to the USB audio driver that VirtualHere adds to the system.
  • People had a blast playing untethered. Gorn and Space Pirate Trainer were the crowd favorites. My own game – Orbital Injection – was great for first-timers.
  • Tracking was a bit spotty, occasional jitter and pause, but nothing game breaking and nobody got sick (I was worried about that).
  • Display was perfect, no artifacts this time
  • Every 30 minutes or so, the TPCast wifi module dies and tracking stops completely. The only fix is a hard restart (unplug battery) and that also requires restarting SteamVR and sometimes VirualHere as well. It takes a few minutes of futzing around on the computer and it’s really annoying.
  • I would rather have 2 or 3 small batteries than a single giant battery. Sadly, there is only one battery that fits into the TPCast right now, and it’s massive.

Conclusion: Fun for events, when it works and you are willing to put in the extra setup time and troubleshooting. The device is not reliable enough for commercial use or businesses.



Microsoft Motion Controllers – Unboxing and first look

Hooray, I just got my Microsoft Motion Controllers, to pair with my Acer “MixedReality” headset (which can’t actually do MR in any way). See my review here and here.

So, Are they any good?

Short Answer

Tracking is superb. Build quality is horrible.

Long Answer

I received two controllers in a plain cardboard box devoid or any marking, text or symbols.


The box contains two controllers and two pairs of AA batteries, and nothing else.

Put in the batteries, hold the Windows button to turn on, and pesto. The first surprise. It uses white visible light LED. For some reason I thought these would be IR.



Moving on to the software, running Mixed Reality Portal greeted me with this screen:


After a bit of digging around I found that the battery compartment hides a secret button, holding it down for a few seconds puts in the controller into pairing mode. You’ll need a Bluetooth capable computer (dongle not included). What isn’t documented anywhere is that the pass code is 0000. Yeah, I guessed it. On my third attempt.


The Good Part

The tracking is surprisingly good. It’s really spot-on, HTC vive quality, as long as the controller is within view of the headset cameras. The FOV is 180 degrees horizontal but only about 100 degrees vertical. In fact, when moving up, the controller will sometimes lose tracking while it’s still in the headset virtual view (because the cameras are tilted down).

When the controller is outside the camera view, it will freeze in position but will still track orientation.

There is a slight lag in this video, but that lag is only in the screen display. The headset view is lag-free.


The Bad Part

The build quality is.. how to put it gently.. cheap-plasticy-chinese-knockoff kind of quality. Here, watch this:


Trigger: It’s binary but offers no click, no resistance, no feedback when you press it. It feels like a $5 RC car controller

Touchpad: Up, down, left and right all have a different “clickiness” to them. Different sound, different feel, different pressure required to trigger. UP is very “clicky”, DOWN barely registers.

Thumb stick: Movement is OK, but also has a cheap feel to it. Clicking on it generates this nasty grinding sensation (clearly audible in the video). Both of my controllers exhibit this problem, but one is much more pronounced than the other. It really is horrible.

Buttons: The main “windows” button and the side button are both fine, but the smaller menu button doesn’t register any click sensation or noise. It also doesn’t do anything in the Mixed Reality Portal. I’m not sure if it’s just broken (on both controllers), or if it’s designed to be a “silent” button, or what. Actually, I don’t care. It’s terrible as well.


I was genuinely surprised how well the controllers track in space, and the headset tracking has also improved since I last tried it. It seems the latest Windows update has refined the tracking algorithms. When moving the controller sideways and down you can to reach pretty far before it loses tracking. Moving it up is problematic, as it goes out of camera view before it even leaves your virtual view.

The build quality is well below the standard I was expecting, and on par with cheap knockoff XBOX controllers.

About the author

Shachar “Vice” Weis is a software developer freelancer and the CTO / Founder of, a software house that builds custom VR applications for the manufacturing and power industry.

How to troll Google Maps on a national level

If you happen to look at Israel on Google maps, on Yom Kippur, you would notice something interesting. The entire country is red with traffic. These are no ordinary traffic jams.


So what is going on? Something weird happens on Yum Kippur in Israel. For 24 hours, from dusk to dusk, nobody drives. The religious and atheists alike take a break from their cars. The roads and highways are empty and everyone takes to the streets. You’ll see kids biking on deserted highways, people just hanging out in the middle of an intersection. It feels post-apocalyptic and it’s amazing. The silence hits you like a wall.

Image result for israel yom kippur night picture


Of course many of these people have cellphones, and none of them are moving very fast, hence, Google thinks it’s a country-spanning traffic jam. And that’s how you troll Google maps in style. I wish every country would adopt this tradition.

Hell, I would make it a monthly event.


Scientifically (in)accurate crane simulator

My company ( is working a lot with the manufacturing and power industry. I wanted to see how I can take a physical controller and bring it into the VR environment. For simplicity, I went with a Throttle Qudrant joystick , often used for flight simulators. I modeled the joystick and mapped it 1:1 with the real device.


And then VR’ed the whole thing. It’s surprisingly fun. I’m going to demo this at Manufacturing Matters next month.

VR for Industry

Geiger Counter

Training and testing on the usage of a hand-held radiation detector.

This is a pilot project for the nuclear power plant in Pickering, Ontario. An operator places a virtual radiation source on the table and the trainee has to find it. Only the operator can see the source on screen. In VR, this test can be conducted in an office or even at home, without expensive equipment and without exposing trainees to a radiation source.

Crane Demo

This is a demo of combining a virtual device (the crane) with a real-life controller (the levers). The user can see the levers in VR and operate the controller with their hands. The virtual and physical controllers are mapped 1:1 (position and scale). It is also possible to show the user’s hands in VR, for easier orientation.

Audio Testing Room

This was is a test developed for the audiology department at the University of Western Ontario. A user is presented with a VR room that includes several audio sources. The sources can be moved in 3D space and their volume / reach adjusted. This VR setup could potentially replace audio testing rooms, which have dozens of custom speakers and cost upwards of $100k.