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Origin of the question

I have recently finished an internship at a (small) game development company which is working on a Virtual Reality game that is also used in medical rehabilitation. From what I have heard and read online, you generally want to achieve at least 90 FPS in a VR game because otherwise the player can get motion sickness or dizzyness, etc.

I am aware that not everyone is getting sick but from what I have read, you still want to aim for that 90 FPS to be sure, although I am unable to find the exact reason for this number. (it feels like an industry standard)

Questions

"Why is 90 FPS an industry standard when developing a VR game?" and "Why can people get sick when playing on lower frame rates?"

When searching online, I keep reading about 'studies' that show that people will be more likely to feel sick or dizzy when playing a VR game below 90 FPS but I never see the actual reason (source) as to why that is.

I can only make assumptions based on what I know already but I would like to know the exact reason (or as much of a reason as possible) and hopefully a source as to why 90 FPS is chosen (and not 80 or 100) and what causes some people to become sick from lower framerates.

sidenote: I am referring to VR games for devices as the 'Vive', 'Rift', etc. (as there are also VR Simulations for pilots, etc. which also have elements such as simulated movement that can impact sickness)

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    \$\begingroup\$ I don't think there's anything special about 90. It's just a matter of "higher is better" and 90 is currently a decent compromise for what our hardware and software can put out. Some sets go up to 120, but it becomes very difficult to render a detailed scene in such a narrow slice of time. \$\endgroup\$ – DMGregory Jun 24 '19 at 11:36
  • \$\begingroup\$ I suspect that 90=60*3/2 plays a relevant role. 60Hz is a very common frame rate as the large majority of people cannot reliably detect higher frame rates anyway (unless they're explicitly pointer out, of course - see 144Hz monitors) \$\endgroup\$ – MSalters Jun 24 '19 at 12:06
  • \$\begingroup\$ @DMGregory I see, but is there any scientific basis for 90 FPS becoming an industry standard for VR games? It is almost like an axiom. I guess that "higher is better" will always be something you aim for but at some point we got statements claiming that 90 FPS should be your main target. \$\endgroup\$ – D.Kallan Jun 24 '19 at 12:06
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    \$\begingroup\$ I don't think you're going to get anything better than "a bunch of people tried a bunch of different framerates and 90 was a nice round number closest to the one they found was the minimum acceptable". I remember John Carmack doing some blogging or tweeting about this in the early days of the Oculus Rift so maybe there's some Google search terms that might help you. \$\endgroup\$ – Maximus Minimus Jun 24 '19 at 12:54
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    \$\begingroup\$ Actually, I suspect @MSalters comment hit the nail on the head - the choice of a specific number is probably motivated more by the hardware than the wetware. You want your image to sync with the monitor's refresh rate to avoid tearing, and hardware generally supports a fixed set of options. If you want to do stereo, then you might also need a rate that works when halved/doubled. If you're supporting haptics then you might need to take that system's rate into account as well. You might find food for thought by researching a similar issue: Why do we make video 60 FPS (60Hz) ... or 29.97, 30, etc \$\endgroup\$ – A C Jun 25 '19 at 4:23
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I'm going to start by answering your second question, which has a more concrete answer.

Why can people get sick when playing on lower frame rates?

In the real world, when you turn your head one direction, objects move across your field of view in the other direction. Consider the drawing below. When the person turns their head to the left, the tree is visible in their right field of view, and vice versa. Your brain expects this to happen whenever you move your head. The response time of your visual system is low enough that we can consider this to be instantaneous.

Turning heads in the real world

Imagine that someone has a stick mounted on their head holding a photograph of a tree in front of their face. In this situation, when the person turns their head left and right, the tree always remains in the center of their field of view, as shown below.

Turning heads with static image

Now imagine if we put a screen on the stick instead of a static photograph. If we can change what is displayed on the screen when the person turns their head, then we can make the image of the tree behave as if it were an actual tree, moving across the field of view in the opposite direction of the turning head, as in the drawing below. This is basically how VR headsets work.

Turning heads with an updating image

However, the visual system is finely tuned to this behavior and knows that it should happen pretty much instantly any time you move your head. If VR frame rates are too low, your head can turn faster than the image can keep up.

When your brain detects this discrepancy between the image it sees and the movement of your head, it thinks, "Hmm, something is poisoning my nervous system; I should get rid of whatever I just ate." This is the nausea and discomfort some people feel in VR, which is exacerbated by low frame rates.

Why is 90 FPS an industry standard when developing a VR game?

Because people tend to feel sick when the frame rate is below 90 FPS.

This is not a magic number; 89 FPS would be fine. I'm sure some neuroscience articles have identified the maximum amount of time that your brain will tolerate between frames, but VR developers most likely just pushed the frame rate higher until most people weren't feeling sick, as suggested in the comments. Also mentioned in the comments is the fact that a 90 FPS VR display will work well in conjunction with a 60 FPS monitor, since usually both are active at the same time.

Unfortunately I was not able to find any authoritative study demonstrating the 90 FPS "standard." However, some articles do mention the 90 FPS figure. If you are really curious, you may have to do some digging. If so, here are some places to start:

Temporal Resolution Multiplexing: Exploiting the limitations of spatio-temporal vision for more efficient VR rendering.

The Problem of Persistence with Rotating Displays.

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