VR is not the promised land anymore. The expectation starts to align with the current state of the technology. Some interest are gone, however, VR is not something that will happen someday. Is something that already happened and is part of the industry.
I've worked with VR for a while but there's a ton of guys talking about VR technology or VR development on Internet so let's try another approach here. Just for a moment, let's think about VR into a design context.
Measures
So let's start talking about Ergonomics.
Some guys from Oculus, Samsung, Google, Apple and other are really engaged in understand what is usability inside VR context and what is different from what we already know about VR.
I've being tuned at this discussion and also did some research about that topic some year ago, usually starting at the ergonomic side and iterating over the results testing efficiency and comfortability.
Eye
Even being one of the most important parts of our study I don't want to extend myself at this part or this could easily become a book, so let's just assume that your eye isn't so uniform and regular.
Color and light are more perceived into the region called fovea and some areas nearby.
Also, in the place where optic nerve passes through the optic disc you're blind. Don't worry, the other eye compensate this lack of vision with the help of your brain, that's why you've never notice that before, however, as a designer you really care about accessibility and that means that you really should avoid to put something very important on that spot.
SMLXL
Here, our measures comes in angles. Beside the natural bio-variance factor, some variance can also occur due the VR lens or due changes in stereoscopic FOV.
Another factor to be noticed here is how much your eyes move and how much of that movement is comfortable.
SMLXL
Other interesting measure is the IPD (Inter-pupilar distance). This variable is very important since it reflects on convergence/divergence in your point of focus. More Inter-pupilar distance usually means more depth as also more miniaturized (everything looks as a miniature if your IPD is very high). This can be a little counter-intuitive and is not my objective to explain all about it here but believe-me, more IPD makes the things looks smaller.
The average adult IPD (53mm - 55mm) is used by manufacturers to create the glasses . An IPD very far from yours can force a little bit your cognitive system. Not a problem at all if you're an adult but a huge problem if you don't have this cognitive system fully developed as happens on Childs (more susceptible to cognitive disorder that can lead to convulsion, as epilepsy).
This can really be a trigger for some major problems and that's one of the reason that current devices are made for adult and not for child. Kids and adults have huge differences between IPD.
SMXLL
Using the default IPD, the focal point (the point where both eyes convergence) is a very important reference. We can say that the element which your user is looking at is your most important spot and you should enhance this experience making this focal point meaningful and comfortable.
For that, using Unity, you should avoid anything less than 5 units from your camera since it provokes a huge eye convergence forcing your eye-muscles. Less than 3 units and the result is a total mess.
The cross-eye phenomenon results in double images, and sometimes, pain. This is the most common mistake on VR design these days. Non-trained eyes can't perceive depth right and will be guided by image size only. So the design make a mistake positioning their element and the user forces their optical nerve, unaware of what is wrong, until some discomfort happen.
Between 5 and 10 units you have the strong 3d area. On my experience, 7.5 units is the optimal distance to comfort/3d perception. As rule of thumb, I like to put focused elements at 7.5 units and the rest of UI at 10 units. Between 5 and 7.5 is used on very special occasions involving effects, movements and pyrotechnical stuff.
Between 10 and 20 Units is mid-ground, where your 3d element still 3d but not a strong one.
More than 20 Units, the perception of depth start to decay and the weak 3d area starts.
More than 30 units and the stereoscopy are gone. Your 3d element are flat as 2d.
A more critical aspect are the refresh rate which can be even more problematic if you're moving. Again, your cognitive system can be overloaded trying to "blend" each frame with the other one. Again, an issue if you are under 12 years old.
VR experience with a refresh smaller than 60 are very stressful. Most VR devices offer 90 to 120fps.
Head
Our head position is very influenced by our personality and mood. Also some variance can occur if you are sitting or standing.
SMLXL
If you're happy or are very self-confident, your head is probably a little higher in comparison with the horizontal line than a sad or less-confident person. Usually, if you look for a middle term, you'll find that most people have their eyes between 10 and 12 degrees downwards. (Some other studies say 15 degrees but on my experience that's too low).
SMLXL
SMLXL
This measure says a lot about how to organize your layout and put things inside the comfort zone.
If you're looking to fill the spatial verticality into VR, you should really bet in lower things because is more comfortable to look down than up.
On the counterpart, we need to understand that any experience can be framed as a two-way interaction (interactivity) and you can really change the mood of someone by placing elements higher. Higher elements also favored exploration and amusement, the most common desired feel from the dev-side.
It's up to you but as rule-of-thumb use your horizontal line as reference and put your most important information on the bottom of each element. You can than push your element higher if amusement is an important aspect of your user experience.
Adding our eye rotation to our head rotation the result is the one below.
Body
Most of your users, if not all, have a body. Most, but not all of them, can move their bodies. Some of them can have difficulties due physical and context factors so use body interaction as an optional UX-enhancement of your experience when is possible but try not to use body movement as the only way to do things.
Also, not all VR devices have body interaction.
When changing focus to an element at your side, you blink. Also, your eyes always goes to the desired place before your head.
Brain
You brain loves to be right and hate to be wrong. When your brain receives conflicting inputs, you 're in trouble. Your brain will say that something is wrong by creating several annoying answers as dizziness and seasickness. Unfortunately, VR is about give a visual input while your body receives another one. That's where we notice several issues as the famous motion-sickness.
The way to go is simple...Never move your player to a direction different from where he's looking at and avoid inertial breaks.
Additional interactions should be aligned to your line of sight and movement.
Hand
Achieving our extremity, there's really nothing very revolutionary to say. Just be simple and be tuned to hardware limits.
Not all hand systems works on the same way so you really should think on actions before interpret those actions as gesture. Also, some VR devices don't support hands so if your interaction depends on this, you're probably creating a very exclusive experience.
Conclusion
Put it all together and we have the result below:
Here are just some rules of thumb. The interesting part of think on VR is to think as a reality being created and not as a experience that should be interesting. Realities are interesting. Experiences can be interesting. You can read more about this here.