The most obvious thing think about when designing a graphical user interface in VR is that has to be diegetic appearing in the game world. But there are also a lot of other details to think about. For one thing, a head mounted display is much lower resolution than a typical phone or computer screen. So you always can easily become pixellated. This can be made worse because the interface is diegetic. So you might be viewing it side on and it would definitely be smaller than the whole screen. VR interfaces are likely to be much harder to read than a phone or computer interface and so small texts won't work well. Another issue is interface devices. We don't have a keyboard, mouse, or touchscreen. We don't even have a screen. VR control is great for 3D interaction but don't have the detailed interaction of a keyboard or touchscreen. Gaze-based interaction without a controller is even harder. Combined with the low resolution this means that UI elements should be bigger than a standard UI and easier to use. Better to have simple buttons than a complex dropdown menu. Unlike touchscreens, virtual UI screens don't have a physical surface. I can tell when I'm touching my phone because I can physically feel it and my finger won't go through the screen. You don't have that kind of touch feedback from a virtual screen. That means it has to have a wide tolerance for detecting when a user is touching it or not. You need to detect a touch in quite a thick box in front of and behind the screen. It's tempting to make interfaces partially transparent. It can look pretty cool but in VR, transparency can cause problems. You can easily get distracted by objects behind the screen. What that means is that you keep switching between focusing on objects at different depths, nearer and further from you. When we focus at different depths in the real world, we did two things. The lenses of our life focus on the object called accommodation and our eyes were tight to point to their object. Since both eyes are turned towards the same object, if an object is far away, your eyes will point nearly straight ahead, if it's close your eyes will be turned towards each other called convergence. In VR on the other hand, we are always accommodating to the depth of the actual screens in the HMD but we are converging at different depths depending on where we are looking in the scene. This can give you a headache and if you are constantly switching between near and far objects, it makes the headache worse. In short, transparent interfaces in VR give you headaches. Don't use them. One final thing, John Carmack, CTO of Oculus, worked a Facebook post saying that Oculus team had found the interfaces that slightly curved around you on the inside of a cylinder feel nicer than completely flat interfaces. And who am I to argue with John Carmack?
