Normally, when you play a 3D game, it will have some form of 2D graphical user interface displaying your health and other statistics, some actions you can do and maybe your map. Usually, the interface is overlaid onto the game screen so it appears flat and in front of the action of the game. It isn't part of the game world. It sits in front of it as part of the user interface. The type of interface is called non-diegetic. The diegesis means this world of the story, so non-diegetic means not part of the story world. Non-diegetic UI works fine for screen-based games but it doesn't make sense in VR. In VR, we don't see a screen in front of us. We are immersed in a virtual world so there is no screen to put 2D UI in front of. We can't see anything outside of the story world so the UI has to be inside the story world. It has to be diegetic. A diegetic interface appears in the world. It would normally appear as a 2D screen in the world that is also possible to touch user interface elements to objects in the world, on a wall, for example, or have free-floating UI elements like text or buttons just floating in space. The interface can contain any sort of user interface element you could have in a standard computer or phone interface, buttons, texts, sliders, images, checkboxes and many other things. There are a couple of ways of positioning diegetic interfaces. The first is to position them at a fixed position in the world. This can be a bit like a touch screen on the wall of a shop or a museum. As a player, you have to move over to the interface in order to see it or use it. This type of interface can be useful for interaction specific to a particular place. For example, the start of an experience. The Unity documentation calls this type of interface spatial UI. Diegetic interfaces can also be attached to moving objects so they move without object. For example, the Unity Fly demo has a spaceship that has an information display around it that shows health, score and the time remaining. Some of the most useful interfaces are attached to the player's body, so they can be with you all the time. For example, I might move my hand up to my face and the user interface appears in front of it. This is a bit like holding and interacting with a phone. This kind of on-body UI is really useful for information or interaction you need at any time. The VR drawing app Tilt Brush uses this type of interface very well. One problem is that an interface that's attached to you all the time can be annoying, especially if it's floating in front of your head. It's normally better to attach UI elements to your hand rather than to mimic a heads-up display that is always in front of you. VR interfaces have to be diegetic. They have to appear in the virtual world but that still gives us a lot of options about where to put them, floating freely on objects or appearing around players' bodies. You have to think carefully about what is most natural and easiest to use for your application.
