The interface plays a vital role in creating immersion in spatial computing. Usually, when we speak about interfaces in interaction design, its more on the visual front, as the current devices and technology are more focused on the visual medium. But in the spatial world, when we are designing for immersion, it is not just limited to visual.
To create content for this medium we can use the concept called “Diegesis” created in greek theatres.
The fourth wall is an imaginary boundary between any fictional work and its audience
To understand diegesis, we need to focus on the concept of “Fourth Wall”. A theatre consists of three walls, two walls on the sides, and one behind. The fourth wall is an imaginary boundary that separates the stage from the audience. The characters are not aware of this imaginary wall or what’s happening on the other side of this wall. If a character interacts with the audience, it is said to be breaking the fourth wall.
Using the concept of diegesis, elements are divided into two categories: diegetic and non-diegetic, based on the existence in relation to the fourth wall, which are not limited to visuals but also include sound and other elements.
Diegetic means it is part of the scene (world space), and Non-diegetic means it exists outside of the scene. It is the difference between a piece of music played by the character in the scene (diegetic)and an external voice-over or background score played to enhance the scene (non-diegetic).
Diegesis theory has been used to analyze user interfaces in the gaming industry for a long time. The interaction elements are categorized based on where they exist, i.e., in space or the story or both.
Non-Diegetic elements usually are 2D elements that are overlays that live outside the space and story. In spatial computing, the usage of non-diegetic should be carefully handled as it will decrease the level of immersion. They are unaffected by the position and orientation of the user. Typical applications of non-diegetic UIs include a gaze reticle, or any system communication or alert that users must not miss.
Diegetic elements are part of the environment (space). The diegetic elements increase realism and the level of immersion. This is the most preferred way of designing interfaces for spatial computing. But there is a limit to realism currently. One should also be aware that if the elements lack intuitiveness, it will result in a decrease of immersion. This is where different natural interactions come handy to increase the level of immersion.
Spatial elements are interactive elements that float in space, and they exist in the space but not part of the real story they are fictional. Though these fictional elements reduce the level of immersion but provide a lot of clarity for the user and are easily recognizable, these are often flat 2D or 3D elements that do not obey the physics of the space.
Meta elements are part of the story and not part of the space. These are effects that are used to depict certain conditions in stories like shake, blur, and discolor. But meta elements reduce the visibility for users, so it is not commonly used in spatial UI.
Understanding the diegesis theory helps us in categorizing the Spatial UI elements better.