# Eye-Tracking in Interactive Virtual Environments: Implementation and Evaluation
## 1 Introduction
### Eye-Tracking in VR  
The present paper explores the possible interpretative value provided by eye-tracking in combination with VR technology. To describe the utility and potential of this combination of methods, an insight into their respective technologies follows.  
While research into eye-tracking has a history of around a hundred years, it has only gained traction over the last four decades because the technology has matured into relatively simple to use, and easily interpretable, tools. The very first eye-tracking solutions were crude, painful extrinsic measures, which attached direct pen-like transcribers to the eyeball. In 1935, [PERSON] published a major eye-tracking study based on manual transcription of eye movements captured on film [14; 15]. Indirect eye-tracking based on tracking light reflections in the eye, also known as Purkinje images [16], was first implemented in the 1970s and 1980s [17]. This technology uses the infrared light spectrum to obtain pupil center corneal reflections (PCCR), and is the most used eye-tracking method to this day.  
Mobile eye-trackers also have a history, albeit a significantly shorter one. Given the technical requirements for processing corneal reflection video feeds and video encoding in such a small mobile device, the technology first had to miniaturize and improve power and performance [18; 19] before it became a feasible tool. Mobile eye-trackers have therefore emerged only in the last decade or so; although, they are still less accurate and have less performance in frames per second captured compared to their stationary counterparts.  
Along with its application in video-capturing mobile glasses [20], mobile eye-tracking technology has also been implemented in VR headsets. This coincides with the so-called \"virtual reality revolution\" of the current generation of VR headsets [21; 22]. The first viable VR headsets of this generation appeared in 2013-2016 (Oculus Rift DK1 and DK2, and the first HTC Vive), and the first VR eye-trackers followed in 2016 (SMI VR ET addon to the abovementioned headsets). Currently, several vendors offer either inbuilt eye-trackers in VR headsets of their own production or eye-tracking add-ons intended as extensions to existing VR headsets. Advancements in 3D graphics rendering and VR display technology [23] allow the visual quality of VR displays to approach the physiological limits of human visual perception.  
What is the practical utility in combining VR and eye-tracking? Perhaps it is the synergy of this combination. VR is known for its ability to render visuals of projected, proposed, or imaginary spaces [24; 25], and facilitate interactivity with these imaginary worlds [26] and other people [27]. VR can thus transcend the limitations of time and space. Once the VR visuals are rendered, they can be combined with eye-tracking for numerous purposes, for example, validation of the spatial arrangement of a virtual environment [28] or user behavior in a virtual scene [29], reduction in graphics performance requirements by applying foveated rendering [30], or the use of eye-tracking as an interactive interface [31].  
This can be effected at a fraction of the cost of preparing a real scenario. The software and application program interfaces (APIs), which facilitate graphics processing, and rendering and spatial algorithms are also computed relatively quickly, some even in real-time. Real spatial data, pre-made 3D objects, and graphics pipeline workflows [32] can be employed to create plausible virtual spatial compositions that can be supplemented with spatial data visualizations and evaluations [33; 34].