# Eye-Tracking in Interactive Virtual Environments: Implementation and Evaluation
## 2 Methodological Issues  
### Eye-Tracking Technology Applied in VR  
To establish the common ground in all video-based eye-tracking technology, whether VR or otherwise, the similarities in the hardware and software of most eye-trackers must first be examined.  
The commonalities in the hardware include the type of technology used and the metrics for sampling rate, field of view, accuracy, and precision. Given the limited space of the interior of a VR headset, the eye-trackers used in VR employ tiny cameras that implement infrared PCCR, which is the most used eye-tracking technology today. The sampling rate, accuracy, and precision of all non-stationary eye-trackers, whether VR or mobile, is limited by the size, layout, and performance constraints of the hardware, resulting in worse parameters than stationary eye-trackers (eye-trackers used in VR sample at 75-120 Hz, at about 3-5 degrees of accuracy, whereas stationary eye-trackers are superior, sampling at, for example, 2000 Hz at 0.15 degrees of accuracy [39]). Most VR headsets use an LED display behind an optical lens to cover a visual field of 100-110 degrees to accommodate most of the clearly perceived human vision; the range covered by eye-trackers used in VR has greater limits due to the optical limits of the system [40].  
Eye-tracking software for VR (APIs, calibration procedures, and 3D engine extensions) is tailored to the virtual 3D world. The captured data contain two levels: the raw X/Y coordinate data, as captured through the eye-tracking cameras, and the transformed 3D data, as projected into the virtual world. Whether the raw data are accessible or correctible, and how it interfaces with third-party applications such as 3D engines, depends on the implementation of an eye-tracker API for VR or a 3D engine-specific software development kit (SDK) that draws functionality from the API. At the level of the 3D engine, it would be beneficial if the VR eye-tracking vendor provided an engine-specific SDK or a preconfigured extension (plugin) so that the API link between the eye-tracker and the engine would be well established, out of the box, with official support. This would allow the hardware capable features to be utilized in the engine without the need for deep understanding of hardware and low-level programming to access the functionality. All Tobii XR eye-trackers (HTC Vive Pro Eye, Pico Neo 2/3 Eye, HP Reverb G2 Omnicept Edition) [31] and enterprise solutions such as VRgineers [41] and Varjo [42] provide engine-specific plugins for eye-tracking in VR. Behavioral data inputs, such as eye-tracking gaze, fixations, dwell time, and object-to-object transitions, can thus be both logged and used as a means of interacting with the objects in a virtual world. Custom programming functionality in 3D engines can expand on the eye-tracker's extensions and functionality. All this, as depicted in Figure 1, provides a full picture of how an eye-tracking solution integrates with a 3D engine and a virtual world. The industry is even showing an inclination to develop standardized eye-tracking interfaces for VR [31] and SDKs [43].