“…Both rigid shapes (e.g., rectangular [107]) or circular [108] and flexible shapes (e.g., [109]) have been used, as well as various display media (e.g., projection on cardboard [17,107]), transparent props [12,98], handheld touchscreens [40,59], or virtual lenses [64,82]. In addition, the combination of eye-gaze with other modalities such as touch [85,86], mid-air gestures [87,97,101] and head-movements [56,103,104] has been recently investigated for interaction in spatial user interfaces. For a recent survey on gaze-based interaction in AR and VR, see Hirzle et al [47].…”