# Parallels of Head-Up Displays and Visual Safety for Driving
## 1 Introduction  
Head-up displays (HUDs), widely employed within the automotive industry, constitute a virtual presentation technique that projects essential information onto the vehicle's windshield. This innovation empowers drivers to access a diverse array of information without the necessity of redirecting their gaze downward, thus effectively reducing potential interruptions in their monitoring of the vehicle's external environment [1]. And it can be integrated with an autonomous driving system to offer real-time information, therebyproviding a more convenient and safer way to drive while interacting with information. Figure 1 shows a schematic diagram of the HUD optical imaging principle and light path. HUD technology has also evolved from the traditional combiner head-up display (C-HUD) and windshield head-up display (W-HUD) to the augmented reality head-up display (AR-HUD). C-HUDs project information onto a semitransparent surface for the driver to see; W-HUDs and AR-HUDs both project information onto the windshield, where the driver sees a virtual image of the display. The main difference between a W-HUD and an AR-HUD is the difference in virtual image distance (VID) and field of view (FOV), with the VID of an AR-HUD being at least greater than 6 m and the FOV being greater than 10\({}^{\circ}\)[2; 3; 4] and having the ability to interactively display driving information. The display information of a W-HUD is mainly concentrated in the center of the field of view, while the AR-HUD is displayed in the whole field of view, including the edge area. Figure 2 shows the difference between a W-HUD and an AR-HUD in terms of VID and FOV, as well as the difference in display information area. HUD systems mainly consist of light sources, phase source projections, reflective surfaces, and glass; their configuration and the human eye's observation of information constitute a typical binocular viewing system [5; 6]. Because of the optical characteristics of the reflective mirror surface and the windshield, when light enters the left and right eyes, a spatial perceptual mismatch (horizontal and vertical) is created and, thus, constitutes horizontal and vertical parallaxes [7]. As depicted in Figure 3, the distinction between the viewing angles \(\alpha\) and \(\beta\) defines the parallax, typically measured in milliradians (mrad). Figure 4 visually demonstrates the perceptual misalignment of the image between the left and right eyes. Despite the existence of a single image, binocular parallax gives rise to horizontal or vertical misalignments in binocular perception [7; 8]. In the context of horizontal parallax, two distinct types exist: convergence and divergence parallaxes,  
Figure 1: Schematic diagram of the HUD optical imaging principle and light path. Figure 2: W-HUD projection distances and the field of view versus AR-HUD. contingent upon the separation distance between binocular viewing points. In the HUD system, horizontal parallax is mainly manifested as convergence parallax. As a binocular system, HUDs inherently introduce binocular parallax within a binocular viewing context, affording HUDs the capacity to impart a sense of spatial depth [7,9]. Specifically, the design of AR-HUDs necessitates the presentation of information at various screen locations, each corresponding to distinct distances, facilitating a hierarchical information display and layout. This is achieved by introducing parallax at different display locations [1,5]. Consequently, when drivers engage with HUD information, they transition between distinct perceptions of visual depth, resulting in visual fatigue and diminished visual performance due to image parallax and variations in visual perception between the left and right eyes, resulting from parallax differences at different positions [8]. This effect is even more pronounced when the perceptual conflicts caused by the superimposition of virtual images on real objects are also considered [10,11]. Horizontal parallax and vertical parallax have obvious differences in visual perception effects.