Table 10.

Guidelines for possible software factors influencing visual fatigue.

ID_factor Evidence level	Software	Description	Guidelines
VF_8 III	Duration of display use	The longer HMD use, the higher visual fatigue (Yuan et al., 2018; Yue et al., 2018; Guo et al., 2019, 2020; Szpak et al., 2020; Marshev et al., 2021)	Visual fatigue symptoms can start after 10 min of use. About 20 min will induce visual fatigue. Therefore, breaks might occur every 15 min to prevent visual fatigue (Yuan et al., 2018; Yue et al., 2018; Chang et al., 2020)
VF_9 IV	Binocular disparity (possible and comfortable fusion)	High disparity can be fused without diplopia, but high disparity induces visual fatigue (Shibata et al., 2011; Patterson, 2015; Fuchs, 2017). Negative parallaxes lead to higher visual fatigue than positive (Sun et al., 2020)	Shibata et al. (2011) assume that the maximum and minimum relative distance of the comfort zone is between 0.8 and 0.3 D Apply ±1.0° disparity to avoid visual fatigue (Bando et al., 2012; Matsuura, 2019; Hibbard et al., 2020) However, according to Patterson (Patterson, 2009), fusion is possible from 80 arc minutes for high spatial frequencies and up to 8° for low spatial frequencies images
VF_10 IV	Motion parallax	Moving objects can induce more visual discomfort (Speranza et al., 2006). The more dynamism in videos, the more visual fatigue (Kweon et al., 2018). Vertical parallax induces visual fatigue (Sugita et al., 2019). However, motion parallax from head movement reduces visual discomfort (Kongsilp and Dailey, 2017) See also CYB_37 and CYB_45	Prefer slow-motion parallax cues in the virtual environment and avoid discontinuity (Speranza et al., 2006; Kweon et al., 2018; Sugita et al., 2019)
VF_11 VII	Texture gradients	Conflicting texture gradient could lead to more visual fatigue as those cues play a role in stereopsis (Lambooij et al., 2009; Su et al., 2018). Too sharp textures, when supposed to be far from the user, would be “unnatural” depth cues. Texture gradients can also inform about object orientation (Leroy, 2016), and if conflicting with other orientation cues and motion, it could participate in visual fatigue See also VF_10 and CY_46	When textures are determinant depth cues, make sure to reproduce gradients close to real visual perception to give orientation information (Leroy, 2016) See also VF_10 and CY_46
VF_12 IV	Occlusion	Objects hiding part of another will make it appear as “closer” to the viewer. If the object is supposed to be behind has other depths cues that make it closer, it could influence visual fatigue (Pietroszek, 2015; Leroy, 2016). The cues are ambiguous. When stereoscopy is displayed, since FoV in HMDs is limited, objects with negative parallax would be partially “cut” by limited FoV	Make sure to avoid ambiguous occlusion. Reducing the number of 3D objects can help. Reducing overlapping objects can help (Sidenmark et al., 2020), especially when you are supposed to reach and touch this object (Yu et al., 2021) Make sure that objects with stereoscopic cues are mainly located in the central vision
VF_13 IV	Blur	Blur can drive vergence and accommodation (Lambooij et al., 2009; Sweeney et al., 2014). Therefore, blurring objects where the visual system is supposed to rely on vergence and accommodation cues could lead to more symptoms	Apply blur in images carefully, not on objects of interest but on other objects in the scene to avoid driving unwanted accommodation (Lambooij et al., 2009; Sweeney et al., 2014) Also see CYB_18
VF_14 IV	Colors	The more frequent color changes, the higher visual fatigue (Kim et al., 2016) Color temperature seems to impact visual fatigue (Wang K. et al., 2020) Stereoscopic acuity can increase with increasing color discrimination ability (Koctekin et al., 2020) Color also has a link with luminance: see VF_6 and VF_7	Avoid highly changing colors Avoid highly saturated colors (Kim et al., 2016) Prefer low luminance colors See VF_6 and VF_7