Vergence-accommodation conflict

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Focal conflict due to chromostereopsis. Look at this image in full screen and feel the eye strain.

A vergence-accommodation conflict is when a brain receives mismatching cues between eyes' focus and angle. It is disorienting and can result in eye and brain pain. The effect can be unpleasant and cause eye strain.

Focal conflict can be encountered when viewing flat-focus stereograms, 3D movies, or virtual reality (VR). It can cause visual fatigue and headaches after a short period of time; It is one of the main contributors to virtual reality sickness. The phenomenon can make it impossible to focus on objects close to the eye in VR, limiting the development of VR software.[1]

This can happen with fixed-focus two-view displays, such as VR headsets and AR headsets including the Meta Quest, Oculus Rift, and Microsoft HoloLens. Focal conflict can occur in binocular flat focus autostereoscopic displays.

Focal conflict is difficult to overcome when designing new types of 3D displays.[1] Overcoming focal conflict requires the use of integral imaging, or holographic displays or sequential lightfield displays.

Effects[edit]

Focal conflict is part of why VR and 3D media causes eye strain and is disorienting.[2]

Focal conflict generally does not cause long term vision damage. Even though this is the case, users of classic stereoscopic gadgets report being unable to look at the 3D screen for a long period of time.[3]

Measurement[edit]

Focal conflict can be quantified; typically, by comparing the optical power required to focus on objects at the vergence distance with the optical power required to focus on objects at the focus distance.[4] In this context, optical power is equal to the reciprocal of distance, with units of Diopter (inverse of meters). Hence the difference between the reciprocal of the vergence distance and the reciprocal of the focus distance characterizes the extent of vergence-focal conflict.

In the example of a virtual reality head-mounted display, the focus distance corresponds to the distance of the virtual image plane. Often the optics is designed to place a virtual screen somewhere between 2 meters and infinity. That is, for a virtual display at a 2-meter distance, the target focus distance expressed in diopters is 0.5 D. In contrast, the vergence distance in a stereoscopic display can change freely based on the location of target content. For example, a virtual object by means of binocular disparity can be placed at a 30 cm distance, corresponding to 3.33 Diopters. In such a case, the magnitude of the vergence-focal conflict for a person with normal vision would be 3.33-0.5=2.83 diopters.

Physiology[edit]

A vergence-focal conflict is caused due to factors in human physiology like the accommodation reflex. Focal conflict occurs when the human brain receives mismatching cues between vergence and accommodation.[5][6][7] It often causes headaches and visual fatigue.[8] The vergence-accommodation conflict is one of the main causes of virtual reality sickness.[9]

There are two ocular responses: vergence of eyes, and focus, which is also known as accommodation. These mechanisms are linked together. Vergence or independent inward/outward rotation of eyes is engaged to fixate on objects and perceive them as single. Incorrect vergence response can cause double vision. Accommodation is the eye’s focusing mechanism and it is engaged to produce a sharp image on a retina. Both of these mechanisms are neurally linked forming the accommodation-convergence reflex of eyes.[10]

One can distinguish vergence (a distance of a point towards which both eyes are converging), and an accommodation (a distance of a region in space towards which the focus or refractive power of the crystalline lens has been adjusted to produce a sharp image on the retina).

Most people can tolerate some extent of vergence-focal conflict, without noticeable onset of adverse effects. While it depends on a particular person and viewing distance, vergence-focal conflict of around up to 0.4 Diopters is within comfort limits of most people.[4]

Causes[edit]

Virtual and augmented reality[edit]

Examples of fixed-focus biscopic gadgets that can cause focal conflict include the Meta Quest 3S, HTC Vive, Valve Index and the Microsoft HoloLens. Focal conflict can be experienced by bringing a virtual object very close to one's eyes in the headset and trying to focus on it.[11]

Stereoscopic displays such as holographic displays and light field displays can remove the problem.[5]

Other causes[edit]

Focal conflict can be experienced when using other technologies, including:

Effect on software[edit]

VR and AR hardware companies often ask software developers not to show virtual content too close to the user.[13][14] However, this is only a software mitigation and often times the effect can still be noticed.

Solutions[edit]

Figure 3. Different solutions for the vergence-accommodation conflict. (Image: roadtovr.com)

In VR and AR, new types of displays have been developed since the 2010s that can minimize or eliminate focal conflict to non-issue levels. These displays include varifocal, multifocal,[15] holographic, pin-mirror and light field displays.[16]

Varifocal displays are a concept explored mainly in VR display solutions. The basic principle relies on dynamically adjusting focal distance of displays based on the gaze direction. The technique requires an eye-tracking solution and means of modulating focal distance of a screen. Modulation of a focal distance can be, for example, physical actuation of the screen in relation to a fixed eyepiece optics, alternatively it can be utilization of varifocal[17][18] lens element(s). While varifocal approach mitigates or entirely solves VAC, it cannot convey realistic monocular focus cues. To try to add realism, these techniques rely on image processing techniques to simulate focus cues.    

Meta Half Dome prototypes addressed the problem with variable focus lenses that matched focal depth to vergence stereoscopic depth.[19] The first prototype used bulky mechanical actuators to refocus the lens. The third prototype used a stack of 6 liquid crystal lens layers where each layer could be turned on and off by applying a voltage, and this creates 64 discrete focal planes.[20] There are currently no production products using this technology.

Multifocal displays are a way of overcoming focal conflict. The principle of operation relies on availability of multiple image focal planes (screens), which from the perspective of a viewer are available simultaneously at all times. This gives the ability to accommodate eyes within the available range of focal distances and perceive realistic monocular focus (image blur) cues similarly to natural viewing conditions. Essentially multifocal displays discretize the depth dimension and split or slice the 3D content according to the available configuration of depth planes to minimize focal conflict. The topic of multifocal displays has been generously researched for at least several decades,[21][22] nevertheless, there is only a limited offering of commercially available multifocal near-eye displays.

Light field displays are one of the best ways to solve the vergence-focal conflict.[16] They share features with integral imaging displays.

CREAL, a near-eye display manufacturer for AR headsets/glasses, developed a light field display technology that projects the light rays just like they exist in the real world. This way, the virtual content has a real depth, and each eye can change focus naturally between the virtual objects, from up close to infinity.

SeeReal Technologies, a manufacturer of displays for 3D-enabled mobile gadgets, claim that their displays can generate visuals that do not have fixed focus.[23] The company developed the display used in the Takee 1 smartphone.[24] However, SeeReal's solution requires eye tracking, which can limit the 3D capabilities of the displays such as the field of view of the multifocality.[citation needed]

See also[edit]

References[edit]

  1. ↑ 1.0 1.1 (in en) Vergence-Accommodation Conflict: Facebook Research Explains Why Varifocal Matters For Future VR, This sequence of clips is taken from Douglas Lanman's talk 'Quality Screen Time: Leveraging Computational Displays for Spatial Computing' at the 33d annual Electronic Imaging Symposium (26-30 January 2020), https://www.youtube.com/watch?v=YWA4gVibKJE, retrieved 2022-09-26
  2. ↑ "Is it normal the 3DS effect hurt my eyes? - Nintendo 3DS". https://gamefaqs.gamespot.com/boards/997614-nintendo-3ds/72524629.
  3. ↑ "Is Your Nintendo 3DS Bad for Your Health?" (in en). https://www.pcmag.com/archive/is-your-nintendo-3ds-bad-for-your-health-262584.
  4. ↑ 4.0 4.1 Shibata, Takashi; Kim, Joohwan; Hoffman, David M.; Banks, Martin S. (2011-07-05). "The zone of comfort: Predicting visual discomfort with stereo displays" (in en). Journal of Vision 11 (8): 11. doi:10.1167/11.8.11. ISSN 1534-7362. PMC 3369815. PMID 21778252. https://jov.arvojournals.org/article.aspx?articleid=2121032.
  5. ↑ 5.0 5.1 "Resolving the Vergence-Accommodation Conflict in Head-Mounted Displays". 2022-09-22. https://3dvar.com/Kramida2016Resolving.pdf.
  6. ↑ Konrad, Robert (2015-11-06). "What is the vergence-accommodation conflict and how do we fix it?". XRDS: Crossroads, the ACM Magazine for Students 22 (1): 52–55. doi:10.1145/2810048. ISSN 1528-4972. https://doi.org/10.1145/2810048.
  7. ↑ 7.0 7.1 "3D technologies and eyesight: use not recommended for children under the age of six, use in moderation for those under the age of 13" (in en). 2014-11-06. https://www.anses.fr/en/content/3d-technologies-and-eyesight-use-not-recommended-children-under-age-six-use-moderation-0.
  8. ↑ Hoffman, David M.; Girshick, Ahna R.; Akeley, Kurt; Banks, Martin S. (2008-03-01). "Vergence–accommodation conflicts hinder visual performance and cause visual fatigue" (in en). Journal of Vision 8 (3): 33.1–30. doi:10.1167/8.3.33. ISSN 1534-7362. PMC 2879326. PMID 18484839. https://jov.arvojournals.org/article.aspx?articleid=2122611.
  9. ↑ Lawson, Ben D.; Stanney, Kay M. (2021). "Editorial: Cybersickness in Virtual Reality and Augmented Reality". Frontiers in Virtual Reality 2. doi:10.3389/frvir.2021.759682. ISSN 2673-4192.
  10. ↑ "Reflex action | Definition, Types and Mechanism and Important solved questions". 11 November 2020. https://www.crackyourtarget.com/reflex-action.html.
  11. ↑ (in en) (Video) Vergence-Accommodation Conflict: Facebook Research Explains Why Varifocal Matters For Future VR, VR Trailers & Clips, 18 July 2020, https://www.youtube.com/watch?v=YWA4gVibKJE, retrieved 2022-09-22
  12. ↑ Digital, Brandon Costa, Director of (2011-10-25). "Vergence-Accommodation Conflict: Why Bad 3D Literally Makes You Sick" (in en). https://www.sportsvideo.org/2011/10/25/why-bad-3d-literally-makes-you-sick/.
  13. ↑ "Comfort - Mixed Reality" (in en-us). 19 October 2021. https://learn.microsoft.com/en-us/windows/mixed-reality/design/comfort.
  14. ↑ "Magic Leap Developer - Game Design: Best Practices". 2019-08-26. https://ml1-developer.magicleap.com/en-us/learn/guides/best-practices-in-game-design.
  15. ↑ Zhan, Tao; Xiong, Jianghao; Zou, Junyu; Wu, Shin-Tson (2020-03-30). "Multifocal displays: review and prospect". PhotoniX 1 (1): 10. doi:10.1186/s43074-020-00010-0. ISSN 2662-1991.
  16. ↑ 16.0 16.1 (in en) Douglas Lanman (Nvidia) - Light Field Displays at AWE2014, https://www.youtube.com/watch?v=8hLzESOf8SE, retrieved 2022-09-26
  17. ↑ Wilson, Austin; Hua, Hong (2019). "Design and demonstration of a vari-focal optical see-through head-mounted display using freeform Alvarez lenses". Optics Express 27 (11): 15627–15637. Bibcode 2019OExpr..2715627W. doi:10.1364/OE.27.015627. PMID 31163757.
  18. ↑ Stevens, R. E.; Rhodes, D. P.; Hasnain, A.; Laffont, P.-Y. (2018-05-21). "Varifocal technologies providing prescription and VAC mitigation in HMDS using Alvarez lenses". Digital Optics for Immersive Displays. 10676. SPIE. pp. 142–158. Bibcode 2018SPIE10676E..0JS. doi:10.1117/12.2318397. ISBN 9781510618787. https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10676/106760J/Varifocal-technologies-providing-prescription-and-VAC-mitigation-in-HMDs-using/10.1117/12.2318397.full.
  19. ↑ VR, Oculus. "Introducing the Team Behind Half Dome—Facebook Reality Labs' Varifocal Prototype | Meta Quest Blog" (in en). https://www.oculus.com/blog/introducing-the-team-behind-half-dome-facebook-reality-labs-varifocal-prototype/.
  20. ↑ "Facebook's Display Research Lead: Varifocal Half-Dome 3 'Almost Ready For Prime Time'" (in en). 2020-07-28. https://www.uploadvr.com/half-dome-3-prime-time/.
  21. ↑ MacKenzie, Kevin J.; Hoffman, David M.; Watt, Simon J. (2010-07-01). "Accommodation to multiple‐focal‐plane displays: Implications for improving stereoscopic displays and for accommodation control" (in en). Journal of Vision 10 (8): 22. doi:10.1167/10.8.22. ISSN 1534-7362. PMID 20884597. https://jov.arvojournals.org/article.aspx?articleid=2191682.
  22. ↑ Rolland, Jannick P.; Krueger, Myron W.; Goon, Alexei (2000). "Multifocal planes head-mounted displays". Applied Optics 39 (19): 3209–3215. Bibcode 2000ApOpt..39.3209R. doi:10.1364/AO.39.003209. PMID 18349886.
  23. ↑ Häussler, R.; Schwerdtner, A.; Leister, N. (29 February 2008). "Large holographic displays as an alternative to stereoscopic displays". Stereoscopic Displays and Applications XIX. 6803. SPIE. pp. 219–227. doi:10.1117/12.766163. https://www.spiedigitallibrary.org/conference-proceedings-of-spie/6803/68030M/Large-holographic-displays-as-an-alternative-to-stereoscopic-displays/10.1117/12.766163.full.
  24. ↑ Kano3D (2020-12-29). "Takee 1: The first Holographic Smartphone gets a new Update after 6 Years!" (in en-US). https://www.tridimensional.info/2020/12/takee-1-the-first-holographic-smartphone-gets-a-new-update-after-6-years/.
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