confusion (e.g., P21"When it did that, I felt like taking a step back, when it closed. Or I was trying to move away from that effect, or that I was taking a step forward and felt unbalanced.") and some did not understand why the effect happened (e.g., P15 thought that theFoV was being changed by the researcher).
Forfixed-FoV, severalparticipants expressed frustration for its existence (e.g., P19
"More than too distracting; I felt like I was losing a lot of information, by not seeing the surroundings. Also, because, if you have this video in the middle of beautiful mountains, and you just can see one tiny bit."; P4 "I interacted less this time, and still felt sick.", P7 "This one was always closed, it was like looking at a TV screen, I didn’t feel as present."). Some participants counteracted the fear of missing out by moving more (e.g., P1 "I couldn’t read everything that was on screen. For example, for the name of the documentary, in the beginning, I had to move my head from one side to the other to be able to read it."). It is also relevant to point out that severalparticipantsreported eye discomfort (e.g., P19
"I noticed there is a bigger difference in brightness, and maybe that is why my eyes feel weird when I take offtheVRheadset", P13"I felt my eyes blinking more.").
Forunrestricted-FoV, severalparticipantsreported using the widerFoVto explore the scene (e.g., P22"In the others, since the field of view was smaller, it was good to focus on that determinate thing, but here, since it didn’t have that, there was more freedom to look around.").
However, they also reported discomfort, specifically in scenes with fast movement (e.g., P18"I felt uncomfortable when they were going down the canyon because the objects moved really fast and they seemed really close to me.", P16"There were some moments when going through the narrow canyons, for example, I noticed the feeling of vertigo a bit more.").
8 . 5 . D I S C U S S I O N
semi-structured interviews reporting eye discomfort (e.g., P19, P13); although we cannot conclude a cause, we posit that the severeFoVrestriction for a long period of time might cause eye fixation leading to eye strain. The severity of simulator sickness symptoms typically increases with time [DSS18]. For example, Min et al. [Min+04] prompted users operating a driving simulator with an oral SSQ at five minute intervals; participants reported nausea and disorientation after ten minutes and oculomotor symptoms after twenty five minutes. Due to a lack of availability of longer360º videos[Dav+18;Nas+19], we exposed users to relatively short stimuli of 8’49” minutes (less than the reported mean of 10 minutes [Sar+20]). The lowSSQ-TSandSSQ-Oscores we observed are consistent with this reduced exposure time and higher values may result from longer exposures. Our current results should be interpreted with this caveat in mind and future work should explore the use of a longer exposure time. We believe this would be particularly valuable as it may exacerbate the differences between conditions.
Considering Presence items, as expected,participantsexperiencingunrestricted-FoV reported higher values for General Presence and Spatial Presence compared tofixed-FoV.
This relationship is expected, since more severeFoVrestriction has been shown to reduce the sense of Presence [Al +19;FF16]. Based on the statistically significant differences in accumulated angles over several types of locomotion, we conclude thatparticipantsin fixed-FoVmoved their head less than inunrestricted-FoV, which may explain the reduced Spatial Presence values for fixed-FoV. However, we note that in the semi-structured interviews, someparticipantsreported trying to use motions to mitigate the limitations of thefixed-FoV(e.g., P1 moving their head to read the video title).
Although Presence items were not significantly different fordynamic-FoV, mean scores were higher than forfixed-FoV, and involvement was higher than forunrestricted-FoVas well. This supports further development of ourdynamic-FoV. FoVrestriction is a no-ticeable effect, as seen by fig. 8.3 and by the statistical significance forVS1 andVS5.
First off, the quantity of visual statements is intended to identify the most noticeable changes (by introducing statements that exist regardless of the condition) but the word-ing is subjective to users; for example, some users identified theFoVrestriction as part of "virtual environment" in VS1, which leads to differences between VS1andVS5. Be-ing noticeable is not necessarily bad, asparticipantsin theMNVCandSMNVCreported these statement’s enjoyability and comfort towards the central or higher levels of the scale.
However, their desire for its future inclusion is more diverse in their responses. Looking at semi-structured interviews can help clarify this disagreement. For someparticipants (e.g., P2, P6, P7, P12), thedynamic-FoVwas considered as part of the experience or as an attention guidance mechanism, especially in intervals with "fast" movement. Uninten-tionally,dynamic-FoVcan work asattention guidancesince the point of interest in a scene is many times something that causes optical flow. For example, when the secondary pilot jumps inI22, the point of interest is the lead pilot, an element that causes more optical flow through its movement, than looking at the faraway trees. FoVrestriction when tran-sitioning to a "fast" interval can also seem like it is intentional. For example, when the
pilots are preparing to jump, there is little optical flow noise and subsequently a small FoVrestriction; as they jump and transition to the "fast" interval, theFoVrestriction is in-creased to its minimum value of 40º, making it seem like it was intentionally introduced by the video. For otherparticipants, thedynamic-FoV was seen as something that was distracting and not helpful, as stated by P1 "It’s adding more visible movement than what was there before". Contemplating on fig. 8.4, we posit that in "slow" intervals, changes toFoVsize might introduce optical flow due to the contraction of the FoVrestriction, making it more noticeable for users. Current results should be interpreted having in mind theFoVsize limits used. While our innerFoVdiameter of 40º is smaller than Fernandes and Feiner’s 50º [FF16], it is of note that our restriction ofdynamic-FoVis mostly applied to "fast" intervals (from the comparison of meanFoVsizes and fig.8.4) and that in those intervals the meanFoVsize (57.64º) is higher than 40º. Since stricter FoVrestrictions can affect bothSSQand Presence scores [Al +19;FF16], future studies should investigate and adjust the minimumFoVsize accordingly, with the goal of maximizing Presence and minimizing symptoms.
8.5.1 Implications for Design, Limitations and Future Work
Aligning peripheral optical flow with motion classification fordynamic-FoV has shown to be effective in mitigating VIMS, while maintaining Presence. Although noticeable, dynamic-FoVwas well accepted byparticipants, allowing for exploration of "stationary"
and "slow" scenes, and protecting the user from optical flow during "fast" scenes. While someparticipantsenjoyed thedynamic-FoVconsidering it to be intentional, some partici-pantsfound it to be distracting due to instability inFoVsize. Previous work [Bon+08] has suggested that vection change like the one caused by theFoVexpanding and contracting alternately can lead to exacerbated symptoms. While we try to reduce this instability by polling optical flow ahead of time and smooth dampening the size ofFoV, these attempts are not fruitful in "slow" intervals. The crux of the issue, in this case, is when should you protect the user: a fast response to optical flow stimuli can causeFoVinstability if the optical flow stimuli is not maintained; a slow response to optical flow stimuli can expose the user to the stimuli for far longer than needed.
A possible solution using motion classification involves having automatic classifica-tion between "slow" scenes with small rotaclassifica-tional or translaclassifica-tional movement and "fast"
scenes with considerable translational movement. This classification would allow for restrictedFoVto be applied only in "fast" scenes, to customize parameters like the time of response to the type of scene, or to customizeVIMSmitigation strategies to the type of scene (e.g., using a less obtrusive strategy in "slow" scenes). Future work on velocity estimation or simultaneous localization using OpenVSLAMcould introduce a plethora of opportunities to augment360º video. While this is not yet possible for videos with multiple scenes due to inconsistencies in measures, single-shot videos can be used. For example, based on the precomputed locomotion direction of a single-shot video, we could