# Subtitling 3D VR Content with Limited 6 DoF: Presentation Modes and Guiding Methods
## 2 Related Work
### Traditional 2D Video Subtitling  
In recent years, research efforts on technological and user experience aspects for subtitles have mostly focused on traditional 2D video, addressing a variety of key aspects, as reviewed in [2]. Relevant examples are: positioning (e.g., [14]); representation of non-speech information (e.g., [15]); reading speed (e.g., [16]); presentation methods for better speaker identification (e.g., [8]); and personalization (e.g., [2]). Although most of these studies focused on TV-related services, a small number also focused on live events, such as opera performances (e.g., [17]). ### Traditional 2D Video Subtitling  
Research studies on VR360 video subtitling have also recently emerged, due to the increasing popularity of this immersive _medium_[1]. First, a study carried out by BBC [18] compared four alternatives for subtitle presentation modes: (i) _Fixed-positioned (evenly spaced)_: subtitles are equally spaced with a separation of 120\({}^{\circ}\) in a fixed position below the eye line; (ii) _Follow head immediately_ or _Always-visible_: subtitles are always displayed in front of the viewer, and follow him/her when looking around; (iii) _Follow with lag_: subtitles appear directly in front of the viewer, and remain there until the viewer looks somewhere else; then, the subtitles rotate smoothly to the new position in front of the viewer; and (iv) _Appear in front, then fixed_: subtitles appear in front of the viewer, and are then fixed until they disappear. The obtained results from the conducted user tests (\(n=24\) participants), using short clips, reflected that the _Always-visible_ mode was preferred by users, mainly because: (i) subtitles were easy to locate; and (ii) viewers did not miss the subtitles when exploring the 360\({}^{\circ}\) environment. However, the particular implementation in [18] resulted in blocking effects (i.e., subtitles blocked important parts of the images and were considered obstructive). Second, the study in [19] also compared the first two previous presentation modes (\(n=34\) participants). Although no clear differences between their appropriateness and users' preferences were identified, the issues of presence, VR sickness, and workload slightly favored the _Always-visible_ mode. Both studies noted the need for further research on the topic, not only considering longer pieces of content and different content genres, but also exploring additional presentation modes. An additional recent study [20] contributed with a VR360 video corpus to characterize commercially available subtitling solutions and practices, by analyzing the YouTube channels of two major providers: BBC and NYT. This analysis indicates that the _Always-visible_ presentation mode, which appeared to be the most preferred by users, has not yet been adopted by these providers, although a few VR360 players have recently started considering it [13]. Furthermore, this analysis confirms that many key requirements for efficiently subtitling VR360 content, as analyzed in [9], are not yet met. The subtitling solutions offered to date mostly rely on burned-in subtitles added at post-processing stages, and have a much stronger focus on enhancing the narrative of the videos than on making the content accessible. Consistent with this, the study in [13] provides an analysis and categorization of the subtitling features provided by the key existing VR360 video players. It is shown that further work on this topic is needed to fulfill key requirements in this context. Finally, the study in [9] identifies key challenges and requirements for efficiently subtitling VR360 content, and reports on a set of user tests to provide insights into the most appropriate solutions regarding key aspects: presentation modes, guiding methods, re-presentation of non-speech information, comfortable fields-of-view (FoVs), and use of easy-to-read language.