All multimedia services must be accessible. Accessibility for multimedia content is typically provided by means of access services, of which subtitling is likely the most widespread approach. To date, numerous recommendations and solutions for subtitling classical 2D audiovisual services have been proposed. Similarly, recent efforts have been devoted to devising adequate subtitling solutions for VR360 video content. This paper, for the first time, extends the existing approaches to address the challenges remaining for efficiently subtitling 3D Virtual Reality (VR) content by exploring two key requirements: presentation modes and guiding methods. By leveraging insights from earlier work on VR360 content, this paper proposes novel presentation modes and guiding methods, to not only provide the freedom to explore omnidirectional scenes, but also to address the additional specificities of 3D VR compared to VR360 content: depth, 6 Degrees of Freedom (6DoF), and viewing perspectives. The obtained results prove that always-visible subtitles and a novel proposed comic-style presentation mode are significantly more appropriate than state-of-the-art fixed-positioned subtitles, particularly in terms of immersion, ease and comfort of reading, and identification of speakers, when applied to professional pieces of content with limited displacement of speakers and limited 6DoF (i.e., users are not expected to navigate around the virtual environment). Similarly, even in such limited movement scenarios, the results show that the use of indicators (arrows), as a guiding method, is well received. Overall, the paper provides relevant insights and paves the way for efficiently subtitling 3D VR content.
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