**4.3 Supporting lines-of-sight**

A straightforward way to enhance the described solution is to support virtual lines-of-sight between participants in the same way as in the describe Hydra approach, i.e. by using an own screen based terminal for each of the (maximum number of) remote participants (cf. Chapter 2.1). This means full mesh connections between peers, but the total bitrate may remain low due to each of the streams being a low bitrate video-plus-depth stream. This enhancement would also enable also earlier referred options for increasing user (plus meeting site) mobility e.g. by grid based geometries and interactive landscapes (cf. Chapter 2.2). The downside of this approach is the increase in the complexity for a meeting setup, and support for only a limited number of participants due to the lack of space around a participant. This version may however be justified in special cases where better spatial and gaze awareness between interactive participants is particularly important.

#### **4.4 Natural eye-focus/accommodation**

Stereoscopic rendering is a further means to improve 3D perception. However, stereoscopic viewing detaches natural eye focus (accommodation) and convergence distances. The resulting vergence-accommodation conflict (VAC) causes discomfort and nausea, and restricts a person's willingness to view stereoscopic content [6, 25, 36]. A related conflict in monocular viewing is the sc. focal rivalry (FR) [37]. In addition to VAC and FR, there are various other error types caused by the

#### *Advances in Spatially Faithful (3D) Telepresence DOI: http://dx.doi.org/10.5772/intechopen.99271*

wrong blur of rendered scene components, e.g. all-in-focus virtual objects, or real objects blurred by camera optics. These types of distortions appear especially when combining content for augmented scenes. Without careful considerations, these distortions continue hindering the quality and acceptance of XR functionalities.

As described in Chapter 2.6, unnatural occlusions are another common type of distortion, which occurs especially in XR, when combining optical views (cf. OST glasses), photorealistic captures, and 3D modeled objects and spaces. In addition to occlusions, supporting natural focus is a big challenge in display design and manufacture, and has not yet been properly solved for either external screens or near-eye displays. Natural focus requires that a content is rendered into a 3D volume, instead of one or few display surfaces. There are volume displays aiming to this, but generally they suffer e.g. from the lack of occlusions, colors and brightness.

There are various means to support natural focus, including for example light field rendering [38]. Supporting occlusions in a light field display has been studied e.g. in [39], and occlusions for its simple variant multiple-focal-plane (MFP) display in [25]. MFP-rendering is essentially light field rendering to one viewpoint, and as such fits well to the above introduced viewpoint-on-demand approach. The writers have studied various ways of forming and rendering MFPs, and have even made own designs for MFP glasses [40].

Implementation of MFP or other accommodative glasses is however very challenging, as for example the efforts by Magic Leap, Inc. has us learned. However, as glasses are superior in supporting immersion and mobility, we expect that major technical problems will be solved, and high quality accommodation support will be available within about a decade. Note that their (eventual) emergence will in general mean a big change to the ways visual information is captured, processed and displayed.
