**2.1 Spatial faithfulness supports naturalness in perception**

3D telepresence solutions aim to support natural perception in 3D – sc. spatial faithfulness – better than video conferencing systems [1]. Basic problem in videoconferencing systems is the lack of support for eye contact [2]. In flat screen based solutions, it stems for example from the displacement or offset between a display (showing a counterparty's face) and camera (counterparty's eyes) of a videoconferencing terminal. Note, that although eye contact is one of the early goals for 3D telepresence solutions, it is still not supported in most of the existing telepresence systems.

Hydra system (**Figure 1**) is an early approach for supporting spatial faithfulness in telepresence [3, 4]. With a mesh of connections and a separate (proxy) terminal for each remote counterpart, it aims to support virtual lines-of-sight between participants. With small displays and small camera-display offset(s), participants are

#### **Figure 1.**

*a) VTT idea for a Hydra system using tablets for a communication space and a computer display for a collaboration space, b) corresponding connections between cameras and displays, c) practical implementation showing all contents on one display (cameras on top corners are indicated by red circles) [3].*

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

able to get an approximate eye contact with each remote participant, and in certain conditions, even have a shared understanding of each other's relative positions.

Note that perceiving eye contact does not mean full gaze awareness, i.e. understanding of also other, intermediate eye directions. In a traditional Hydra system, terminals for each remote party can be placed independently by each local participant, which easily results with inconsistent meeting geometries across meeting sites, and thus also inconsistent positions and eye directions of the parties.

Note that in principle, any position of displays can support virtual lines-of-sight between participants. The situation can be compared to private house residents seeing their neighbors through windows, which, however, is the more unlikely the more of neighbors like to communicate with each other.

The easiest solution unifying a virtual meeting geometry between participants is to position proxy terminals in the same relative order into vertices of an equilateral polygon (e.g. a triangle, square, pentagon, hexagon, etc.). This naturally restricts the seating of participants more than in a face-to-face meeting.

A more recent screen based solution is Viewport by Zhang et.al [5]. In the Viewport system, high-quality 3D models are formed for each user in real time, and extracted and embedded into a common virtual geometry. Using 3D models enables correcting camera-display offset, and supporting depth perception by stereoscopy. The system supports eye contact between three sites, with one user at each site. In particular, limiting the number of sites into only few is a factor limiting the usability of corresponding solutions.

Natural perception of depth and distances belongs to the factors of spatial faithfulness. Note that this is not possible using 2D displays lacking depth, and strictly speaking not even with stereoscopic 3D (S3D) displays, whether multiplexed, polarized, or autostereoscopic, due to their incapability to support natural focus/accommodation (suffering from the sc. vergence-accommodation conflict, VAC [6]).

#### *2.1.1 Advances by XR technologies and computer games*

Note that spatial faithfulness is an inherent requirement in XR visualization, which aims at replacing, in a seamless way, parts of a physical view with virtual elements (or vice versa). XR visualization can as well be used for rendering models (cf. avatars) or visual reconstructions of human participants into a participant's view. Correspondingly, for more than two decades, developing enablers for XR has also advanced 3D telepresence solutions. These enablers include e.g. sensors for 3D capture, coding and streaming methods, low latency networks, tracking and detection for XR, camera and user positioning, motion capture and tracking methods, new display technologies, and general advances in algorithms and processing power.

In the same way, developing game technologies has advanced 3D telepresence solutions based on virtual modeling and rendering, here denoted as Virtual World (VW) approaches. Traditionally, in VW approaches, visual content has been modeled/produced in advance, and rendering of the content is based on real-time transfer of parameters for viewpoint and object positions, dynamic 3D shapes and poses (animation), etc. Although VW approaches are rather common and have their specific benefits, their description is omitted in our presentation, focusing on rendering of photorealistic real-time captures. This focus is reasoned in more detail in Chapter 2.5.

A recent example of a photorealistic telepresence solution based on advanced 3D displays is Google Project Starline (https://blog.google/technology/research/ project-starline/). A good example of a 3D telepresence system based on AR/VR (XR) visualization is MS Holoportation [7] (cf. https://www.youtube.com/ watch?v=7d59O6cfaM0). Both of them are quite impressive but obviously also

complicated and costly. In this article, we aim to define a more economical solution with good sides of both photorealism and XR visualization. Note that even a lone talking head on a screen - whether camera captured or 3D modeled - may well be a value-adding functionality. Communication and attractiveness may namely be supported by using e.g. a speech-controlled, look alike or anonymous virtual head (cf. https://remoteface.ai/ and a video at: https://www.youtube.com/ watch?v=prpPqwV5Weo).

## **2.2 Remarks on mobility and serving with viewpoints**

In above, Hydra system was described as an early attempt towards spatially faithful 3D telepresence. Using such full-mesh approach, and by making restricting assumptions on participant positions ("seating order"), all participants may perceive eye directions and participant positions consistently, however, apart from solving the disturbing camera-display offset. Furthermore, a regular setup with fixed camera and display positions naturally limits the mobility of participants within their meeting sites.

Note, that although a participant position is fixed, a whole meeting room with its occupant may move virtually. In [8], a solution is described for compiling captures of regular sensor and display setups into a landscape, enabling participants to mingle together with their meeting spaces within each other, like people in a cocktail party (**Figure 2**). For example, a capture setup in a square or hexagonal formation can be used. Writers of this paper are however not aware if someone has implemented and tested such arrangement.

Let us consider a Hydra setup a little bit further. If a Hydra system with all its terminals was in one large hall or open space, a participant is able to switch (walk) between different sites and perceive spatial faithfulness in each of them separately, i.e. participant mobility is supported in discrete locations.

User mobility may be supported in principle at any viewpoint when using near-eye glasses (NEDs) for viewing. Limitations set by fixed camera positions can be relieved using a setup of multiple 3D sensors, or multiple cameras in an array. For the latter, solutions based on wall-mounted camera arrays or moving cameras are described in [9, 10], correspondingly. Arbitrary viewpoints can be supported to remote spaces, provided that complete and real-time enough 3D reconstructions of those spaces are available, or that one of the multiple cameras provides the viewpoint along a desired line-of-sight (**Figure 3**).

For serving viewpoints from varying positions, i.e. receiving viewpoints ondemand, the system needs to deliver participant positions between sites in unified coordinates (a unified geometry). Renderings of remote participants need to be

#### **Figure 2.**

*Fixed capture setups can support collaboration in dynamic 2D landscapes: a) capture setup in hexagonal grid, b) captures arranged into a tessellated landscape, enabling c) moving with user spaces like people in a cocktail party [8] (image c) is creative commons image by Lucas Maystre from Renens, Switzerland - 053/365: Apéro au forum).*

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

**Figure 3.**

*Idea of bringing real-world meeting sites into a common geometry: a) three sites with users (dots) captured by RGB-D cameras in local coordinates, b) lines-of-sight between users in a unified coordinate system, and c) supporting lines-of-sights (new viewpoints) for a moving participant.*

compiled in each participant view correspondingly. A viewer's head orientation needs to be detected and tracked to serve him/her with a correct part (frustum) of the compiled scene.

The writers have disclosed several inventions using the above described approach [8–10]. Note that instead of delivering visual information as large and bitrate consuming 3D volumes, a visual stream may be a video, a stereoscopic video, or a video-plus-depth stream (V + D, [11]) allowing forming a stereoscopic video. When using the viewpoint-on-demand approach in this way, the bitrate requirement may be much lower than when streaming 3D volume data. Note that Chapter 2.4 will describe the viewpoint-on-demand approach in more detail.

As a summary, spatially faithful telepresence solutions aim to support realworld-like geometries among participants. However, as will be explained in Chapter 3.2, both virtual and photorealistic approaches may be feasible even without such support, and even much easier. For example, perception of depth and motion parallax can be supported without forming and maintaining perfectly unified virtual meeting geometries.

#### **2.3 Transmitting and displaying volume videos (3D streams)**

Ideally, 3D reconstructions are coded, delivered and displayed as real-time 3D streams. However, this is very challenging e.g. due to high computation power, and very high bitrate it requires. The benefits of volume videos include more freedom in choosing one' viewpoint (cf. motion parallax and alternative viewpoints) and support for multiple local viewers. However, both capturing participant spaces and supporting viewing with glasses bring considerable complexity to this approach.

Viewing from various viewpoints may be supported also using multi-view streaming and display methods. However, without capture and delivery of user positions (cf. knowledge of a mutual meeting geometry), users need to choose their position accurately among a priori specified locations e.g. in order to perceive correct eye contact(s). Several approaches are using this approach, although simplified by reducing the volume being supported. Multi-view video coding methods and standards are available and applicable for this [12], but more advanced (realtime) 3D coding method are still under development e.g. by MPEG. Special 3D displays are already available, supporting different 3D viewpoints for multiple local viewers.

In the future, by advances in transmission and display (e.g. using light fields), real-time streaming and display of 3D volumes becomes more feasible. An apparent benefit of these solutions is that simultaneous viewers can see the 3D content from their individual viewpoints, like in the real world.


*3.1. Chemicals and Reagents*

LC–MS-grade water (H2O), acetonitrile (ACN), formic acid, methanol, and DMSO were purchased from Merck Life Science (Merck KGaA, Darmstadt, Germany). Taxifolin, rutin, 4-caffeoylquinic acid, ishorhamnetin, quercetin, and kaempferol-3-glucoside were also obtained from Merck Life Science (Merck KGaA, Darmstadt, Germany). Unless indicated otherwise, all chemicals were purchased from Sigma-Aldrich (Milan, Italy).


**6. Conclusions**

This review first analyzed current evidence of a potential relationship between EGIDs and IEI. According to recent data, EGIDs seem more common in IEI patients than was already reported in the literature. It is reasonable to speculate that EGID can worsen the course of IEI, and vice versa. For this reason, early diagnosis is crucial to prevent complications and define the best personalized treatment. In this context, several unmet needs are still to be clarified. The literature data are still limited, and more research is needed to understand the pathogenetic relationship between these two chronic and invalidating clinical entities. Multicentric prospective studies should be performed to establish the real epidemiology of EGID in IEI patients, the disease-course phenotype, and the response to available treatments.

**Author Contributions:** Conceptualization, M.V. and R.C.; methodology, M.V. and R.C.; writing original draft preparation, M.V. and M.N.; writing—review and editing, M.V., M.N., I.B. and R.C.; supervision, I.B., M.D.F., S.C., A.L., G.L.M. and R.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank Daniela Montagna and Lorenza Montagna (Pediatric Unit, Department of Clinical, Surgical, Diagnostic, and Pediatric Sciences, University of Pavia, Pavia, Italy) for their invaluable support to the research in the field of inborn errors of immunity.

**Conflicts of Interest:** The authors declare no conflict of interest.


**2. Etiology**

There are various etiologies of mid-GI bleeding, but their frequency depends on patient's age and underlying comorbidities [6]. Below the age of 40, the most common causes include Crohn's disease, Dieulafoy's lesion, small bowel tumors, Meckel's diverticulum, and polyposis syndrome. Small bowel tumors could be benign or malignant [7]. Benign ones include small gastrointestinal stromal tumors (GIST), benign neuroendocrine tumors (particularly small carcinoid), hemangioma, adenoma, leiomyoma, lipoma, and neurofibroma. Malignant ones include large GIST, adenocarcinoma, lymphoma, malignant neuroendocrine tumors, leiomyosarcoma, and metastatic tumor from melanoma, lung, or breast [8–11].

Rarely, polyposis syndromes involving the small bowel may present with mid-GI bleeding. These include familial adenomatous polyposis, Peutz-Jeghers syndrome, and generalized juvenile polyposis. Over the age of 40, the most common causes of mid-GI bleeding include angioectasia, nonsteroidal anti-inflammatory drug (NSAID)-induced ulcers, Dieulafoy's lesion, and small bowel tumors. On rare occasions, other small bowel lesions can cause gastrointestinal bleeding. These include small intestinal diverticuli, small intestinal varices, hereditary hemorrhagic telangiectasia, Kaposi sarcoma, intestinal tuberculosis, blue rubber bleb nevus syndrome, hematobilia, hemosuccus entericus, aortoenteric fistula, infectious enteritis, radiation enteritis, ulcerative jejunoileitis due to celiac disease, cryptogenic multifocal ulcerous stenosing enteritis [12], amyloidosis, Behcet's disease, pseudoxanthoma elasticum, and Ehlers-Danlos syndrome [13]. The incidence of small bowel neuroendocrine tumors (SBNET) has been increasing over the last few decades, and they are now considered as the most common primary malignancy of small bowel. Adenocarcinomas are generally seen in the proximal small bowel, whereas SBNET and lymphoma are commonly located in the distal small bowel. Sarcomas (GIST and non-GIST mesenchymal tumors: leiomyosarcoma, liposarcoma, fibrosarcoma, Kaposi's sarcoma, angiosarcoma) are evenly located throughout the small bowel.


*2.3. Generations of Biofuels: A Brief Review*

Fuels produced by the biological process are known as biofuel. Primary biofuels are mostly fuelwood, wood chips, pellets, and organic materials, generally used for heating generation, cooking, or electricity purposes in a crude state. Secondary biofuels come from energy crops, agricultural and forestry wastes and by-products, manure, or microbial biomass used extensively in transportation and industrial purposes [35,36]. Innovative research on biomass for secondary biofuel production has been continuous for the last five decades of biofuels production. A wide variety of potential feedstock from all around the world is utilized for biofuels production. Based on the feedstock, biofuels are classified into four groups, namely: first, second, third, and fourth-generation biofuels [37–39].

However, this type of biomass has disadvantages such as its high initial investment for its production. The biofuel produced from algae is less stable than that produced from other sources, mainly because the oil generated by the algae is highly unsaturated, which means it is more volatile at high temperatures, so it is more likely to degrade. Furthermore, the high water quantity is also a problem when lipids have to be extracted from the algal biomass, requiring dewatering via either centrifugation or filtration before extracting lipids [46].

• Fourth generation. These biofuels, which are still in the developmental stage, use bioengineered microorganisms as microalgae, yeast, fungi, and cyanobacteria. Genetically altered crops used to consume more CO2 from the environment than they emit. These microorganisms are used to produce different fuels, including ethanol, butanol, hydrogen, methane, vegetable oil, biodiesel, isoprene, gasoline, and jet fuel [39]. Fourth-generation biofuel research started in 2006, and significant results have not been published yet in peer-review journals [35]. Figure 1 shows the schematic representation of the integrated biorefinery for 1G to 4G.

**Figure 1.** Schematic representation of integrated biorefinery for first, second, third, and fourth generation of biomass.