DissLiteratur/storage/WDWR52HH/.zotero-ft-cache

computers
Review
An Overview of Augmented Reality
Fabio Arena * , Mario Collotta , Giovanni Pau and Francesco Termine
Faculty of Engineering and Architecture, Kore University of Enna, Cittadella Universitaria, 94100 Enna, Italy; mario.collotta@unikore.it (M.C.); giovanni.pau@unikore.it (G.P.); francescogaetano.termine@unikore.it (F.T.) * Correspondence: fabio.arena@unikore.it
Abstract: Modern society is increasingly permeated by realities parallel to the real one. The so-called virtual reality is now part of both current habits and many activities carried out during the day. Virtual reality (VR) is, in turn, related to the concept of augmented reality (AR). It represents a technology still in solid expansion but which was created and imagined several decades ago. This paper presents an overview of augmented reality, starting from its conception, passing through its main applications, and providing essential information. Part of the article will be devoted to hardware and software components used in AR systems. The last part of the paper highlights the limitations related to the design of these systems, the shortcomings in this area, and the possible future ﬁelds of application of this extraordinary technological innovation.
Keywords: augmented reality; virtual reality; Industry 4.0; vehicle maintenance

Citation: Arena, F.; Collotta, M.; Pau, G.; Termine, F. An Overview of Augmented Reality. Computers 2022, 11, 28. https://doi.org/10.3390/ computers11020028
Academic Editor: Paolo Bellavista
Received: 25 November 2021 Accepted: 16 February 2022 Published: 19 February 2022
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁliations.
Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

1. Introduction
The term augmented reality (AR) was coined in 1992 by Boeing researcher Thomas Preston Caudell, who developed an AR application for industrial use to view some assembly diagrams. Today, there are several deﬁnitions of AR [1], although the most shared is that provided by Paul Milgram (Department of Industrial Engineering, University of Toronto) and Fumio Kishino (Department of Electronics, Information Systems, and Energy Engineering, Osaka University). They theorize the existence of different types of reality that create a continuum, which, starting from the real world, lead to a completely virtual world [2].
In their deﬁnition, the following environments are distinguished:
• Real Environment (RE): it is the environment in which we live and is governed by the laws of physics;
• Augmented Reality (AR): physical reality in which participants also see virtual elements; • Augmented Virtuality (AV): it is a virtual reality in which the participants also see
real elements; • Virtual Reality (VR): represents a synthetic world in which the participant is
completely immersed.
In Figure 1 we can see the schematic representation of the Reality–Virtuality Continuum [3].
AR’s basic functionality consists of creating links, direct or triggered by user interaction with the device, between the real world and the information generated by a device or electronic information. This circumstance provides an interface to the user of an electronically enhanced physical world. AR is the technology that aims to digitally integrate and expand the physical environment or the user’s world, in real time, by adding layers of digital information. This integration can be applied to various display technologies capable of overlaying or combining information (numbers, letters, symbols, audio, video, graphics) with the user’s view of the real world.
AR can superimpose computer-generated information on real-world views, amplifying human perception and cognition in new and extraordinary ways. There are different

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types of technologies in which AR can be applied, each pursuing different objectives or applications, such as those mentioned below:

• marker-based AR; • AR not based on markers; • AR based on projections; • AR based on overlaps.

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This work presents an overview of the main applications of AR, paying particular attention to its application to the industry of tomorrow. This technology can provide significant support in various fields of industry [4,5]; it is possible to think, for instance, of predictive maintenance in the automotive sector or the development of AR applications in the world of video games. A section of the article will be dedicated to hardware and software systems that support AR systems. Another chapter, as already mentioned, will

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2. Applications in AR

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2.1. A Brief History of AR

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uunniiqquueeAffuRunn,cctctiiooonmnssbaaninnddessdeerrvwviiccieetshs tththrhrooeuuggIhohTtthh,eeeuxusspeelaoannidtdinmmgaanntahaggeeemmineentnettgoorffammtieeoddniiccaawll didtaahttaabWbaassSeesNs,, (Wire ssNuuccehhtawassotthrheker)reemamnoodtteeWmmooBnnAititooNrriinn(ggWoofifrppeaaltetieisensnttssB,,iodiddeenynttiiﬁAficcraaettiaioonnNaanendtdwpproreervvkee)nntntiiooenntwooffoccrriktitiscic,aalhllyyaislillimprov ppsaaetticieetnnottsrs,s,aasnnuddcohoffaaansnyyhppeaaatthlhtoohllocogagiiereses,aansndcdhssouupopplpoeordrttfufoocrraeetldildoeenrrl,lyyeppnaatttieieernnttatssiwnwhmhooeuunssete,atanoniuninrtteiesllmilgige,enanttnd the eiepnlnniavfavteihsriroohionnloonmrgmtgeyteenimnnitnt[ee1ra[8t1ha]8s.rl]ho.T.ouhTrgethhutesitmheueoesfuetAsheoRrfoiufAnagRtphhpeitlnhmiceaethtduieoiscneamslo,sefiedncaictcploauprldlaiisclnleaogctwti“oosEnrfysoa,erlDiltnohecwceliusdddiefai”ong,rgntoot“hsfEieasycodeilfDiiatpaegatcnteihdoboselli”oos,ogdotyof sfaamcipliltianFtegotrbhlrionoousdgtahsantmhceep,ulisniengottfhhpreorumtgabheldethiscecaaunl snfeeieroslfdthp,aoatrpptarpbolljeicctsacttahinoenneexsrasctththpaaottsilpteirovonejerocaft gtthheeevthexienascI,toT aim tpouofnsaiictqilouintaetoeffusutnhrgceticivaoelninnsst,earntvoednftasicoeinlrisvtatithcereosuugtrhhgirtcoharluegein-hdteirmtvheenstuiosnesalatvhnirsdouuamglihzaanthioarnegeoe-fdmtihmeenepntasirootfnoafml edica tvhsieusubcaohldizyaasttoiotbhneeoofprteehrmeaptoeadtretoonmf, tahonendbimtoodurycihntomgboeoroefp[p1e8raa]t.tieednotns,, ainddemntuicfhicmatoiroen[1a8]n. d prevention of 2p.4a.tOiethnetrsA, papnlidcatoiofnsany pathologies and support for elderly patients who use an envIinrothnemscehnotol[e1n8v]i.roTnmheentu, sAeR otefchAnoRlogiines tchane hmelpedstiucdaelntssetcotuonrdearlsltoawndssofmore the d tpecahtnhiocallocgoynceinptsabesthteor.rFtotriminsetanthcer,othurgohughththeeuusseeoof fsimapplpelsimcaatritophnosn, eisnecqluuipdpinedg “Eye wfaitchilAitRataeppblilcoaotidonss,athmepenlitnirge sttrhurcotuurge hof tthheehuumseanobfodpyoorrtathbelemoslceacunlnaresrtsrutchtuarte projec

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2.4. Other Applications
InInthtehescchoomolmeenrvciiraolnsmecetnotr,,AteRchtencohlnooglioegsiethsactanexhpellopitstAuRdewntosutlodubnedberesnteafnidciasol mtoemore teecahsniliycaslecllonacseppetscibfiecttperr.oFdourcitnostratnoceh,ethlprotuhgehutsheer udsueroinf gsimthpelepsumrcahratpsehothnerosuegquhipthpeeduse of waipthplAicRataipopnlsictahtaiotnasl,lothwe evniteiwreinstgruicntu3rDe owf tihthe hthuemuasneboofday coarmtheerma;oilnectuhliasrwstaruyc,ttuhree ochf osen apcrhoedmuicctacl oaugeldntb, eorseeevneninthreeasltrtuimcteurwe iothf tdhieffseorleanr tsycostleomrscoarnabneyvaicecweesdsoirnie3sDb[e1f9o]r.e purchase ea[vap1sap9irll]iyiI.ocnIausntetsihlotlsehneaecrssvottpmiohceuaemctrsiiﬁearsrlcemlclopiaawsrtlioensvcdegtiucoetctwroot, ritoI,nhortgeTetcosianhtahpn3tepouDlllpsoicwogtahfiiteteifhsoliuntgthshsheaetcrtsoudeamsxuneprbdiolinonbfigeatadgtAchgawReamgipwteeuhro,raouAc;rhlRidfanosbcrteoehatuibshslserdwionsuebtaaﬁgeyn,chucitahteshleedtedocuuhtrmosoinesopgeorrneftorvaivdeel pwroidthuicnt acnouuldnkbneosweennciintyr,etahlrtoiumgehwthiteh udsieffeorfenatppcolilcoartsioonrsatnhyatacaclelossworvieieswbeinfogreinpteurra-ctive cmhaaspes[,1b9u].seIns, tahnedtmouertirsomlisneecst,oor,rIaolTlowaptprlaicvaetlieornsstocoamutboimneadtiwcailtlhy AboRockoautladxbi ebausseeddotno their pprorevfiedrerevdarlioocuastisoenrvaincdesprreelfaetrinengcteost[h2e0]s.tNatouws oadf aﬂyigsh, tths earnedarbeagsegvaegrea,lofrieflodrsainsswisthaincchethese dtuercihnngotlroagvieelswairtheinalarenaudnyktnaokwinngcihtyo,ltdh.roFuogr hexthaemupslee,ofAaRppislicuastieodnsinthmatuaslelouwmvsiefowringguided intoteurrasctoirveinmgaepnse,rbaul sIoesT, anpdplmicaetrionlisnues,edorinallporwivtartaevheolemrsetsoaanudtosmmaatritcacliltyiebso[o2k1,2a2t]a.xi based on their preferred location and preferences [20]. Nowadays, there are several ﬁelds in w3h. iAchRthSeyssetetemchsnologies are already taking hold. For example, AR is used in museums for g3u.i1d.eHdatroduwrasroerainndgSeonfetwraalrIeoT applications used in private homes and smart cities [21,22].
3. ARTShyestnemums erous applications of AR require specific software and hardware. Generally, 3h.1o.wHeavrdewr,arteheansdoSftowftwaraereused exploits the coordinates of the real world through cameras andTthraecnkuinmgedroeuvsicaepsp; ltihceataioimnsiosftAo Rtrarenqsufeirrethspisecpioﬁscitsioofntwinarfoeramndathioanrdtwo aarne.XGMenLefrialell,yu, sing htohwe esvoe-rc,atlhleedsoAftRwMarLe u(Aseudgmexepnloteitds tRheeacliotoyrdMinaarkteuspofLtahnegrueaagl ew).orTlhdethfruonucgtihoncaaml berloacsks of aAndRMtraLckainllgowdevthiceesf;utshieonaimbeitswtoeetnranresafelratnhdis pvoirstiutiaolnwinofrolrdmsabtiyonsptoecainfyXinMgLtﬁhlee,cuosninngection thbetswo-eceanllethdeAmR; MthLis(AalulogwmsenthteediRnceoalriptyorMataiorknuopfLvainrtguuaalgoeb).jeTchtse ifnunthcteiorneaall bwloocrklsd.ofThese AoRbMjecLtsaallroewcothnetrofullseiodnbbaseetwd eoenntrheealmaonvdemviretnutaslimwoprlledms ebnytesdpebcyifythinegutsheer (cFoingnuerceti6o)n. Most boeftwtheeenhathrdemwa; rtehicsoamllopwonsetnhtes iunsceodrpionrAatRionapopflviciartuioanl soabrjeecItos Tindtehveicresalthwaotrcladn. Tbehedsievided oibnjetoct:s are controlled based on the movements implemented by the user (Figure 6). Most of th•e haIrndpwuatredceovmicpeosn:enthtsesueseddeivniAceRs aaplploliwcatiuosnesrsaretoIoiTndteervaiccteswthiatht caAnRbesydsivteidmesd. inTthoe: AR • Ininptuetrdfaecveiceasc:tsthaesseademveicdeisaatollrowbeutsweresetno tinhteesreacdt ewviitcheAs RansdysttehmesA. TRhesyAsRteimnt.erAfacteypical
acetxsaamspalemmedaiyatobrebethtweeienntetrhfeasceeduevseicdesinantdhetheVAORMsAyRsteamp.plAicatytipoinc.alTehxeamupsleer can mraeyarbraentghee itnhteerffuarcneituusreedininhthisehVoOmMe AuRsinagppgleicsatutiroens. tThhate suusbesrecqaunernetalyrratrnagnesltahtee into facuapcbrnopunmlibiiltcteuma-irutneaisoniesnndde. nshAisinisnonvohrstaoth,hrmeiweoreuhaeusixpcsaApihnmlRgicpcagaalneeptisobpotfeulniarc.uneaAsstieintnodhponauitstnhtsfedovurerabvrsediiexrcoqaaeuumwisesinpAngtllglReoy,voagterfpasampnawlnsiiilnctaihagtnet,bpiaioununnittldost-dfmcinoeoravmsniedcmynresaamowinsrodsir,gnsewlgi.on,hTvgtiehhcasheemwinitgh, inapnudtsmcaannybemoofrvea. rTiohuesintyppuetssacnadnnbaetuorfev: afrroiomusgetystpiceuslaatnindgntaotubrlien:kfirnogm, tgoeuscthicinuglattoing to spbeliankkiningg[,2t3o,2u4c]h. ing to speaking [23,24].

FFigiugruer6e. 6F.uFncutniocntaiol bnlaolckbsloAcRkMs LA[R20M]. L [20].
• Sensors: these devices, useful in the tracking mechanisms, allow determining of the generic user’s position or the generic object. This operation is essential for the visual recording of the physical environment and its digital information; in this way, it will

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• Sensors: these devices, useful in the tracking mechanisms, allow determining of the generic user’s position or the generic object. This operation is essential for the visual recording of the physical environment and its digital information; in this way, it will allow a fusion of images of the physical world and the digital world. The composition of the scene is possible through the use of data traced by cameras or 3D models. The tracking devices and techniques are different (GPS, ultrasound, to name a few) and have different settings and ranges; they improve the tracking accuracy of the AR system.
• Display: these devices allow users to interact with the AR system. Examples are HMDs, monitors, and wearable devices (such as glasses, gloves, and clothing). HMD includes one or more cameras based on holography and optics (diffraction and reﬂection) techniques. These devices are typically placed on the user’s forehead during use in a speciﬁc application (from videogames to the medical sector to the engineering ﬁeld. Another device slightly different from the one just mentioned is the HUD (Head-Up Display). It is a very compact and lightweight device that provides additional information during any user’s activity. Finally, Google Glass, Hololens, and smartphones also fall into this category [25,26].
The main feature of an AR system is that of offering a service in real time. This technology does not just improve the user experience. It also provides a great business opportunity for service providers and, therefore, for companies. The development of smartphones, sensors, high-end camera quality, tracking technology, and wireless networks allowed the implementation of AR applications even in mobile environments. However, there are some attributes to consider when designing an AR system and its related architecture. Such quality attributes are classiﬁed as common attributes and high priority. Considering the immersive property of AR, the attributes of high priority include latency in rendering and monitoring, the Quality of Service (QoS), or parameters such as the bitrate, packet loss, ping, management of multiple devices, reusability, and integration of new architectures with those already existing. On the other hand, low priority attributes include the reconﬁgurability, security, uptime of the system, fault tolerance, user preferences, and multiple hardware support.
As for the software part, the two most important players in the market, Apple and Google, have interpreted AR through their development platforms, ARKit and ARCore [27].
ARKit is an iOS app development kit. Apple unveiled ARKit at WWDC’17, and, almost immediately, it was available to developers around the world. ARKit’s advanced features are limited to devices with A12 Bionic or higher chip variants. Currently, the A12 Bionic powers Apple’s iPhone XR, iPhone XS, and iPhone XS Max, while the more robust A12X Bionic is available in the 11-inch iPad Pro and the third-generation 12.9-inch iPad Pro. AppStore presents applications built with ARKit in different categories: productivity, education, business, and, above all, games. The overall download of AR applications has exceeded more than 13 million times, and in 2020, the number of AR users has a value of around 1.7 billion.
ARCore is, on the other hand, a development kit for Android apps. When Apple launched ARKit, Google already had some experience with AR technology. Tango, a so-called AR platform from Google, lasted nearly four years from summer ‘14 to spring ‘18, but it has not enjoyed the amount of hype that ARKit has made. Eventually, Google wrapped up Tango and rolled out a completely new AR SDK. In March 2018, ARCore was ofﬁcially released, and Tango was ofﬁcially passed. However, the ﬁrst stable version was only released in December of the same year.
One of the most popular applications for Android with ARCore is Google’s Playground.
3.2. Design Limits
Systems that adopt AR make it possible to have contextual processing information. It is possible to design applications and systems based on context awareness, thus greatly

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simplifying their daily lives. However, there are some practical problems related to the design of such systems [28,29]:
• Interoperability: in AR, objects and devices should communicate with each other without problems, producing a signiﬁcant result for users regardless of their heterogeneous nature; precisely for this reason, one of the leading design problems of an AR system is the interoperability between such devices. To overcome this problem, we should focus on semantic communication between objects and devices and between devices and devices so that they can interact without necessarily knowing their structure, thus working towards a common goal. This goal can be achieved using the semantic web, which can enrich the digital content and be viewed through the user interface [30,31]. The term “semantic web” was coined by Tim Bernes-Lee. With it, we mean the transformation of the World Wide Web into an environment where published documents, such as HTML pages, ﬁles, and images, are associated with metadata that speciﬁes their semantic context suitable for automatic processing, such as querying and interpretation by search engines. It will thus be possible to carry out searches based on a speciﬁc keyword and the construction of networks of relationships and connections between the various documents, according to more elaborate mechanisms than the simple hypertext link currently used by the normal WWW [32].
• Security and trust: since each device is unknown to the other devices, it is necessary to implement mechanisms for verifying the object’s authenticity through, for instance, certiﬁcates [33]. Another crucial point is the security of communication and the guarantee of information delivery. It is inadmissible that an untrusted device, a virus, or the loss of packets containing information compromise the correct functioning of the entire AR system. For this purpose, various encryption techniques (with symmetric and asymmetric keys) can be implemented through security algorithms such as AES (Advanced Encryption Standard), RSA, Difﬁe-Hellman, RC4 Double, Triple DES (Data Encryption Standard), and many others [34–36].
• Context sensitivity: communication between devices should be in real time, up-todate, and relevant to the context of user requests. Consequently, applications must be designed in such a way as to be sensitive to the context and to correctly process and deliver the information relating to the latter in order to avoid possible conﬂicts between the various intelligent devices on the network.
• Minimal user intervention: another problem of AR applications is their dependence on the user and their intervention. Each IoT device should be self-sufﬁcient and responsive. Its work should be invisible to the user, thus allowing them to have a much more autonomous system, even in any failures [37].
• Hardware problems: an IoT system uses a wide range of intelligent devices, from the least powerful, such as 8-bit devices, to the most powerful, such as 64-bit devices, which work in different environments and platforms, such as Atmel, Cortext, and Arduino. Therefore, one of the main objectives is the reduction in the energy consumption of these devices to make them more efﬁcient. In this regard, various techniques can be used, such as the use of recharging mechanisms using kinetic energy for moving devices, using sunlight, and much more [38–40]. Two other main problems found at the hardware level are the device’s failures and heaviness caused by its size. In particular, to overcome this last problem, thanks to technological advancement, we are already able to develop compact and low-weight devices, gradually guaranteeing greater portability.
• Software problems: given that AR applications can be run on hardware platforms with different computational characteristics, the software level’s main problem is interoperability and compatibility. The operating system used to be versatile must be optimized in terms of code, size, and power. Examples of operating systems that can be practiced in the AR context are TynyOS, FreeRTOS, and OpenWSN. There are also speciﬁc browsers for AR, such as “Firefox Reality” launched by Mozilla, which allows the display of content through the use of viewers (HTC Vive or Google Daydream), and

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various other devices that are still in the process of development. Therefore, it appears evident that it is necessary to develop appropriate toolkits capable of supporting different devices and applications running on multiple platforms and an interface capable of interacting as multiple interfaces where possible [41,42].
The IoT and the service network include sensor nodes, objects, RFID, and smart devices whose function is to interconnect objects around the user to provide continuous communications and contextual services. Through it, the user’s life is more comfortable.
Furthermore, in order to have more services, advanced technologies are used, such as AR. Combined with an IoT system, it makes life and user actions even more accessible, making the system more autonomous and “user friendly”. Indeed, the primary purpose of an IoT–AR system is to bridge the gap between the real world and digital world by interacting with the natural world in real time. The information-acquired context is the central pillar of these systems; therefore, context and management are critical processing features. All components of the entire system, from its architecture and the device’s applications used by the user, must be context-sensitive and must be empowered to refer and deliver the various services based on the priorities of the moment. Another fundamental pillar in an IoT–AR system is the technologies of communication and gateway protocols, such as CoAP (Constrained Application Protocol) and MQTT (Message Queuing Telemetry Transport), as they are required to connect IoT–AR devices and use cases ofﬂine to the outside world via the Internet. All data relating to the user, preferences, and the surrounding environment are stored, managed, and processed in the cloud. Therefore, they are essential advanced and speciﬁc security algorithms for such applications. If, on the one hand, such systems have a high potential, on the other hand, they are weak in their structure as they are very complex to implement and require a high degree of interoperability with a number of devices. There are, in fact, various related design problems to them, such as scalability, context and data management, performance, control access, and security and privacy, which require additional developments and research.
Thus, suppose the system is already at the hardware level. In that case, there are in the market products that are already widely used by users, and there are still some challenges to the software design of such a complex system to face and some objectives to be achieved. Despite this, being that the development of such systems is already making great strides year by year and that AR and IoT applications are growing dramatically as they bring more and more innovations, technological advancement in this sense and the design of such systems appears to be promising. If it were possible to manage to overcome the various design problems, in the future, the whole real environment would become interconnected from houses to cities. There would no longer be the gap between the physical world and digital, thus allowing the user to interact with the data and have services with minimum effort and minimum intervention.
4. AR in Industry 4.0
AR is increasingly being described as one of the leading technologies of the 21st century and as one of the pillars of the new industrial revolution envisaged by Industry 4.0. Numerous articles in the literature outline some of the main speciﬁc applications of AR to develop its potential in numerous ﬁelds of the various industrial sectors. However, the current limitations of this technology are not particularly contemplated in the literature, especially in the case of its introduction into a natural work environment, where daily activities could be carried out by operators using AR-based approaches. However, this paper will examine this later.
The hardware and software needed to implement an AR application typically depend on the complexity of the virtual scene to be reproduced. In general, the minimum hardware required to run an AR application is composed of a camera that frames the outside world, a screen or a lens that is used to project a streaming video, and computational resources (PC, smartphone, or microcontrollers in general) necessary for the management of the video part and the detection of the objects present in the actual scene. Some application examples

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outside world, a screen or a lens that is used to project a streaming video, and computa-
tional resources (PC, smartphone, or microcontrollers in general) necessary for the management of the video part and the detection of the objects present in the actual s1c0eonf e15. Some
application examples of the technologies proposed in Industry 4.0 are represented by an-
alyzing big data, the Internet of Things (IoT), additive manufacturing, intelligent sensor neotwf tohrektse,cmhnaoclhoigniees, apnrodpsoeslefd-minonInitdoursintrgy n4.e0twaroerrkespr[e4s3e]n. ted by analyzing big data, the
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Industry 4.0 [45,46].

Figure 7. Trend of scientiﬁc articles published on Scopus in the last 5 years relating to the keywords
“Augmented Reality”, “Industry 4.0”, and “Augmented Reality” AND “Industry 4.0”.
F“Aigouufgrtmeh7Aee.nnTntoeurtedmhnRedbreeoiarnflitosteycfri”ees,scnt“itieIninfngidtcisuﬁatsractttiriacsyrlteti4ics.c0cpl”eaus,nbatblnhiesdahot“ebsAdthauooignnwmeSdtce,honaeptgeukadsienRyinwbeatyohlicretdhyle”a“csAAkt i5NunyggDemtah“erIesnnStrdeCeudlOasttRPrinyUega4Slt.i0odty”at.”thaewbkaiesthey,words
another keyword that is indicated in the left column in Figure 8 (for example, “Design”
or “AGnaomthee”roirnetveernes“tIinndgussttarytis4t.i0c”.cTanhebreesoublttaoibntaeidn,eadg, vaiisnibbleyinchFeicgkuirneg8,tlheendSsCiOtsePlUf tSo databanseu,moefrtohuesnrueﬂmecbteiornosf. scientific articles that show the keyword “Augmented Reality” with anotheTrhkiesygwraoprhdatlshoaatlilsowinsduicsatoteadnainlytzheealsecfetncaorliuominnwihnicFhigliuttrlee h8a(sfoyretebxeaemn apnlea,ly“zDedesiingn” or “GAaRmaep”polircaetivoenns, “suInchduastIrnydu4s.0tr”y. 4T.0h.e result obtained, visible in Figure 8, lends itself to numeroAultshroeufglehcntiuomnse.rous scientiﬁc articles have now addressed the issues relating to the
devTehloispmgreanpthofaAlsRo taelclhonwoslougsietso, eavneanlytozdeaay,sncuemnaerrioouisnliwmihtaictihonlisttslteillheaxsisyteitnbtheeenlaargnea-lyzed in sAcaRleadpipssleicmaitnioantiso,nsoufcthheams ,Inesdpuesctiarylly4i.n0.the industrial ﬁeld. In particular, deepening the
cboenArteelsntohtlsovoeudfgshsoomnthueamotfetthrheoesueasrttesiccchlieensnoctlioitfgeicdieasinratrtiheclemebsoibrhelaiwovgeirdaneposhpwyre, aanddu.dmreersosuesdptrhoebliesmsusesstilrlenlaeteidngtoto the develoSppmeceinﬁtcoalflyA, Rrefte[c1h3]ndoleosgcriiebse,setvheenctuordreanyt, sniutumateioronuisnlriemlaittiaotniotnostshteilslperxeiasdt ionftAheRlargescatelechdniossloegmieisn.aItniopnarotfictuhleamr, b, yesmpeackiianlglyaninotvheerviniedwuostfrtihael frieesleda.rIcnh pacatrivtiictuielsairn, cdlueedpedenining the con96ternetlesvoafnstodmoceuomf etnhtes,aprtuibcleisshceidtefdroimn t2h0e11btiobl2io01g8r,aipt hisyp, onsusmibleerotouosbpsreorvbelehmows sAtilRl niseed to benroetsoyelvt eredasdoy tfhoratintdhuesetritaelcihmnpolleomgeienstaatrioenminorseomweidaeresapsr,ewadh.ile it is in other sectors. In

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Specifically, ref [13] describes the current situation in relation to the spread of AR
technologies. In particular, by making an overview of the research activities included in 96 relevant documents, published from 2011 to 2018, it is possible to observe1h1 oofw15 AR is
not yet ready for industrial implementation in some areas, while it is in other sectors. In
the same document, the authors present the state of the art, current challenges, and future dirtehcetsioamnseodforceusmeaenrct,hthine athuethfoiresldproefseAnRt thteecshtantoeloofgtihees.art, current challenges, and future
directions of research in the ﬁeld of AR technologies.

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inteInlli[g2e5n]c,eth(CeIc) oinllarebaolrtaimtioenthbreotuwgheeAnRh. uAmlthaonuogphedraigtiotirzsaitsioannhaalyszbeedenwuistehdcionmrepcuentattional intyaenelaldirgsheutnomcmeano(CsdecIr)anniinzpereermfaoolrdtmiemrtnheeintmhdruboseuttrtgieehrst,AhsaoRnm. mAe maltchahoninuuefgash.ctTduhrigiinsigtsitzouapdteyioracntoinohndasuscrbetsemeaansinyusmsteeamdnauintaiclr, ecent yeraervsietwo mofothdeerrnecieznet mlitoerdaeturnreionndAusRtraipeps,liscaotmioensmdaevneulofapcetdufroinr ginoteplleigreantitomnsanruefmacatuinrinmga. nual, and humans can perform them better than machines. This study conducts a systematic rev5i.eCwonocfluthsieonrescent literature on AR applications developed for intelligent manufacturing. This article has presented an overview of AR and its main applications. It seems
worthwhile, at this point, to describe some other existing limitations, which still determine
5. Ca noonnc-lmuassisoinves diffusion of these systems, as well as the possible future application scenarios faolrreTAahdRiys. mAaRertniitsciloaenwehdealpsl-reepsvtraieobsuleissnlhyt,eesddevcaoenrmalpoiuvmteeprrrvogivreaewpmheoincfstsAtienRchbnaoitnqhduseoiftitnswrmaersaeeiaanrncdahphaapprlpdiclwiacaatirtoeionsnhsso..uIAtldsseems
wobrethinwtrhoidleu,caetdtbheisfopreoiint tis, twoiddeeslycriinbterosdoumceedoitnhethreexinisdtuinstgrileismoiftatotimonosrr,owwh. iTchhesatirltliﬁdceitaelrmine a vniosnio-nmmasosdivuele dreifpfruessieonnts othf ethmeasien sliymstiteamtios,nsasidewnetilﬁl eadsdtuhreinpgotshseibcloencfeupttuuraelizaaptipolnication scenarios for AR. AR is a well-established computer graphics technique in research

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of some AR applications; it cannot always provide results in real time. These modules suffer from great extension due to the volume of the input data; in particular, when it increases, the computation time increases exponentially. Therefore, it appears evident that the hardware must be equipped with particular performing and capacious elements, possibly also resorting to clusters of computers operating simultaneously.
AR technology can ﬁnd applications in various industrial areas. An emerging problem in the ﬁeld of industry is the wide range of AR-compatible devices. The properties of these devices vary greatly, and the requirements for industrial use are different from those for consumer products. Aspects such as ergonomics, weight, and brightness must be considered according to the target environment and the speciﬁc use scenario in order to be able to make a good choice on which AR device is best suited to the speciﬁc use case.
Finally, integrating AR data into business workﬂows is an essential aspect. AR software is composed of different components, which excessively complicates the customization process within a company. However, this process is necessary to adapt the AR to the desired scenario and data integration. A modular software architecture could be a possible solution to increase the efﬁciency of personalization. This goal would reduce the costs of AR software within a company’s IT infrastructure.
Despite having exposed some of the limitations present in the diffusion of technologies in AR, numerous applications appear to rely heavily on these technologies in the future. First of all, the industrial sector, that of the so-called Industry 4.0. In this context, AR applications may provide for the use of so-called predictive or preventive maintenance. It can introduce many advantages both from an economic point of view and efﬁciency. The massive diffusion of preventive maintenance will allow avoiding annoying and unexpected machine stops due to sudden technical or structural problems, which, on the contrary, can be foreseen and planned for in due time. In this direction, the automotive sector, in general, appears to be the one in which this emerging technology can ﬁnd widespread diffusion.
Unlike virtual reality, augmented reality is designed for constant use on the move, which gradually replaces the smartphone. No one will ever be willing to use these technologies in tandem, e.g., such as cyborgs. For this reason, many companies are proceeding in the opposite direction: they are not creating bulky AR viewers. However, they are instead marketing devices similar in all respects to a regular pair of glasses but equipped with some initial smart functions [47].
In this sense, Facebook’s Rayban Stories are limited to taking photos, shooting videos, having integrated earphones, and little else. Amazon’s Echo Frames are designed above all to communicate with Alexa; again, Snapchat’s Spectacles are the ﬁrst glasses that integrate real augmented reality functionality [48,49].
Therefore, the future world could become more and more similar to what the digital artist Keiichi Matsuda imagined: streets that digitally signal the passage of cars, buses that indicate when our stop has arrived, supermarket shelves that indicate where this is. Alternatively, as John Hanke, founder of Niantic Labs, said: “AR is where the real Metaverse will happen” [50].
From this point of view, the real metaverse would no longer be an environment that limits our movements, alienates us from the physical world, and forces us to interact with avatars but our usual digitally enriched world. Some companies are also experimenting with AR fashion garments (for example, a leather jacket that is displayed in ﬂame by anyone wearing an AR headset) or its applications for smart working (during a remote meeting, it would be possible, for example, to view a colleague’s hologram instead of seeing their face on a computer screen). This also poses several problems regarding privacy, surveillance, data collection, tracking of all our behavior, constant availability, and more. All themes should be explored one by one.

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Author Contributions: F.A., M.C., G.P. and F.T. contributed equally to this work. 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: The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.
Acknowledgments: The research activities of this work are funded by the Kore University of Enna, through the research project SAMANTA-PON I&C 2014–2020 Fondo per la Crescita Sostenibile— Sportello “Fabbrica Intelligente”. The authors are members of CENs LAB—Computer Engineering and Networks LABoratory—Kore University of Enna.
Conﬂicts of Interest: The authors declare no conﬂict of interest.
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