This paper deals with two aspects of virtual world creation – consistency of data and isolation of independent
concepts. These concepts include among others 3D graphics rendering, physical simulation, audio processing, or
user interaction. Current systems for virtual world creation focus more on concrete concepts and less on their
independence thus limiting extensibility. They usually define static core functionality that quickly becomes
outdated due to the fast development in computer industry. Adapting XML on the other hand focuses on
extensibility, defining rules for cooperation (data sharing) and providing mechanisms for concept isolation. This
makes it suitable as the base layer for extensible virtual world languages

Hudec, Bohuslav

Miléř, Vlastimil

University of West Bohemia Digital Library

Creating virtual worlds using adapting XML

DSpace at University of West Bohemia

Creating virtual worlds using adapting XMLVlastimil MiléřPostgraduate StudentCzech Technical UniversityKarlovo náměstí 13 121 35, Praha 2, Czech Republicxmiler@fel.cvut.czBohuslav HudecAssociate ProfessorCzech Technical UniversityKarlovo náměstí 13121 35, Praha 2, Czech Republichudec@cslab.felk.cvut.czABSTRACTThis paper deals with two aspects of virtual world creation – consistency of data and isolation of independentconcepts. These concepts include among others 3D graphics rendering, physical simulation, audio processing, oruser interaction. Current systems for virtual world creation focus more on concrete concepts and less on theirindependence thus limiting extensibility. They usually define static core functionality that quickly becomesoutdated due to the fast development in computer industry. Adapting XML on the other hand focuses onextensibility, defining rules for cooperation (data sharing) and providing mechanisms for concept isolation. Thismakes it suitable as the base layer for extensible virtual world languages.KeywordsVirtual World, XML, 3D Graphics, Data Adaptation1. MOTIVATION ANDINTRODUCTIONBuilding a working virtual world is hard. It is so hard,because there is a large amount of concepts in thevirtual world, which must be synchronized andcooperate.It is a trivial task to model a tree today. A bit harderis to simulate its growth. New entities are required –sun light, substrate, water and nutrients, gravity orwind to name a few. This problem has potentialexponential characteristics. Each new element mayinfluence the existing ones. Therefore adding it into avirtual world may as a consequence causeinvalidation of current data structures and algorithms.Our goal is to minimize this unpleasant property. Oneof the possibilities to reduce the exponentialcharacteristics of the interactions in virtual world is toreuse data and algorithms. Once the algorithms forrendering, occlusions, collision detection, and eventrouting or physical simulation are developed, theymust be applicable on as much real world data aspossible without compromises. Each algorithmshould work with most suitable data.In this paper, we describe a data storage mechanismthat mimics the properties of the real world and that:• Keeps the data and concepts as muchisolated as possible and thus lowers theexponential growth in complexity.• Allows sharing of selected chunks of data toprevent duplication and inconsistency.2. ADAPTING XMLAdapting XML is based on best features of currenttechnologies for virtual world description(XML+DTD [1], XML Schema [4], VRML-97 [5],X3D [6], MPEG-7 [7]). These features are:• Transparent structure.• Selection of most important concepts.While it tries to eliminate the problematic ones:• Missing references support in parser.• Missing custom atomic types.• Poor isolation of concepts.• Monolithic architecture.To accomplish this, we employ following techniques:• References are processed in parser. Theirexistence is transparent for client code.• Type-libraries – a type-library is a modulecooperating with the parser providing meansPermission to make digital or hard copies of all or part ofthis work for personal or classroom use is granted withoutfee provided that copies are not made or distributed forprofit or commercial advantage and that copies bear thisnotice and the full citation on the first page. To copyotherwise, or republish, to post on servers or to redistributeto lists, requires prior specific permission and/or a fee.WSCG POSTERS proceedingsWSCG’2004, February 2-6, 2004, Plzen, Czech Republic.Copyright UNION Agency – Science Pressfor storing, parsing, serialization andvalidation of atomic types. No atomic typesare defined by the parser – there are onlycustom atomic types in type-libraries.• Adapting engine – adapting engine is a layerabove the parser that extracts the relevantbits of data from the whole file and creates a“writable view” on the data.Architecture of adapting XMLThe adapting XML inserts a new layer between XMLparser and the application. The new layer is calledadapting engine and its task is to adapt the data in theXML file using converters (Fig. 2-1).The typed and adapted DOM is similar to the XMLDOM [1] interface. But instead of using simpleelements, the elements implement two interfaces – acommon interface and a type-specific interface.Using the common interface, the element can beserialized or de-serialized from a stringrepresentation in the XML file. Also, the element canbe uniquely identified using this interface.The type-specific interface provides access to theinternal data For example, if an atomic typerepresents a triangle mesh, it can have methods formanipulation and rendering of the mesh. Figure 2-2shows the role of adapting engine in an application.Each part of an application receives suitable datathrough the typed and adapted DOM. This separationis a way to achieve one of our primary goals – theseparation of concepts and minimization ofdependencies.The components in the application can cooperate atdifferent levels [10]. Among others, they may use thefile in the file system and serialize access to it or theymay use the typed and adapted DOM.3. TYPES AND ELEMENTS INADAPTING XMLAtomic typesXML defines one atomic type – a string (CDATA).For some applications, this may be not suitable. Theyrequire multiple atomic types optimized for differenttasks. These applications would usually encode thesetypes into a string. But each application that uses thistype must provide the validation code on its own. Itwould be desirable to prevent the code duplication bymoving it into a shared type library. This type libraryis then used by the adapting engine. The libraryneeds to be written in a full featured programminglanguage that is able to interoperate with the adaptingengine.The adapting engine instantiates the type from thelibrary and uses it to parse and validate the string. Anapplication interfacing with Adapting engine usingthe Typed and adapted DOM and would see this typeinstead of the generic XML element.By allowing custom atomic types and defining theway of connecting them to the parser and adaptingengine, the mechanism for concept separation wasgiven a solid foundation. Application may parse,serialize and validate files while not explicitlyunderstanding the semantics and syntax of theircontents.Structured typesThe ability to define transparent structured types isparamount for flexible virtual world description.Fig. 2-1 Adapting engine in the system.ApplicationFig. 2-2 Adapting engine separates conceptsand promotes componentized architectureinstead of monolithic architecture.XML-formatted data fileXMLparser/serializationAdapting engineApplicationXML DOMTyped andadapted DOM3D renderingSound playerPhysical sim.App specificParser and adaptingengineData fileXML Schema definition of structured data types issufficient for our needs.In the examples in the rest of this article, XMLSchema syntax will not be used due to limited space.The structured types will be represented graphicallyas on figure 3-1.4. CONVERTERSThe problem of data adaptation may have differentcomplexness. Sometimes it is enough to ignoreportion of the data, rename data fields or fill missingfields with default values. Alternatively, theconversion may be very complex.A converter is an object cooperating with theadapting engine. Its declaration specifies a sourcetype and a destination type. When given an elementof source type, it creates another element ofdestination type. The created element is a “view” onthe source element through destination type. If itmakes sense for given pair of types, the changesmade to the created element should be reflected in theoriginal one.Adapting XML defines two types of converters –scripted and native converters. They differ inimplementation. Native converters are declared inXML file and implemented in type library. Scriptedconverters are completely defined in the XML file.5. VIRTUAL WORLD CONCEPTS INADAPTING XMLThis chapter describes the so far verified conceptsand enumerates the alternatives how each conceptmay be implemented if constraints are different.3D GraphicsFigure 5-1 shows types in a commonly usedhierarchical 3D scene composed of Segments. Thisdefinition was used for testing of adapting XML.The described scene definition was used due to itssimplicity and similarity with already existingdefinitions. It uses only discrete atomic types (int,float) and does not make use of advanced features ofadapting XML.Since 3D graphics data are usually large in volumeand it would be ineffective to have object for eachfloating point number of each vertex, an atomic typefor vertex can be defined in a type library. With therecent movement to pixel shaders and opaque vertexdata [8], this will suit most applications.In each application, there must be a module able tointerpret the semantics of the Segment. The definitionof types, the type library and the interpreting moduleform logical system component. It makes sense todistribute them together as a package. Support ofsimilar functionality is one of the X3D goals.Grip PointsIt is virtually impossible to create 3D graphics andother complex data used in virtual worlds without theFig. 5-1 Structure of a simple hierarchical scene.The arrows mark dependencies between types.VERTICESV:VERTEX*TRIANGLEINDEX1:intINDEX2:intINDEX3:intTRIANGLEST:TRINAGLE*GEOMETRYV:VERTICEST: TRIANGLESTRANSFORMATION_:float[16]MATERIALR:floatG:floatB:floatA:floatSEGMENTCH:SEGMENT*T:TRANSFORMATIONG:GEOMETRYM: MATERIALHierarchy ofsegmentsVERTEXX:floatY:floatZ:floatFig 3-1 Definition of structured typesVERTEX, VERTICES and TRIANGLE.VERTICESV:VERTEX*TRIANGLEV:VERTEX[3]Structured type nameChild nameChild typeAny numberof instancesConstantnumber ofinstancesuse of suitable tools. Grip points [9] is a simplemethod for editation of data transformable to pointsin 3D space (typically vertices positions or normals).Types GripPoint and GripPoints (vector ofGripPoints) were defined. An accompanying librarywas able to draw them as 3D points and user was ableto move them in 3D space.ExampleCooperation of isolated concepts using scriptedconverters was tested on a simple example. A usertype – Box – was defined. It consisted of 6 floatingpoint numbers named X1, Y1, Z1, X2, Y2, Z2. Theserepresented the two points in 3D space, where thebox was positioned (fig. 5-2).Two scripted converters were defined:• Box to Geometry – this converter maps aBox node to a Geometry node. Thegeometry node contains 12 triangles and 8vertices. The child nodes in Verticesreference the children of Box.• Box to GripPoints – the Box node isconverted to a GripPoints node containingtwo GripPoint nodes (X1, Y1, Z1) and (X2,Y2, Z2).This example, however simple, demonstrated thatfeatures of adapting XML fulfill our primary goals.The two concepts (Segment, GripPoints) wereindependent and the user defined Box type made useof them.6. SUMMARY AND CONCLUSIONIn this paper, the basic principles of adapting XMLwere described – architecture, definition of atomicand structured types and features of converters. Thenit was demonstrated, how some of virtual worldconcepts could be implemented in adapting XML.The adapting XML simplifies the creation of virtualworlds in by isolating concepts and thus allowing topostpone decision making. At its current stage it isnot meant to replace X3D or MPEG-7 as they aremore specific and suitable for today’s industry. Thevirtual world creation is still a very immaturediscipline and many virtual world implementationscontain few distinct concepts and sometimes the datain the file are misused for inadequate tasks (forexample using graphics data for collision detection).As the number of concepts in virtual world rises, theadvantages of adapting XML become more visible. Inthe future, set of independent libraries will beimplemented (for 3D rendering, collision detection,user interaction, physical simulation, soundprocessing). The creation of virtual world will besimplified to selecting appropriate libraries,implementing own concepts and connecting themusing scripted converters. Adapting XML tries tofollow a famous quote of Albert Einstein: “Makeeverything as simple as possible, but not simpler”.7. REFERENCES[1] Bray T., Paoli J., Sperberg-McQueen C.M., MalerE. (editors): XML+DTD specificationhttp://www.w3.org/TR/REC-xml[2] Clark J. (editor): XSLT specificationhttp://www.w3.org/TR/xslt[3] Clark J., DeRose S. (editors): XPath specificationhttp://www.w3.org/TR/xpath[4] XML Schema specificationhttp://www.w3.org/XML/Schema[5] VRML97 specification (ISO/IEC 14772-1:1997)http://www.web3d.org/technicalinfo/specifications/vrml97/index.html[6] X3D specificationhttp://www.web3d.org/x3d.html[7] MPEG-7 specification (ISO/IEC 15938-x:200x)[8] OpenGL architecture review board: OpenGLExtension Registry http://oss.sgi.com/projects/ogl-sample/registry/[9] Frey D.: AutoCAD 2000, Sybex 1999, ISBN 07-821249-8-4[10] Miler V., Hudec B.: Communication inComponentized System with 3D Graphics andMultimedia, WSCG 2003 – Posters, ISBN 80-903100-2-8[11] Gamma E., Helm R. Johnson R., Vlissides J: DesignPatterns, Addison-Wesley Pub. Co. 1995, ISBN 02-016336-1-2Figure 3-2 User defined type Box is rendered asGeometry and edited as GripShapes(independent concepts cooperate).Box in its raw formconsists of 6 floatingpoint numbers.Box as Geometry(12 triangles)Box as 2GripPointsRendered Segment(data adaptation istransparent to app).

English

http://dspace5.zcu.cz/bitstream/11025/949/1/J23.pdf

Creating virtual worlds using adapting XML

Abstract

Similar works

Full text

Available Versions

University of West Bohemia Digital Library

DSpace at University of West Bohemia