The invention relates to a method and a system for generating virtual scenes intended to be displayed on at least one display device.

Methods for generating virtual worlds are already known. The VRML (Virtual Reality Modelling Language) international standard provides a list of descriptions of nodes that can be used to build a virtual world. In a virtual world constructed by the use of VRML, each node must be separately defined. The VRML standard provides only very limited support for virtual world parameterisation (DEF/USE statements). Although, building complex objects can be simplified by the use of prototypes (PROTO statements), they cannot be connected by the inheritance relationship. This makes the description of a virtual world more complex than necessary. Also, there are no standard means of accessing databases from a VRML system.

The extension proposed recently by the VRML Consortium Enterprise Technology WG "Recommended Practices for SQL Database Access" especially in the "Server Redirect" section, has serious limitations. It allows retrieval from a database only values of fields of existing nodes. It can be done directly only for supported types and must be done indirectly - by the use of Script nodes - for not-supported data types. Elements like nodes, prototypes, routes, and fields (not field values) cannot be retrieved from a database. This limitation particularly precludes the possibility to keep models of virtual worlds or their elements inside a database.

The invention overcomes these drawbacks by a method based on W3C Recommendation specifying XML syntax. This method extends the VRML International Standard.

The method according to the invention is characterised by the following steps:

• generating a virtual scene description in conformance with a description model based on a document structured markup language, this description model provides at least loops, conditional statements, assigning values to variables, inserting values, connecting to a database, retrieving data from a database, and use of classes;
• translating said description into a virtual scene modelling language;
• displaying said scene on at least one display device.

According to another characteristic of the invention, the description model is based on a document structured markup language and comprises non-nested data records representing modelling language tags which have names and sets of attribute specifications.

According to another aspect of the invention, the virtual world data used during the translation process, including nodes, prototypes, routes, fields, and all types of field values may be provided by at least one database.

An advantage of the invented method resulting from the use of concepts like variables, loops, and conditions, consists in simplification of the description of virtual worlds and a possibility of the description parameterisation. Particularly efficient is coding of elements that have a repeating structure.

Another advantage of this method results from the possibility of generation of modelling language descriptions on different levels of details of the virtual scenes or with different contents (e.g., depending on user privileges).

The method allows also building inheritance hierarchies of classes, which greatly improves efficiency of the coding virtual scenes, and makes the description of these scenes more concise. Furthermore, it enables code reuse.

The method allows database access at the time the virtual scenes are being dynamically generated in response to a user request. Any type of virtual scene data can be retrieved from a database.

A better understanding of the invention will be obtained by reference to the detailed description below, in conjunction with the following drawings in which:

• Figure 1 is a generalised schematic diagram of a system used for implementing the method according to the invention;
• Figure 2 illustrates an array of binary arithmetic operators used in method according to the invention;
• Figure 3 illustrates an array of relational arithmetic operators used in method according to the invention;
• Figure 4 illustrates an array of Boolean operators used in method according to the invention;
• Figure 5 illustrates binary string operator used in method according to the invention;
• Figure 6 illustrates an array of relational string operators used in method according to the invention;
• Figure 7 illustrates an array of operators used in method according to the invention

The following description concerns a particular mode of implementation of the method using the VRML language. For more convenience, the extension of the VRML language obtained by the method of the invention will be designated by the term X-VRML.

Figure 1 represents a system for implementing a method for generating virtual images of virtual scenes intended to be displayed on at least one display device and described by the use of nested XML elements containing at the lowest level VRML code.

By reference to Figure 1, a memory 2 comprising files, which contain X-VRML virtual scene descriptions, is connected to an XML parser 4. Said files are read by the parser 4 and parsed at the time of user request. The parser 4 constructs a map of XML elements representing X-VRML scene description components. This map is then analysed, interpreted, and converted to its final VRML form by an X-VRML processor 6. During this conversion process the X-VRML processor 6 can retrieve data from at least one database 8 used as a data repository. Said data can be included in the processing outcome and include attribute values, objects, class definitions, and fragments of VRML or X-VRML code. These data can also influent the process of generating the final VRML form.

According to one important aspect of the invention, the method comprises the following steps:

• generating a X-VRML virtual scene description in conformance with a description model based on XML or SGML language, this description model provides at least: loops, conditional statements, assigning values to variables, inserting values, connecting to a database, retrieving data from a database, and use of classes;
• translating said description into the VRML language;
• displaying said scene on at least one display device.

In a possible implementation of the method according to the invention, the VRML language may be replaced by the MPEG 4 language.

The X-VRML language defines data records expressed by XML tags, which have names and attribute specifications according to XML syntax. An attribute specification has a name and a value which may be a constant or an expression that must be evaluated prior to tag interpretation.

There are three types of tags: empty tags, start tags, and end tags. Empty-tags are commands used to denote empty elements, i.e., elements that do not contain X-VRML code. In one particular embodiment of the invention, tags are always delimited by special characters like the signs '〈' and '〉' for example, and cannot be nested. Every '〈' character must be accompanied by '〉' character before a new '〈' character can appear. Empty tags are indicated by the slash '/' character at the end of the tag. Examples of empty tags in X-VRML are "〈CONNECT/〉" and "〈INSERT/〉".

The beginning of every non-empty XML element is marked by a start-tag. Examples of start-tags in X-VRML are "〈ITERATION〉" and "〈QUERY〉".

The end of every element that begins with a start-tag must be marked by an end-tag. An end-tag has the same name as the corresponding start tag and is indicated by a slash '/' character in front of the tag name, e.g., "〈/ITERATION〉" and "〈/QUERY〉". A pair of start- and end-tags defines a non-empty element which may be nested. The text that is between a start-tag and its complementary end-tag forms element body whose meaning depends on the element type. Valid element nesting is defined by the Document Type Declaration (DTD). X-VRML descriptions of virtual scenes must consist of valid XML documents according to the X-VRML Document Type Declaration (DTD).

For coding elements that have repeating structure, the method according to the invention uses a number of tags in X-VRML implementing different types of loops:

• FOR, used for numerical values; the loop is repeated with a numerical control variable varying from a beginning to an ending value, every time changed by a step value;
• ITERATOR, repeated for each value in a specified list of values,
• WHILE, repeated as long as a condition is satisfied,
• QUERY, used to get data from a database; repeated for every row of data selected from a database.

The FOR loop may be used to repeat a fragment of code an arbitrary number of times with a numerical control variable. The variable takes values from a specified range (defined by FROM and TO attributes). During the first loop traversal it has assigned the value defined by the FROM attribute. During each of the next loop traversals its value is being modified by the STEP attribute. When the value declared in TO attribute is exceeded, the loop is not being repeated anymore.

The syntax of the FOR loop is the following:

〈FOR NAME="..." FROM="..." TO="..." STEP="..."〉

〈/FOR〉

The FOR element contains the X-VRML code that should be traversed a number of times with the changing control variable value. The FOR tag has four attributes: NAME, FROM, TO, and STEP.

The meaning of the attributes is the following:

NAME is mandatory and represents the name of the loop control variable. The value of the loop control variable may be accessed inside the loop.

FROM is mandatory and represents the initial value for the loop control variable. During the first loop traversal, the loop control variable is set to the value defined by this attribute. After each loop traversal the value of the control variable is being modified by the value specified by the STEP attribute.

TO is mandatory and represents the ending value for the control variable. When the control variable exceeds the value specified by the TO attribute, the loop is not repeated anymore.

STEP is optional and represents the value by which the control variable is modified after each loop traversal. The value of the STEP attribute may be positive indicating that the control variable should be increased after each loop traversal or negative indicating that the control variable value should be decreased. If the STEP attribute is omitted, the default value "1" is taken.

The ITERATION in X-VRML is a loop that is repeated an arbitrary number of times, once for each value specified in a list. For each of the loop traversals, the control variable is set to next value from the specified list of values. This control structure is particularly useful for repeating a fragment of code in a case when the list of occurrences is known during the design time. Nevertheless, since the list can contain not only constant values, but also expressions that are evaluated before the values are assigned to the control variable, this construct can be used also for values that are calculated during the parsing process. Values in the list are preferably separated by the comma sign ' ,'.

The syntax of the iteration loop is the following:

〈ITERATION NAME="..." LIST="...,...,..."〉

〈/ITERATION〉

According to the invention, the ITERATION tag denotes a non-empty XML element containing the X-VRML code that should be repeated a number of times, once for each value in the value list. This tag has two attributes: NAME and LIST.

NAME is mandatory and represents the name of the loop control variable. The value of the control variable may be accessed inside the loop.

LIST is also mandatory and represents the list of values for the loop control variable. During the first traversal of the loop, the control variable value is set to the first element of the list. During each of the next traversals, the control variable is set to the succeeding elements in the list. The list can contain as well numeric as non-numeric values. These can be constant values entered during the design-time or expressions. Values of expressions are calculated before assigning them to the loop control variable.

The WHILE LOOP is a loop that is repeated as long as a condition is satisfied and does not have loop control variable. It is the responsibility of the programmer to organise the control inside the loop in such way, that the loop finally exists.

The syntax of the while loop is the following:

〈WHILE CONDITION="..."〉

〈/WHILE〉

The while loop has one mandatory attribute: CONDITION. It should be an expression evaluating to a Boolean value. The loop is repeated as long as the condition evaluates to TRUE.

The QUERY loop is a special command for accessing data in a database. It is a loop that is being repeated for each row or object (depending on the type of the database used) obtained from a database in result of a query execution. Instead of one control variable, this loop uses a set of control variables. This set of control variables can be interpreted as a record with several fields. To simplify the notation, it is denoted as a set of separate names.

In the loop start-tag a list of names of variables and a query are specified. The number of names of variables should be equal to the number of attributes selected from the database by the query. For each of the loop traversals, values of selected attributes are assigned to variables identified by the provided list of names.

The syntax of the query loop is the following:

〈QUERY NAMES="...,...,..." SQL="..."〉

〈/QUERY〉

The QUERY tag requires two attributes:

NAMES is mandatory and represents a coma-separated list of names of loop control variables. Values of attributes, selected from the database, are assigned to variables identified by these names.

SQL is mandatory and represents the text of the SQL query to be executed in the database.

In a configuration of the system where the parser is not permanently connected to a database the query loop should be preceded by a CONNECT element. A connection established by one CONNECT element may be used by multiple QUERY elements.

As said above, the method according to the invention allows for conditional inclusion of fragments of X-VRML code. Conditional statements are expressed by the IF element. It is a non-empty XML element containing a part of the X-VRML code that is included in the outcome only if the specified condition is satisfied and optionally part that is included when the specified condition is not satisfied.

The syntax of the IF statement is the following:

〈IF CONDITION="..."〉

〈THEN〉

〈/THEN〉

〈ELSE〉

〈/ELSE〉

〈/IF〉

The IF tag has only one attribute: CONDITION. This mandatory attribute specifies an expression string that is evaluated to a Boolean value.

The IF element can contain only THEN and ELSE elements. If the CONDITION evaluates to TRUE, the THEN element is parsed. Otherwise, the ELSE element is parsed.

The method of the invention allows for assignment of a value to a variable by the use of the SET element. This is an empty element since it does not contain X-VRML code.

The SET element has the following syntax:

〈SET NAME="..." VALUE="..."/〉

The SET tag uses two attributes:

NAME is mandatory and represents the name of the variable. The variable can be later referenced by the use of this name.

VALUE is optional and represents the value that should be assigned to the variable. This can be as well a simple constant value as an expression that must be first evaluated. If the VALUE attribute is omitted, the default value "TRUE" is assigned.

Calculated values of X-VRML expressions can be included into the outcome VRML file by the use of the INSERT empty element.

The syntax of the INSERT element is the following:

〈INSERT VALUE="..." TYPE="..."/〉

The processor 6 replaces the INSERT tag with the value calculated from the contents of the VALUE attribute. This attribute can contain a simple constant value, a variable reference, or an expression that must be evaluated by the processor 6 prior to inserting. The VALUE attribute is mandatory.

The TYPE attribute is optional and indicates the type of the value to be inserted. This attribute is used only for numerical and Boolean values. Possible values of the TYPE attribute are "FLOAT" and "INTEGER". If not specified, the "FLOAT" value is taken by default. The "INTEGER" type may be used with Boolean expressions to convert TRUE to "1" and FALSE to "0".

Before the processor 6 can connect to a database, it must be provided with some connection parameters. These parameters include DNS/IP of the computer the database is running on, the driver name, the Port number, the Database name, and the User/password.

Connection to a database is specified by the CONNECT element. The required connection parameters are concatenated into one connection string attribute.

The syntax of the CONNECT element is the following:

〈CONNECT CONN_STRING="..."/〉

The CONNECT element is required only in a case when the X-VRML processor 6 requires data from a database to which it is not permanently connected.

In some implementations the processor 6 may be permanently connected to a database and in such case the CONNECT elements may not be required. The CONNECT element is not used when the processor 6 does not require data from a database (i.e., the X-VRML scene descriptions do not use the QUERY elements)

If the CONNECT element is required, it must precede all QUERY elements. More than one CONNECT element may appear in an X-VRML file. It allows retrieval of data from multiple databases. Each QUERY element uses connection parameters specified by the most recently parsed CONNECT element.

The method of the invention allows including X-VRML files in other X-VRML files. An empty element called INLINE accomplishes this task. During the parsing process, this tag is being replaced by the contents of the X-VRML file it references. Included X-VRML file may include other X-VRML files. The depth of the inclusion graph is not limited. The inclusion graph may contain cycles as long as the files are parameterised appropriately to assure that the process of inclusion is finite. All variables set in the main X-VRML file are visible in the included files, and all variables set in the included files are visible in the including files

The syntax of the INLINE element is the following:

〈INLINE FILE="..."/〉

The INLINE element has one mandatory attribute: FILE, which indicates location and name (URL) of the X-VRML file to be included.

The method of the invention allows also definition of X-VRML classes. Some of the X-VRML classes may be translated by the X-VRML processor 6 to VRML prototypes. X-VRML classes, however, are a much more powerful tool than VRML prototypes themselves. In particular, X-VRML classes provide inheritance, multiple inheritance, abstract classes, object typing mechanism and the possibility of passing any elements as attributes (e.g. class names).

An X-VRML class is defined by a set of XML elements containing X-VRML code. The syntax of a class definition is the following: CLASS is the main element of the class definition. It may contain only two other elements: INTERFACE and IMPLEMENTATION.

The CLASS tag has three attributes:

NAME which is mandatory and which represents the name of the class. The name of the class must be unique in the whole X-VRML file and recursively in all files that are included in this file and those which include this file.

EXTENDS which is optional and which represents names of class or classes this newly created class extends. This can be a single class name if the class inherits only from one superclass, or a coma-separated list of class names if the class inherits from more than one superclass. The value "NONE" indicates that the class does not inherit from other classes. "NONE" is the default value.

ABSTRACT which is optional and which indicates that the class is abstract. An abstract class can use in its implementation abstract components that must be defined in subclasses. Default value for the ABSTRACT attribute is "FALSE", which means, that if the attribute is omitted, the class is assumed to be non-abstract.

The CLASS element may contain only two other elements, INTERFACE, and IMPLEMENTATION.

The INTERFACE element is optional and contains the interface part of the class definition. It contains declaration of attributes: fields, exposedFields, eventIns, and eventOuts in standard VRML syntax. If a class inherits from another class (extends it) it must declare only the attributes that are new or overridden in this new class. The attributes, which are inherited from a superclass, do not have to be listed in a subclass.

The IMPLEMENTATION element contains the actual X-VRML implementation of the class. Its contents corresponds to prototype implementation - in VRML contained in curly brackets ( "{" and "}" ). The implementation part of the class definition after processing by the X-VRML processor should be conformant to the VRML syntax.

If a class does not inherit from other X-VRML classes, the implementation part should contain the whole code of the class. If the class extends other classes, the implementation may contain the whole code of class or only parts of the code - parts that are defined, replaced, or added in this new class.

A class defined in X-VRML can be abstract. Abstract class is a class that is not fully defined and thus cannot be instantiated. An abstract class can use components that are not defined. These components can be specified later in subclasses. A subclass that defines all of its superclass components may be non-abstract. If it specifies only a subset of these components, it remains abstract.

Components in abstract classes are denoted by the COMPONENT element. It is an empty tag.

The syntax of the component element is the following:

〈COMPONENT NAME=^"..."/〉

The component tag requires one attribute: NAME that represents the component identifier. It can be used by subclasses to define the component implementation. Only abstract classes can use the COMPONENT element.

A subclass of an abstract class may define a component used in its superclass by the use of DEFINE element. It has the following syntax:

〈DEFINE NAME="..." CLASS_NAME="..."〉

〈/DEFINE〉

The DEFINE element is a non-empty XML element containing the X-VRML code that defines implementation of a component used by an abstract superclass. It has the mandatory attribute NAME which is the identifier of the component that is being defined. The code contained in a DEFINE element is used in place of a COMPONENT element of the same name in a superclass implementation.

An optional CLASS_NAME attribute must be used when the class inherits from more than one superclass to indicate the name of the superclass that contains the COMPONENT element to be replaced by the contents of the DEFINE element.

A class in X-VRML can extend an implementation of a superclass, in the sense that it can use the superclass implementation and add some additional code to it. A code of the superclass implementation can be referenced by the use of the SUPER element.

The syntax of the SUPER element is the following:

〈SUPER CLASS_NAME="..."/〉

SUPER is an empty element that is substituted by the processor 6 with the actual implementation of a superclass. The SUPER tag has one optional attribute: CLASS_NAME that represents the name of the superclass the SUPER element is referencing. If the class inherits only from one superclass there is no need to specify the CLASS_NAME attribute. In case of multiple inheritance, where there are a number of superclasses for the class, the CLASS_NAME attribute is mandatory. There can be multiple references to a superclass in a class implementation. It means that the code of a superclass can be used multiple times in the subclass.

A class in X-VRML may override a part of the code of its superclass. To override a part of the implementation code the REPLACE element should be used. It is a non-empty element that contains the code of the replacement.

The syntax of the REPLACE element is the following:

〈REPLACE NAME="..." CLASS_NAME="..."〉

〈/REPLACE〉

The REPLACE element has two attributes: NAME and CLASS_NAME.

The mandatory attribute NAME indicates the part of the super-class definition that should be replaced by the contents of the REPLACE element. The NAME attribute refers to a VRML element named by the use of standard VRML DEF statement in the super-class.

The optional CLASS_NAME attribute must be used when the class inherits from more than one superclass to indicate the superclass that contains the part of the code to be replaced by the contents of the REPLACE element.

The method of the invention allows use of a number of operators inside expressions. There are five types of operators:

• binary arithmetic operators
• relational arithmetic operators
• Boolean operators
• binary string operators
• relational string operators

There are four binary arithmetic operators in X-VRML as shown in Figure 2: addition, subtraction, multiplication, and division. All of them use two arithmetic arguments and give arithmetic value as a result. There is no distinction between integer and floating point values during the expression calculation. A conversion can be made when the value is being inserted into the final VRML code.

Relational arithmetic operators are shown in Figure 3 and Boolean operators are shown in Figure 4. There is one binary string operator as shown in Figure 5 and a number of relational string operators as shown in Figure 6.

Relational arithmetic operators use two arithmetic values as arguments and return a Boolean value as the result. These operators are used for comparison of arithmetic values.

Boolean operators use one or two Boolean values as parameters and return a Boolean value in result. These operators can be used for a value computation and for comparison.

As shown in Figure 5, there is only one binary string operator, concatenation. It uses two string values as arguments and returns a string value in result. The operator appends the value of the second string at the end of the first string and returns the concatenated string as the result.

Relational string operators shown in Figure 6 are used for comparison of strings. These operators use two string values as arguments and return a Boolean value as the result.

X-VRML expressions are evaluated according to operator precedence shown in Figure 7.

Since the "( )" operator has the highest precedence, parenthesis "(" and ")" can be used inside expressions to change the order in which the expression is evaluated.

String constant used in expressions must use apostrophes " ' " as string delimiters.

Variables may be used inside expressions and text of elements. The X-VRML processor 6 replaces the variable reference with its actual value. To use a variable the variable name must be preceded with dollar sign " $ ". Two consequent dollar signs " $$ " denote a dollar sign.

The XML Syntax of X-VRML is represented below: