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WebCGM 2.0 — WebCGM DOM


5. WebCGM Document Object Model (DOM)

This section and its subsections are normative, unless otherwise indicated.

Contents

5.1 Overview

This subsection is informative (non-normative).

This chapter defines a set of objects and interfaces for accessing and manipulating WebCGM documents. The functionality specified in this section enables script writers to manipulate WebCGM documents and access information found in standard WebCGM XML companion files. The WebCGM 2.0 DOM API focuses its methods on: tree traversal, style changes, and providing access to metadata.

5.2 Relationship with XML DOM

This subsection is informative (non-normative).

Although inspired by the XML DOM specifications, the WebCGM DOM remains oriented towards WebCGM specific functionality. Since WebCGM uses a tree structure to group graphical primitives, it was therefore appropriate, to use a set of interfaces similar to the XML DOM Node, Element and Document interfaces. However, since WebCGM is expressed in a non-XML syntax, several changes had to be made to commonly known interfaces and methods in order to improve the user experience of WebCGM script writers.

The WebCGM DOM could almost be perceived as a 'readonly' DOM. Some interface methods allow users to change the visual appearance of Application Structures, but unlike the XML DOM specification, it does not allow for removal or insertion of WebCGMNodes into the object model. This constitute a significant difference between the specifications.

While WebCGM 1.0 offers interactivity support via hyperlinking and highlighting; the WebCGM 2.0 DOM takes it to the next level. The WebCGM 2.0 DOM borrows concepts from the DOM3 Events specification, and introduces the concept of EventListeners and mouse Events in order to meet the requirements of WebCGM users.

5.3 Relationship with XML companion file

The WebCGM DOM is designed to provide access to XML metadata found in XML Companion Files. Practice has shown that some CGM illustrations are easier to maintain if some of the non graphical information remains outside the illustration. An example of such information could be; language sensitive screentips. The WebCGM DOM can then be used to 'apply' the information from the XML companion file to the WebCGM document (see Example 5.3) . For more information on XML companion file syntax, please refer to Chapter 4, WebCGM XML Companion File.

Another benefit of the XML Companion File is to carry application specific data (or metadata) concerning a WebCGM illustration (see Example 4.2). This information is expressed using namespace attributes and elements in the XML Companion File. The WebCGM DOM provides a method for loading the XML metadata into the user agent's object model. Using the WebCGM DOM, a user can gain access to the metadata. Here is a detailed example to better illustrate the concept.

Example 5.1a: This WebCGM document (expressed in clearText encoding) will be updated by an XML companion file.

BEGMF 'example.cgm';
...
BEGPIC 'Picture 1';
...
BEGAPS 'L1' 'layer' STLIST;
  APSATTR 'layername' "14 1 'Standard layer'";
  BEGAPSBODY;
  BEGAPS 'G1' 'grobject' STLIST;
    BEGAPSBODY;
    LINE 210,265 210,200 300,200;
    LINE 300,200 300,265 210,265;
  ENDAPS;
ENDAPS;
...
ENDPIC;
...
ENDMF;

The in-memory tree representation of this illustration should be similar to the illustration found below. The metafile contains a picture, the picture contains a child node Application Structure of type layer, and the layer contains a child node Application Structure of type grobject, as illustrated in Figure 7.


Figure 7. Original tree structure
Figure 7. Original tree structure

Example 5.1b: XML companion file to be 'applied' on example.cgm of example 5.1a.

<webcgm id="example" xmlns:wiring="http://www.example.org">
  <grobject apsId="G1" screentip="A new screentip">
    <wiring:data wire-bundle="E132-NAV"/>
  </grobject>
</webcgm>

The WebCGM DOM provides methods for 'applying' an XML Companion File, like the one shown in example 5.1b, to a picture in a WebCGM document. A conforming user agent is expected to load and parse the XML Companion File and possibly 'apply' updates from the XML Companion File to the user agent's object model. A user may want to apply a companion file for the following reasons:

i) To replace standard Application Structure Attribute values present in the WebCGM instance with new values from the XML Companion File.

ii) To supply standard Application Structure Attribute values to Application Structures which do not contain attribute values with values from the XML companion file.

iii) To transiently modify the Style Properties (stoke-color, text-size, etc) with which an object (APS or picture) is displayed.

iv) To add XML metadata to the user agent's object model to be retrieved at a later stage using WebCGM DOM APIs.

Once the user agent has loaded the XML Companion File into its memory model, the tree should resemble this:


Figure 8. New tree structure
Figure 8. New tree structure

The overall set of rules that a user agent must follow when applying an XML Companion File is as follows:

  1. Verify that root element is <webcgm>, else stop further processing and throw FILE_INVALID_ERR exception.
  2. Process unknown attributes if any on root element, see below about processing namespace attributes.
  3. Process all child elements using a depth-first algorithm.

More specific rules for processing namespace attributes are:

  1. If the target APS is not present in the CGM file, all attributes of the current element are ignored.
  2. If the attribute is not part of the base WebCGM DTD, it must be an extended namespace attribute, else the attribute is ignored.
  3. An attribute that already exists on the corresponding APS must be updated with the new attribute value.
  4. In the case where an attribute with the same local name and namespace IRI is already present on the APS, its prefix is changed to be the prefix part of the qualifiedName, and its value is changed to be the attribute value. If the attribute does not exist on the APS, the namespace attribute is appended onto the APS.

More specific rules for processing child elements are:

  1. The target APS (the parent element) must be present in the CGM file, if that is not the case, all child elements of the current element are ignored. The target APS must not be of type 'grnode'. 'grnode' elements are not accessible via DOM calls.
  2. If the element is not part of the base WebCGM DTD, it must be an extended namespace element. Namespace elements and their attributes are appended at the end of the target's list of child elements.
  3. Elements that are defined in the WebCGM DTD are processed as follows:
  4. If the element is a <linkuri>, the following rules apply:
  5. If the element is a <bindById>, only namespace attributes and attributes relevant to the target APS type are added to the target (i.e., if the <bindById> has a 'screentip' attribute and the target APS is of type 'layer', the 'screentip' attribute will be ignored).
  6. If the element is a <bindByName>, the user agent has to find all Application Structures that have a matching 'name' or 'layername' attribute. All found Application Structures are then subject to new attribute values (refer to the <bindById> description above).

5.4 Inheritance of APS Attributes and of Style Properties

This section describes how APS Attributes are inherited in a WebCGM structure tree. It also describes how Style Properties are inherited, which is similar but differs in a few key details. The inheritance models are based closely on the inheritance model of CSS 2.0. Some details have been adapted to the particulars of the WebCGM format. This chapter is the normative reference for inheritance of APS Attributes and of Style Properties in WebCGM.

5.4.1 Specified, computed, and actual values of Style Properties

WebCGM 2.0 user agents are required to support the inheritance model of Style Properties defined in this section. Once a user agent has loaded a document and constructed a document tree, it must assign, for every Application Structure in the tree, a value to every Style Property.

Very similar to the CSS model, the final value of a Style Properties is the result of a four-step calculation: the value is determined through specification (the "Specified Value"), then resolved into a value that may be used for inheritance (the "Computed Value"), then converted into an absolute value if necessary (the "Used Value"), and finally transformed according to the limitations of the local environment (the "Actual Value").

5.4.1.1 Specified Values of Style Properties

User agents must first assign a Specified Value to each Style Property based on the following mechanisms (in order of precedence):

  1. If the Style Property is assigned a value (via a DOM call, or via an XCF), use it.
  2. Otherwise, if the Style Property is inherited (i.e., its definition includes "Inherited:yes") and the Application Structure is not the root of the document tree, use the Computed Value of the parent Application Structure.
  3. Otherwise use the Style Property's Initial Value. The Initial Value of each Property is indicated in the Style Property's definition.

Note: In the context of WebCGM inheritance, the root of the document tree is the Picture node.

5.4.1.2 Computed Values of Style Properties

Specified Values are resolved to Computed Values after the document tree is created.

The Computed Value exists even when the property doesn't apply, as defined by the 'Applies To' line.

5.4.1.3 Used Values of Style Properties

In the CSS2 model from which the WebCGM model is derived, Computed Values can be relative to each other; for example a width could be set as a percentage, which is dependent on the containing block's width. The Used Value is the result of taking the Computed Value and resolving these dependencies into a final absolute value used for the actual display. In this version of WebCGM, there are no examples where Used Value differs from the Computed Value. This may change in a future version of the specification.

5.4.1.4 Actual Values of Style Properties

A Used Value is in principle the value used for rendering, but a user agent may not be able to make use of the value in a given environment. For example, a user agent may only be able to render borders with integer pixel widths and may therefore have to approximate the computed width, or the user agent may be forced to use only black and white shades instead of full colour. The Actual Value is the used value after any approximations have been applied.

5.4.2 Specified, computed, and actual values of Application Structure Attributes

WebCGM 2.0 user agents are required to support the inheritance model of Application Structure (APS) Attributes defined in this section. Once a user agent has loaded a document and constructed a document tree, it must resolve, for every Application Structure in the tree, if an Attribute has a value (i.e., no value is possible for some attributes).

Very similar to the CSS model, the final value of an APS Attribute is the result of a four-step calculation: the value is determined through specification (the "Specified Value"), then resolved into a value that may be used for inheritance (the "Computed Value"), then converted into an absolute value if necessary (the "Used Value"), and finally transformed according to the limitations of the local environment (the "Actual Value").

5.4.2.1 Specified Values of Application Structure Attributes

User agents must first assign a Specified Value to each APS Attribute based on the following mechanisms (in order of precedence):

  1. If the Attribute is assigned a value in the CGM document or assigned a new value via a DOM call or an XCF, use it.
  2. Otherwise, if the Attribute is inherited (i.e., its definition includes "Inherited:yes") and the Application Structure is not the root of the document tree, use the Computed Value of the parent Application Structure.
  3. Otherwise use the Attribute's Initial Value. The Initial Value of each Attribute is indicated in the Attribute's definition.

In the context of WebCGM inheritance, the root of the document tree is the Picture node. For the purposes of this inheritance model, the Picture root node is treated as if it were an Application Structure.

5.4.2.2 Computed Values of Application Structure Attributes

In this specification, Computed Values of Application Structure Attributes, with the exception of the 'inherit' value are identical to the Specified Values. When the Specified Value is 'inherit', it must be replaced for the Computed Value as defined below in the section on inheritance. The Computed Value exists even when the Attribute doesn't apply (as defined by the 'Applies To' line in the Attribute's definition).

5.4.2.3 Used Values of Application Structure Attributes

In the CSS2 model from which the WebCGM model is derived, Computed Values can be relative to each other; for example a width could be set as a percentage, which is dependent on the containing block's width. The Used Value is the result of taking the Computed Value and resolving these dependencies into a final absolute value used for the actual display. In this version of WebCGM, there are no examples where Used Value differs from the Computed Value. This may change in a future version of the specification.

5.4.2.4 Actual Values of Application Structure Attributes

A Used Value is in principle the value used for rendering, but a user agent may not be able to make use of the value in a given environment. For example, a user agent may only be able to render borders with integer pixel widths and may therefore have to approximate the computed width, or the user agent may be forced to use only black and white shades instead of full colour. The Actual Value is the Used Value after any approximations have been applied. In this version of WebCGM, there are no examples where Actual Value differs from the Used Value. This may change in a future version of the specification.

5.4.3 Inheritance

Some values are inherited by the children of an Application Structure in the document tree, as described above. Each Style Property and Application Structure Attribute defines whether it is inherited or not. As a general rule, when inheritance occurs, Application Structures inherit Computed values of Style Properties and Application Structure Attributes (unless implicitly stated in the Property or Attribute definition).

5.4.3.1 The 'inherit' value

Application Structure Attributes and Style Properties may also have a Specified Value of 'inherit'; which means that, for a given Application Structure, the Property or Attribute takes the same Computed value of the Application Structure's parent. The 'inherit' value can be used to strengthen inherited values, and it can also be used on Style Properties that are not normally inherited. There are no examples of the latter in this version of Web CGM.

5.5 Basic data types

WebCGM consists of three components where data type definitions need to be considered: metafile instances, DOM, and XCF.

5.5.1 Metafile data types

WebCGM instances are binary files. Metafile data types and encodings are fully defined in the CGM:1999 standard, together with chapters 3 and 6 of this WebCGM specification.

5.5.2 DOM data types

5.5.2.1 IDL types and binding types

Each interface of this DOM definition is normatively specified by a section of IDL code. The IDL uses basic data types such as unsigned short, boolean, long, etc. DOM applications are written in some programming language binding of the IDL, such as ECMAScript or Java. The only normatively specified binding for WebCGM DOM is the ECMAScript binding.

The ECMAScript binding unambiguously associates ECMAScript language data types with the IDL data types, which provides all the data type information needed to write WebCGM DOM applications in ECMAscript.

Null return value. WebCGM DOM attributes and method return values of type WebCGMNode and WebCGMNodeList sometimes have to represent the case of no data, i.e., zero nodes. In the DOM functional specification of this chapter, the term null is uniformly used to used to represent this case. In DOM bindings such as the ECMAScript binding the null return value maps naturally to an ECMAScript null reserved keyword. (For example, myNode.childnodes==null evaluates to true, and myPicture.getAppStructureById()==null evaluates to true.)

5.5.2.2 The WebCGMString type

One heavily used data type in the IDL definition is the WebCGMString, and some of the DOM interfaces do specify substructure or sub-types for some WebCGMString attributes and parameters.

WebCGMString

A WebCGMString is a sequence of 16-bit units in WebCGM DOM.

IDL Definition

valuetype WebCGMString sequence<unsigned short>;        

In WebCGM DOM, like XML DOM Level 3, the UTF-16 encoding was chosen because of its widespread industry practice. For ECMAScript and Java, WebCGMString is bound to the String type because both languages also use UTF-16 as their encoding. The WebCGM DOM has many interfaces that imply string matching. For XML, string comparisons are case-sensitive and performed with a binary comparison of the 16-bit units of the WebCGMStrings.

Empty string. WebCGM DOM attributes and method return values of type WebCGMString sometimes have to represent a string that has no data, i.e., zero characters. In the DOM functional specification of this chapter, the term empty string is uniformly used to used to represent this case. In DOM bindings such as the ECMAScript binding, the WebCGM empty string maps naturally to an ECMAScript string of length zero, i.e., zero characters. (For example, myEmptyString.length==0 evaluates to true, and myEmptyString=="" evaluates to true.)

5.5.2.3 WebCGMString sub-types

The WebCGM DOM has a number of WebCGMString attributes or parameters that in fact encode other data — such as numbers, colors, sub-strings, etc — into the string format. For the purposes of this specification, we define the following rules for how WebCGMString sub-types are encoded and represented within WebCGMString objects.

Number

A real number value encoded in a WebCGMString. The representation of the number can be either decimal notation or scientific notation. Decimal notation consists of either an integer, or an optional sign character followed by zero or more digits followed by a dot (.) followed by one or more digits. Scientific notation consists of a decimal-notation representation immediately followed by the letter "e" or "E" immediately followed by an integer.

Percent

A percent, encoded in a WebCGMString, is a number followed by a percent-sign character, "%".

Color

A color, encoded in a WebCGMString, is a "#", followed by exactly six hex digits, [0-9a-fA-F]. The first two digits represent the red component, the second two represent the green component, and the last two represent the blue component. Conceptually: #RRGGBB. Examples: #FF0000 is full red, #e1e1e1 is the gray background of the IDL definitions in this chapter, #00FFFF is full cyan, etc.

List-of-number

The following EBNF defines list-of-number (with number as defined above):

list-of-number  ::= number | number Wsp list-of-number
Wsp             ::= (#x20 | #x9 | #xD | #xA)+

Delimited String (list-of-string)

Some WebCGMString attributes may encode multiple substrings, e.g., the APS Attributes, 'name' and 'linkuri'. For historical reasons, this is known as Delimited String sub-type (although functionally it is a "list-of-string").

A Delimited String conforms to the following notation:

DelimitedString ::= ListOfX | ListOfXX
ListX           ::= '"'Name'"' | '"'Name'"' Wsp ListX
ListXX          ::= "'"Name"'" | "'"Name"'" Wsp ListXX
Wsp             ::= (#x20 | #x9 | #xD | #xA)+
Name ::= (ValidChar)*

The definition of ValidChar depends on the particular WebCGM entity that is being encoded. For example, in the APS Attributes table for DOM access (in section 5.7.6, Interface WebCGMAppStructure), the valid characters for each of the APS Attributes that are encoded by Delimited String are determined by the WebCGM datatype of the particular APS Attribute (linked from the table), in combination with the Character Repertoire rules of section 3.1.1.3.

In the case of the 'linkuri' APS attribute, the value always contains 3 * n strings, n representing the number of 'linkuri' attributes specified on the Application Structure. When meaningful values are not supplied for some of the components, the components must be represented by an empty string. The restriction of 3 * n strings simplifies scripts aimed at manipulating Delimited Strings.

Example: to set a 'region' APS Attribute that consisted of two subregions:
setAppStructureAttr("region", "''1 0 0 100 100' '1 25 25 75 75'")

Example: a multilink consisting of two links could be represented with the following delimited string:
'http://www.w3.org/' 'W3C' '_blank' 'http://www.cgmopen.org/' 'CGMOpen' '_self'.

A Delimited String is a list of wsp-separated substrings. If the Delimited String only contains a single substring, then it is coded as a simple string.

Example: to set a 'region' APS Attribute that consists of a single subregion:
setAppStructureAttr("region", "1 0 0 100 100")

Note. This Delimited String syntax, when combined with handling of string parameters in languages such as ECMAScript, imposes some constraints on the content of Delimited String substrings. In particular, it would not be possible to have a substring that contained both a QUOTE character and an APOSTROPHE character.

5.5.3 XCF data types

The XML Companion File (XCF) provides access to many of the APS Attributes and Style Properties that are accessible via the WebCGM DOM. APS Attributes and Style Properties that occur in WebCGM DOM as attributes and method parameters are represented in XCF as XML attributes. The values are encoded in the XML attribute strings of XCF exactly as they would be encoded in the WebCGMString type or a WebCGMString sub-type of the corresponding WebCGM DOM parameter or attribute.

5.6 Coordinate values — Normalized VDC (NVDC)

In a WebCGM instance, the representation of coordinates (VDC) is influenced by several CGM elements: VDC TYPE, VDC EXTENT, and SCALE MODE. WebCGM requires that SCALE MODE be 'metric', but places few other constraints. Therefore VDC (times some scale factor) are equivalent to millimeters, but otherwise the coordinate system could have a lot of variability: upper-left or lower-left origin, right-handed or left-handed, integer values or real values (floating or fixed), etc.

To simplify working with coordinates, the WebCGM DOM defines and uses a canonical, normalized coordinate system, Normalized VDC (NVDC).

NVDC units are millimeters, in a coordinate system whose origin corresponds to the lower left corner of the VDC extent, with the X axis pointing to the right, and the Y axis pointing up. The following examples illustrate the correspondence between NVDC and VDC values for several WebCGM instances.

Example 1: Simplest possible example, the VDC and the NVDC are identical

The picture's VDC have lower-left origin, X increases to right, Y increases up, picture is 150 mm wide and 100 mm high. The NVDC are identical,: (0.,0.) for lower-left corner, (150.,100.) for upper-right corner. If (x,y) are VDC and (x',y') are NVDC, then:

Example 2: The VDC define an upper-left origin, and correspond to a U.S. paper size of 8.5x11.0 inches:

In VDC space, the origin is the upper-left corner, X increases to right, Y increases down. In NVDC space, the lower-left corner coordinates (as always) are (0,0) and the upper right corner is (215.9,279.4). If (x,y) are VDC and (x',y') are NVDC, then:

Example 3: In the general case, if VDC Extent coordinates are (xll, yll), (xur, yur), and Scale Factor is 'metric', s, then (x',y') NVDC is derived from (x,y) VDC by:

5.7 Fundamental Interfaces

The interfaces within this section are considered fundamental, and must be fully implemented by all conforming implementations of the WebCGM DOM. The WebCGM DOM presents WebCGM documents as a hierarchy of WebCGMNode objects that also implement other, more specialized interfaces. Some of the node types may have child nodes of various types, and others are leaf nodes that cannot have anything below them in the WebCGM document structure. WebCGM has the following node types:

WebCGMMetafile — contains a list of WebCGMPicture nodes.

WebCGMPicture — may contain child WebCGMAppStructures or child XML metadata nodes.

WebCGMAppStructure — may contain child WebCGMAppStructures or child XML metadata nodes.

WebCGMAttr — no children.

The WebCGM DOM also specifies several other interfaces to facilitate access to WebCGM attributes. The GetWebCGMDocument interface is the medium between the host environment and the WebCGM functionality. The WebCGMNodeList interface enables the handling of ordered lists of WebCGMNodes. The WebCGMEvent interface provides contextual information regarding mouse events. WebCGMNodeList objects in the DOM are live; that is, changes to the underlying document structure are reflected in all relevant NodeList objects. For example, if a DOM user gets a WebCGMNodeList object containing the children of an WebCGMAppStructure, then changes one of its children in the tree, all changes are reflected in the NodeList objects and in fact to all references to that Node in NodeList objects.

The WebCGMPicture node is the root of the document tree for DOM and inheritance model purposes:

5.7.1 Exception WebCGMException

WebCGM operations only raise exceptions when an operation is impossible to perform.

IDL Definition
exception WebCGMException {
  unsigned short   code;
};

// ExceptionCode
const unsigned short      INDEX_SIZE_ERR                 = 1;
const unsigned short      WEBCGMSTRING_SIZE_ERR          = 2;
const unsigned short      INVALID_CHARACTER_ERR          = 3;
const unsigned short      NO_DATA_ALLOWED_ERR            = 4;
const unsigned short      NO_MODIFICATION_ALLOWED_ERR    = 5;
const unsigned short      NOT_SUPPORTED_ERR              = 6;
const unsigned short      INVALID_ACCESS_ERR             = 7;
const unsigned short      FILE_NOT_FOUND_ERR             = 8; 
const unsigned short      FILE_INVALID_ERR               = 9;        
Basic example

EXAMPLE:

This simple example shows how to catch a WebCGMException from HTML & ECMAScript. In a WebCGM-DOM enabled browser, correct execution of this HTML-ECMAScript code should cause an alert box to appear that an attempt to change a readonly attribute is made.

<html>
<head>
<title>Example, WebCGMException interface</title>
<script type="text/ecmascript">
  function causeException(evt) {
    alert("Try to change the readonly attribute layername");
    try {
      var webcgm = document.getElementById('ivx1').getWebCGMDocument();
      var pic = webcgm.firstPicture;
      var obj = pic.getAppStructureByID("A");
      obj.setAppStructureAttr("layername","anotherName");
    } catch (e) {
      alert("Catch the exception: " + e.description);
    }
  }
</script>
</head>

<body onload="causeException()">
<table border="1" rules="cols">
  <tr style="text-align:center;">
    <th>Example, causeException interface</th>
  </tr>
  <tr>
    <td><object id="ivx1" data="ex_Exception.cgm" 
                type="image/cgm;Version=4;ProfileId=WebCGM" 
                width="480" height="360"></object>
    </td>
  </tr>
</table>
</body>
</html>
Definition group ExceptionCode

An integer indicating the type of error generated.

Defined Constants

INDEX_SIZE_ERR; if index or size is negative, or greater than the allowed value.

DOMSTRING_SIZE_ERR; if the specified range of text does not fit into a WebCGMString.

INVALID_CHARACTER_ERR; if an invalid or illegal character is specified, such as in an XML name.

NO_DATA_ALLOWED_ERR; if data is specified for a node which does not support data.

NO_MODIFICATION_ALLOWED_ERR; if an attempt is made to modify an object where modifications are not allowed.

NOT_SUPPORTED_ERR; if the implementation does not support the requested type of object or operation.

INVALID_ACCESS_ERR; if a parameter or an operation is not supported by the underlying object.

FILE_NOT_FOUND_ERR; if the reference document could not be accessed

FILE_INVALID_ERR; if the reference document was not well-formed or was in error.

5.7.2 Interface GetWebCGMDocument

Since WebCGM documents are often embedded within a host document such as XHTML, WebCGM user agents are required to implement the GetWebCGMDocument interface for the element which references the WebCGM document (e.g., the 'object' tag).

IDL Definition
interface GetWebCGMDocument { 
  WebCGMMetafile              getWebCGMDocument ( ) raises ( WebCGMException ); 
  WebCGMString                getAppName();
  WebCGMString                getAppVersion();
};
Basic example
See any of the examples in the other interface subsections. They all use and illustrate getWebCGMDocument.
Attributes
No defined attributes.
Methods
getWebCGMDocument
Returns the WebCGMMetafile object for the referenced WebCGM document. If no WebCGM document is open in the viewer, the WebCGMMetafile object will still be returned.
Parameters
No parameters.
Return value
WebCGMMetafile; The WebCGMMetafile object for the referenced WebCGM document.
Exceptions
WebCGMException; NOT_SUPPORTED_ERR: No WebCGMMetafile object is available.
getAppName
Returns the name of the viewer application.
Parameters
No parameters.
Return value
WebCGMString; The name of the viewer application.
Exceptions
No exceptions.
getAppVersion
Returns the version of the viewer application.
Parameters
No parameters.
Return value
WebCGMString; The version of the viewer application.
Exceptions
No exceptions.

5.7.3 Interface WebCGMMetafile

The WebCGMMetafile interface is the entry point to the entire WebCGM document. The interface exposes information regarding the metafile and provides access to the first WebCGMPicture of the WebCGM document.

IDL Definition
interface WebCGMMetafile { 
  readonly attribute WebCGMString  metafileDescription;
  readonly attribute WebCGMPicture firstPicture;
  readonly attribute WebCGMString  metafileID;
  readonly attribute unsigned short metafileVersion;
           attribute WebCGMString  src;
  void               addEventListener(in WebCGMString type, 
                                      in WebCGMEventListener listener);
  void               removeEventListener(in WebCGMString type, 
                                         in WebCGMEventListener listener));
};
Basic example

EXAMPLE:

This simple example shows how to use a method of the WebCGMMetafile interface from HTML & ECMAScript. In a WebCGM-DOM enabled browser, correct execution of this HTML-ECMAScript code should display the metafileID parameter of the BEGIN METAFILE element in a message below the picture.

<html>
<head>
<title>Example, Metafile interface</title>
<script type="text/ecmascript">
  function metafileInfo() {
    try {
      var webcgm = document.getElementById('ivx1').getWebCGMDocument();
      document.getElementById("_1").firstChild.data = "The metafileID is \"" + webcgm.metafileID + "\"";
    } catch (e) {
      alert(e.description);
    }
  }
</script>
</head>

<body onload="metafileInfo()">
<table border="1" rules="cols">
  <tr style="text-align:center;">
    <th>Example, WebCGMMetafile interface</th>  </tr>
  <tr>
    <td><object id="ivx1" data="ex_Metafile.cgm" 
                          type="image/cgm;Version=4;ProfileId=WebCGM" 
                          width="480" height="360"></object>
    </td> </tr>
</table>
<p id="_1">Metafile ID is:</p>
</body>
</html>
Attributes
metafileDescription of type WebCGMString, readonly

Returns the Metafile Description of the WebCGM document, which is a string consisting of QUOTE-delimited substrings, as defined in the WebCGM PPF. For example: "ProfileId:WebCGM""ProfileEd:2.0""Source:A software vendor""Date:20040602""ColourClass:monochrome". Also as specified by the WebCGM PPF, a valid metafileDescription will always contain the ProfileId: and the ProfileEd:, other information such as Source, ColourClass etc... is optional. If no WebCGM document is open in the viewer, an empty string is returned.

firstPicture of type WebCGMPicture, readonly

Returns the first WebCGMPicture element of the WebCGM document. Subsequent WebCGMPictures can be accessed using the WebCGMPicture interface. A WebCGM 2.0 document contains exactly one WebCGMPicture. If no WebCGM document is open in the viewer, null is returned.

metafileID of type WebCGMString, readonly

Returns the Metafile Identifier, which is the parameter of the BEGIN METAFILE element in the CGM document. If no WebCGM document is open in the viewer, an empty string is returned.

metafileVersion of type unsigned short, readonly

Returns the Metafile Version of the WebCGM document. If no WebCGM document is open in the viewer, the value zero is returned

src of type WebCGMString
The IRI of the current document. On setting, the new document pointed to by the IRI is loaded by the user agent. The user agent must fully parse the fragment identifier (if any) in the IRI and execute the indicated behavior. The "Picture behaviors" rule does apply to the 'src' attribute — if a IRI fragment contains a picBehavior, the viewer shall ignore the picBehavior. If the CGM resource pointed to by the IRI is currently loaded for the object, the user agent shall not reload the CGM (similar to the specification of a same-CGM IRI for the _replace behavior on a CGM-to-CGM link.) On retrieval, if no WebCGM document is open in the viewer, an empty string is returned.

EXAMPLE. The 'src' attribute is an IRI, with possible fragment containing object selection and object behavior terms. It is not a full 'linkuri' (APS Attribute) data record, and any fragment does not contain picture behavior terms. So to open myCGM.cgm using (ECMAscript) DOM calls that reference an HTML <object> element with ID of 'myObjElt':

Correct examples:

document.getElementById('myObjElt').getWebCGMDocument().src= 'myCGM.cgm#myId';
[...].getWebCGMDocument().src= 'myCGM.cgm#id(myId,full)';

Incorrect:

[...].getWebCGMDocument().src= 'myCGM.cgm#"myId" "myTitle" "_blank"'
[...].getWebCGMDocument().src= "myCGM.cgm#id(myId).picseqno(1,_blank)"
Methods
addEventListener
This method allows the registration of event listeners on the WebCGMMetafile. If a WebCGMEventListener is added to the WebCGMMetafile while it is processing an event, it will not be triggered by the current actions. If multiple identical WebCGMEventListeners are registered on the WebCGMMetafile with the same parameters the duplicate instances are discarded. They do not cause the WebCGMEventListener to be called twice.

Note: Although all WebCGMEventListeners on the WebCGMMetafile are guaranteed to be triggered by any event which is received, no specification is made as to the order in which the WebCGMMetafile will receive the event with regards to the other WebCGMEventListeners on the WebCGMMetafile.

Parameters
type of type WebCGMString

The event type for which the user is registering, (for example: "click", "mouseover").

listener of type WebCGMEventListener

The listener parameter takes an interface implemented by the user which contains the methods to be called when the event occurs.

Return value
No return value.
Exceptions
No exceptions.
removeEventListener
This method allows the removal of event listeners on the WebCGMMetafile. If an WebCGMEventListener is removed from the WebCGMMetafile while it is processing an event, it will not be triggered by the current actions. WebCGMEventListeners can never be invoked after being removed. Calling removeEventListener with arguments which do not identify any currently registered WebCGMEventListener on the WebCGMMetafile has no effect.
Parameters
type of type WebCGMString

Specifies the event type of the WebCGMEventListener being removed (for example: "click", "mouseover").

listener of type WebCGMEventListener

Indicates the WebCGMEventListener to be removed.

Return value
No return value.
Exceptions
No exceptions.

5.7.4 Interface WebCGMNode

The WebCGMNode interface is the base datatype of the WebCGM Document Object Model. The WebCGMNode object is the basis of several other interfaces. the WebCGMNode object is the basis of several other interfaces, including interfaces to WebCGM specific elements (eg: WebCGMAppStructure), and to non-WebCGM elements such as Metadata nodes. The WebCGMNode interface specifies the attributes and methods to perform simple and generic tree traversal.

IDL Definition
interface WebCGMNode {
  const unsigned short PICTURE_NODE           = 1;
  const unsigned short APP_STRUCTURE_NODE     = 2;
  const unsigned short XML_METADATA_NODE      = 3;
  const unsigned short TEXT_NODE              = 4;
  const unsigned short ATTR_NODE              = 5;

  readonly attribute WebCGMString        nodeName;
  readonly attribute WebCGMString        nodeValue;
                                         // raises(WebCGMException) on retrieval

  readonly attribute unsigned short      nodeType;
  readonly attribute WebCGMNode          parentNode;
  readonly attribute WebCGMNodeList      childNodes;
  readonly attribute WebCGMNode          firstChild;
  readonly attribute WebCGMNode          lastChild;
  readonly attribute WebCGMNode          previousSibling;
  readonly attribute WebCGMNode          nextSibling;
  readonly attribute WebCGMPicture       ownerPicture;
  boolean                                hasChildNodes();
  boolean                                hasAttributes();
  readonly attribute WebCGMNodeList      attributes;

  readonly attribute WebCGMString        namespaceIRI;
  readonly attribute WebCGMString        prefix;

  readonly attribute WebCGMString        localName;

  WebCGMString                           getAttributeNS(in WebCGMString namespaceIRI, 
                                                     in WebCGMString localName);
  void                                   setAttributeNS(in WebCGMString namespaceIRI, 
                                                     in WebCGMString qualifiedName,
                                                     in WebCGMString value);
  WebCGMNodeList                         getElementsByTagNameNS(in WebCGMString namespaceIRI, 
                                                             in WebCGMString localName);
};
Basic example

EXAMPLE:

This simple example shows how to use an attribute on the WebCGMNode interface from HTML & ECMAScript. In a WebCGM-DOM enabled browser, correct execution of this HTML-ECMAScript code should display a message below the picture, with a 2nd line indicating nodeType "1".

<html>
<head>
<title>Example, WebCGMNode interface</title>
<script type="text/ecmascript">
  function getNodeType(evt) {
    var webcgm = document.getElementById('ivx1').getWebCGMDocument();
    if( webcgm ) {
      var pic = webcgm.firstPicture;
      if( pic ) {
        var elem = document.getElementById('result').lastChild;
        var text = elem.nodeValue;
        text = text + pic.nodeType;
        elem.nodeValue = text;
      }
    }
  }
</script>
</head>

<body onload="getNodeType()">
<table border="1" rules="cols" width="480">
  <tr style="text-align:center;">
    <th>Example, WebCGMNode interface</th>  </tr>
  <tr>
    <td><object id="ivx1" data="ex_Node.cgm" 
                type="image/cgm;Version=4;ProfileId=WebCGM" 
                width="480" height="360"></object>
    </td>  </tr>
  <tr style="text-align:left;" >
    <td id="result">The <strong>WebCGMNode.nodeType</strong> 
                    value of...<br/><em>getWebCGMDocument().firstPicture</em> is: </td>  </tr>  
</table>
</body>
</html>
Definition group NodeType

An integer indicating which type of node this is.

Defined Constants

PICTURE_NODE; the node is a WebCGMPicture.

APP_STRUCTURE_NODE; the node is a WebCGMAppStructure.

XML_METADATA_NODE; the node is XML companion information attached to a CGM element.

TEXT_NODE; the node contains character data.

ATTR_NODE; the node is a WebCGMAttr.

Attributes
nodeName of type WebCGMString, readonly

The name of this node, depending on its type; see the table below.

nodeValue of type WebCGMString, readonly

The value of this node, depending on its type; see the table below.

Exceptions on setting

WebCGMException; NO_MODIFICATION_ALLOWED_ERR: Raised when the node is readonly and if it is not defined to be null.

Exceptions on retrieval

WebCGMException; DOMSTRING_SIZE_ERR: Raised when it would return more characters than fit in a WebCGMString variable on the implementation platform.

nodetype of type unsigned short, readonly

A code representing the type of the underlying object.

The values of nodeName and nodeValue vary according to the node type as follows:

Interface nodeName nodeValue
WebCGMAppStructure WebCGMAppStructure type:
"layer" | "grobject" | "para" | "subpara" | "grnode"
empty string
WebCGMAttr WebCGMAttr.name empty string
WebCGMPicture "#picture" empty string
Character Data "#text" content of the text node
XML Metadata prefix + localName empty string

parentNode of type WebCGMNode, readonly

The parent (immediate ancestor node of a node) of this node. All nodes, except WebCGMPicture and WebCGMAttr, may have a parent.

childNodes of type WebCGMNodeList, readonly

A WebCGMNodeList that contains all children of this node. If there are no children, this returns null.

firstChild of type WebCGMNode, readonly

The first child of this node. If there is no such node, this returns null.

lastChild of type WebCGMNode, readonly

The last child of this node. If there is no such node, this returns null.

previousSibling of type WebCGMNode, readonly

The node immediately preceding this node. If there is no such node, this returns null.

nextSibling of type WebCGMNode, readonly

The node immediately following this node. If there is no such node, this returns null.

ownerPicture of type WebCGMPicture, readonly

The WebCGMPicture object associated with this node. When the node is a WebCGMPicture node, this returns null

attributes of type WebCGMNodeList, readonly

A WebCGMNodeList containing all attributes (WebCGM and namespaced) of this node or null if the WebCGMNode doesn't have any attributes. The 'apsid' parameter of the Begin APS element is considered to be an attribute of its APS for DOM purposes, and the 'pictid' parameter of the Begin Picture element is considered to be an attribute of its Picture for DOM purposes. This table summarizes the contents of 'attributes' for the various node types:

Node type attributes
PICTURE_NODE pictid (always); NS attributes
APP_STRUCTURE_NODE apsid (always); NS attributes; APS Attributes
XML_METADATA_NODE NS attributes
ATTR_NODE none (always null)
TEXT_NODE none (always null)

namespaceIRI of type WebCGMString, readonly

The namespace IRI of this node. For example, on the element foo:someElement, returns the IRI of the (xmlns) namespace declaration that associates the prefix foo with the namespace. This is not a computed value that is the result of a namespace lookup based on an examination of the namespace declarations in scope. It is the namespace IRI given at creation time. This returns empty string if the WebCGMNode is not of type XML_METADATA_NODE or ATTR_NODE.

prefix of type WebCGMString, readonly

The namespace prefix of this node (e.g., foo:elementName, returns "foo"). This returns empty string if the WebCGMNode is not of type XML_METADATA_NODE or ATTR_NODE.

localName of type WebCGMString, readonly

Returns the local part of the qualified name of this node (e.g., foo:elementName, returns "elementName"). This returns empty string if the WebCGMNode is not of type XML_METADATA_NODE or ATTR_NODE.

Methods
hasChildNodes
Returns whether this node has any children.
Parameters
No parameters.
Return value
boolean; true if this node has any children, false otherwise.
Exceptions
No exceptions.
hasAttributes
Returns whether this node has any attributes. (For more information, see the attributes attribute.)
Parameters
No parameters.
Return value
boolean; true if this node has any attributes, false otherwise.
Exceptions
No exceptions.
getAttributeNS
Returns the node attribute value by local name and namespace IRI.
Parameters
namespaceIRI of type WebCGMString

The namespace IRI of the attribute to retrieve.

localName of type WebCGMString

The local name of the attribute to retrieve.

Return value
WebCGMString; The WebCGMAttr value as a string, or the empty string if that attribute does not have a specified value.
Exceptions
No exceptions.
setAttributeNS
Adds a new attribute. If an attribute with that name is already present on the node, its value is changed to be that of the value parameter.
Parameters
namespaceIRI of type WebCGMString

The namespace IRI of the attribute to create or alter.

qualifiedName of type WebCGMString

The qualified name of the attribute to create or alter.

value of type WebCGMString

The value to set, in string form.

Return value
No return value.
Exceptions

WebCGMException; INVALID_CHARACTER_ERR: Raised if the specified qualified name contains an illegal character. The legal-character constraints of the qualified name match those of the attribute name construct of XML 1.0 Third Edition.

getElementsByTagNameNS
Returns a WebCGMNodeList of all the descendant XML companion file elements (application specific metadata) with a given local name and namespace IRI in the order in which they are encountered in a preorder traversal of the WebCGMNode tree. Returns null if there are no such elements.
Parameters
namespaceIRI of type WebCGMString

The namespace IRI of the XML elements to match on.

localName of type WebCGMString

The local name of the XML elements to match on.

Return value
WebCGMNodeList; A list of matching XML element nodes.
Exceptions
No exceptions.

5.7.5 Interface WebCGMPicture

The WebCGMPicture interface allows for access to the Application Structures of the WebCGM document. It also specifies how to load and apply an XML Companion File (XCF) to a WebCGM document.

IDL Definition
interface WebCGMPicture : WebCGMNode { 
  readonly attribute float            width;
  readonly attribute float            height;
  readonly attribute WebCGMString     pictid;
  boolean            applyCompanionFile(in WebCGMString fileIRI);
  WebCGMAppStructure getAppStructureById(in WebCGMString apsId);
  WebCGMNodeList     getAppStructuresByName(in WebCGMString apsName);
  void               highlight(in WebCGMNodeList nodes,
                               in WebCGMString type);
  void               clearHighlight();
  void               setPictureVisibility(in WebCGMString visibility);
  void               setStyleProperty(in WebCGMString style,
                                  in WebCGMString value);
  void               reloadPicture();
};
Basic example

EXAMPLE:

This simple example shows how to use a method of the WebCGMPicture interface from HTML & ECMAScript. In a WebCGM-DOM enabled browser, execution of this HTML-ECMAScript code should cause the initial view of the technical illustration to appear with a blue-gray background, instead of a white background:

<html>
<head>
<title>Example, WebCGMPicture interface</title>
<script type="text/ecmascript">
  function changeBackground(evt) {
    var webcgm = document.getElementById('ivx1').getWebCGMDocument();
    if( webcgm ) {
      var pic = webcgm.firstPicture;
      if( pic ) {
        pic.setStyleProperty("background-color","#A0A0D0");
      }
    }
  }
</script>
</head>

<body onload="changeBackground()">
<table border="1" rules="cols">
  <tr style="text-align:center;">
    <th>Example, WebCGMPicture interface</th>
  </tr>
  <tr>
    <td><object id="ivx1" data="ex_Picture.cgm" 
                type="image/cgm;Version=4;ProfileId=WebCGM" 
                width="480" height="360"></object>
    </td>
  </tr>
</table>
</body>
</html>
Attributes
width of type float, readonly

Represents the WebCGMPicture width in millimeters. Please refer to Coordinate Values section for more information.

height of type float, readonly

Represents the WebCGMPicture height in millimeters. Please refer to Coordinate Values section for more information.

pictid of type WebCGMString, readonly

Represents the WebCGMPicture id, which is the id parameter in the BEGIN PICTURE element in the CGM document.

Methods
applyCompanionFile

Reads an XML Companion File (XCF) into the user agent's object model. If application-specific metadata is found in the companion file (in the form of namespace attributes and namespace children elements), the user agent will create new namespace application structures as children of existing WebCGM Application Structures within it's object model. This information will then be accessible using methods found on this WebCGMPicture interface, on WebCGMAppStructure and on WebCGMNode. If the fileIRI parameter of this method is a relative IRI, then that relative IRI is resolved similarly to resolving relative IRIs for XCF resources referenced in the WebCGM IRI fragment syntax, i.e., the IRI is resolved relative to location of the CGM resource to which the XCF resource is a companion.

Please refer to the Relationship with XML companion file section for more discussion.

Parameters
fileIRI of type WebCGMString

The file name and location of the XML companion file to load and apply into the object model.

Return value
boolean; true if the implementation was able to load and parse the XML companion file into memory as requested; false otherwise.
Exceptions
WebCGMException; FILE_NOT_FOUND_ERR; if the referenced document could not be accessed.

WebCGMException; FILE_INVALID_ERR: if the referenced document was not well-formed or in error.

getAppStructureById
Returns the Application Structure whose ID is given by apsId. If no such Application Structure exists, returns null. Behavior is not defined if more than one element has this ID. Only WebCGM Application Structures may be retrieved using getAppStructureById, it does not retrieve foreign namespace elements (application-specific metadata elements).
Parameters
apsId of type WebCGMString

The unique id value for an Application Structure.

Return value
WebCGMAppStructure; a WebCGMAppStructure object containing the Application Structure with the matching id.
Exceptions
No exceptions.
getAppStructuresByName
Returns the list of Application Structures whose names are given by apsName in the order in which they are encountered in a depth-first-order traversal of the WebCGMPicture tree. If no such Application Structures exists, returns null. Only WebCGM Application Structures may be retrieved using getAppStructuresByName, it does not retrieve foreign namespace elements (application-specific metadata elements).
Parameters
apsName of type WebCGMString

A non-unique name value for an Application Structure.

Return value
WebCGMNodeList; A WebCGMNodeList object containing all the matching Application Structure WebCGMNodes.
Exceptions
No exceptions.
highlight
Highlights a collection of Application Structures (APSs). WebCGM also allows for highlighting of APSs using the IRI fragment syntax. The exact method of highlighting is viewer dependent. The highlight method provides a way for WebCGM script writers to highlight APSs in the same way a IRI fragment would. It also allows for highlighting of entire layers. Highlighting is not defined for WebCGMPicture nodes or XML Metadata nodes.
Parameters
nodes of type WebCGMNodeList

A WebCGMNodeList of APP_STRUCTURE_NODEs to highlight.

type of type WebCGMString

Denotes a behavior identical to the corresponding highlighting object behavior keywords of the fragment syntax. Values: { add | new }.

Return value
No return value.
Exceptions
No exceptions.
clearHighlight
Clears highlighting for all currently highlighted APSs in the picture. The behavior is identical to the special-form object behavior fragment, id(*,clearHighlight), that is defined in the enumeration of behaviors of the fragment syntax.
Parameters
None.
Return value
No return value.
Exceptions
No exceptions.
setPictureVisibility
Sets the visibility on or off for the whole picture. Note that for the purposes of the inheritance model, the Picture node behaves like an Application Structure, and the visibility as set by this method behaves like the 'visibility' APS Attribute.
Parameters
visibility of type WebCGMString

Value for the visibility of the picture, {on | off}.

Return value
No return value.
Exceptions
No exceptions.
setStyleProperty

Set a style property at the picture level by name.

The following table and text describe in more detail each of the style properties, their scopes and allowed values:

Rules & encodings for DOM manipulation of style properties
Style Property
Name
Picture
level
APS
level
Attribute value(s) Initial
value
Example
background-color yes no absolute RGB or relative intensity (0..100%) 100% "#000000" or "75%"
text-size yes yes absolute NVDC or relative scale (both > 0) 100% "225%"
fill-color yes yes absolute RGB or relative intensity (0..100%) 100% "#FF0000" or "75%"
intensity yes yes intensity (0..100%) 100% "75%"
stroke-color yes yes absolute RGB or relative intensity (0..100%) 100% "#FF0000" or "75%"
stroke-weight yes yes absolute NVDC or relative scale (both > 0) 100% "225%"
text-color yes yes absolute RGB or relative intensity (0..100%) 100% "#FF0000" or "75%"
text-font yes yes WebCGMString "metafile" "Helvetica"
raster-intensity yes yes relative intensity (0..100%) 100% "75%"

Common specifications. The following common specifications, related to the inheritance model, apply to all of the Style Properties:

  • Inherited: yes: Each Style Property may be inherited.
  • Value "inherit": Each Style Property may take the value "inherit", in addition to the values listed for the individual property.
  • Computed Value: The Computed Value of each Style Property is the same as the specified value, with the exception of the elimination of the value "inherit" as specified in the inheritance model.

Units in the table. RGB colors are expressed as hexadecimal values. Relative scale values are expressed as a positive or non-negative number (depending on the property) followed by a '%' unit designator. Relative values of some properties can exceed 100%. Relative intensity values are expressed as a number followed by a '%' unit, ex: "75%". Relative intensity values cannot exceed 100%.

Color representation. Absolute RGB colors are expressed using a hexadecimal representation for all three RGB channels, #RRGGBB. Examples of colors expressed in hexadecimal representation: red is expressed as #FF0000, and cyan which uses both full green and full blue is expressed as #00FFFF. The representation must be exactly 6 digits, 2 each for R, G, and B. Shorthand hexadecimal notation, e.g., the 3-digit #RGB notation, is not supported in this specification.

Replacement mode. When Style Properties have values of "%" (percent), the respective attribute value used for display is adjusted by applying the appropriate formulae to the attribute values in the metafile (for the appropriate target object). For example, stroke-weight of 60% means that the metafile-defined LINE WIDTH and EDGE WIDTH attributes are multiplied by 0.6. Successive setting of the same Style Property replaces any previous setting of the same Style Property (rather than accumulating with it). So, for example, stroke-weight 60% followed later by stroke-weight 40% results in stroke-weight 40%, not stroke-weight 24%.

Order counts. Some Style Properties have overlapping effects. For example, intensity and fill-color both affect the color of filled areas. When both properties are defined for a target APS, the latter definition supersedes and replaces the earlier definition. So for example intensity 40% followed by fill-color 60% results in 60% fill color, while fill-color #FF0000 followed by intensity 40% results in fill-color 40% (of the filled-areas colors in the metafile, for the target object).

Style Property Definitions. The following are the detailed functional definitions of each of the Style Properties:

background-color is the color of the rendering surface for the entire picture, on which all elements are drawn. It corresponds to the BACKGROUND COLOUR attribute of the CGM standard. Example: a value of #000000 for the background-color style property will override what is in the WebCGM instance, and display a black background for all elements to render over.

text-size redefines the size of all text in the target object. If text-size is "%", then it adjusts the text restriction boxes (heights and widths) and the CGM CHARACTER HEIGHT attribute by that amount. If the text-size value is NVDC, for each text element in the target object, compute the ratio (effectively, a percent) of the new NVDC value and the restriction box height, and apply the resulting ratio as would be done for the same "%" value.

fill-color is the style property applied to a closed area inside the path of a shape. It corresponds to the CGM attribute FILL COLOUR, and will override the current values of that attribute within the target object (APS or picture), if the fill-color Style Property is applied to object.

intensity is a way to make the current color fade towards white. An intensity value of 0% applied to an Application Structure (APS) will make its contents completely white while a value of 100% will keep the current colors intact. The intensity equation is as follows:

      normalizedNewRed = 1 - intensity * (1 - normalizedOldRed)
      normalizedNewGreen = 1 - intensity * (1 - normalizedOldGreen)
      normalizedNewBlue = 1 - intensity * (1 - normalizedOldBlue)

Example: Here is an example of the computations when applying an intensity of 40% to the color orange #FFA500:

normalizedNewRed = 1 - 0.4 * (1 - 1) = 1
normalizedNewGreen = 1 - 0.4 * (1 - 0.647) = 0.859
normalizedNewBlue = 1 - 0.4 * (1 - 0) = 0.5

The new color is %FFDB99.

Setting a relative intensity value is allowed on a number of individual style properties, see table above. The 'intensity' style property, however, represents a convenience property that simultaneously controls the intensity value of the following four properties: fill-color, stroke-color, text-color and raster-intensity.

stroke-color defines the color for the lines and edges within the target object (APS or picture) to which the property is applied. Stroke-color overrides the CGM attributes LINE COLOUR and EDGE COLOUR. This style property will apply an absolute or a relative intensity color change to metafile-defined values of those CGM attributes within the target object.

stroke-weight redefines the thickness of the pen strokes for drawing of lines and edges within the target object (APS or picture) to which it is applied. Stroke-weight overrides CGM attributes LINE WIDTH and EDGE WIDTH. This stroke-weight property can apply a relative scale change to the metafile-defined value of those attributes, or can provide an absolute (NVDC) replacement for those current values.

text-color redefines the color for the graphical text within the target object (APS or picture) to which the property is applied. Text-color overrides the CGM attribute TEXT COLOUR. This style property will apply an absolute or a relative intensity color change to metafile-defined value of that CGM attribute within the APS.

text-font specifies a replacement font for all text in the target object. If the characters that are needed for all text in the target object are available in the specified replacement font, and if the specified font is available, then use it for all text in the target object. Otherwise, ignore the specified replacement font. The initial value of text-font, which is the reserved keyword "metafile", means that the font specifications of the metafile are used.

raster-intensity is a way to make the current color fade towards white in a raster element. It applies to the colors within CELL ARRAY, TILE, and BITONAL TILE elements within the target object (APS or picture) to which it is applied. An intensity value of 0% applied to an Application Structure (APS) will make its raster contents completely white while a value of 100% will keep the current raster colors intact. The equations for computing new color values are the same as for the intensity property, above.

Parameters
style of type WebCGMString

The name of the style property to modify.

value of type WebCGMString

The new value for the given style. Note that "inherit" is a valid value for every Style Property, and per the inheritance model, it has the effect of restoring the style property to its initial (load time) value (as determined by the inheritance model).

Return value
No return value.
Exceptions
No exceptions.
reloadPicture
Notifies the user agent to immediately redraw the entire WebCGMPicture. The user agent will reload the WebCGMPicture while preserving the current user agent's zoom and pan level. The reloading of the WebCGMPicture also discards any existing companion information that may have been loaded into memory via the applyCompanionFile method.
Parameters>
No parameters.
Return value
No return value.
Exceptions
No exceptions.

5.7.6 Interface WebCGMAppStructure

The WebCGMAppStructure interface offers methods for setting and retrieving Application Structure (APS) attributes. The main methods for accessing Application Structure attributes are getAppStructureAttr and setAppStructureAttr. It is important to note that some attributes, like 'name' and 'linkuri', may have multiple values. In that case, a Delimited String is returned. Delimited String is also used for 'region', which may contain several sub-regions.

The following table identifies which APS attribute values can be expressed as a Delimited String. Each entry in the table points to the detailed description of the attribute, as it appears in WebCGM content.

Rules & encodings for DOM access to APS Attributes
APS Attribute Name read/write Delimited strings Example
content yes no, single string "car engine transmission"
interactivity yes no, single string "on"
layerdesc yes no, single string "This layer contains English instructions"
layername readonly no, single string "English instructions"
linkuri yes yes, multiple 3-tuples possible '"http://w3.org" "W3C" "_blank"'
name readonly yes, multiple names possible '"firstName" "anotherName"'
region yes yes, multiple subregions possible '"1 0 0 100 100" "1 25 25 75 75"'
screentip yes no, single string "This is a screentip"
viewcontext yes no, single string (two corner points) "0 0 100 100"
visibility yes no, single string "on"

The WebCGMAppStructure interface, like the WebCGMPicture interface, also provides methods for modifying Style Properties at the Application Structure level. For more information about available Style Properties, refer to the Style Properties Table.

IDL Definition
interface WebCGMAppStructure : WebCGMNode {
  readonly attribute WebCGMString  apsId;
  readonly attribute unsigned long nameCount;
  readonly attribute unsigned long linkuriCount;

  WebCGMString getAppStructureAttr(in WebCGMString name);
  void         setAppStructureAttr(in WebCGMString name, in WebCGMString value)
                         raises( WebCGMException );
  void         removeAppStructureAttr(in WebCGMString name)
                         raises( WebCGMException );
  void         setStyleProperty(in WebCGMString style, in WebCGMString value);
  WebCGMNodeList toNodeList();
                            
};
Basic example

EXAMPLE:

This simple example shows how to use methods of the WebCGMAppStructure interface from HTML & ECMAScript. In a WebCGM-DOM enabled browser, correct execution of this HTML-ECMAScript code should display "Layer name is fleet" under the picture.

<html>
<head>
<title>Example, WebCGMAppStructure interface</title>
<script type="text/ecmascript">
  function OnBtnDOM() {
    try {
      // Get layernname
      var cgmDoc = document.getElementById("ivx1").getWebCGMDocument();
      var cgmPic = cgmDoc.firstPicture;
      var result = document.getElementById("_1");
      var gr = cgmPic.getAppStructureById("fleet");
      var i = gr.getAppStructureAttr("layername");
      result.firstChild.data = "Layer name is " + i ;
    }
    catch (e) {
      alert("Catch the exception: " + e.description);
    }
  }
</script>
</head>

<body onload="OnBtnDOM()">
<table border="1" rules="cols">
  <tr style="text-align:center;">
    <th>Example, WebCGMAppStructure interface</th> </tr>
  <tr>
    <td><object id="ivx1" data="ex_AppStructure.cgm" 
                type="image/cgm;Version=4;ProfileId=WebCGM" 
                width="480" height="360"></object> </td> </tr>
</table>
<p id="_1">Layer name is ...</p>
</body>
</html>
Attributes
apsId of type WebCGMString, readonly

The unique identifier of the Application Structure.

nameCount of type unsigned long, readonly

Represents the number of 'name' attribute values present on this Application Structure.

linkuriCount of type unsigned long, readonly

Represents the number of 'linkuri' attribute values present on this Application Structure.

Methods
getAppStructureAttr

Retrieves an Application Structure attribute value by name. Please refer to the Application Structure Attributes table for more detailed information on retrievable and modifiable Application Structure attributes.

Parameters
name of type WebCGMString

The name of the Application Structure attribute to retrieve.

Return value
WebCGMString; the Application Structure attribute value as a string, or the empty string if that attribute does not have an explicitly set value (see the inheritance model for further related discussion). The value may be a Delimited String.
Exceptions
No exceptions.
setAppStructureAttr

Adds a new Application Structure attribute. If an attribute with that name is already present in the APS, its value is changed to be that of the value parameter. Please refer to the Application Structure Attributes table for more detailed information on retrievable and modifiable Application Structure attributes.

Parameters
name of type WebCGMString

The name of the Application Structure attribute to create or alter.

value of type WebCGMString

Value to set in string form. The value may be a delimited string.

Return value
No return value.
Exceptions
WebCGMException; NO_MODIFICATION_ALLOWED_ERR: Raised if the Application Structure Attribute is readonly.
removeAppStructureAttr

Removes an Application Structure attribute. Please refer to the Application Structure Attributes table for more detailed information on retrievable and modifiable Application Structure attributes.

Parameters
name of type WebCGMString

The name of the Application Structure attribute to remove.

Return value
No return value.
Exceptions
WebCGMException; NO_MODIFICATION_ALLOWED_ERR: Raised if the Application Structure Attribute is readonly.
setStyleProperty

Set a style property by name on the given Application Structure. Please refer to the Style Properties Table for more detailed information on style properties.

Parameters
style of type WebCGMString

The name of the style attribute to modify.

value of type WebCGMString

The new value for the given style.

Return value
No return value.
Exceptions
No exceptions.
toNodeList

Creates a new WebCGMNodeList object and inserts the current Application Structure node into the list. The list count is 1.

Parameters
No parameters.
Return value
WebCGMNodeList; A WebCGMNodeList object containing the Application Structure.
Exceptions
No exceptions.

5.7.7 Interface WebCGMNodeList

The WebCGMNodeList interface provides the abstraction of an ordered collection of nodes. WebCGMNodeList objects in the WebCGM DOM are live. The index with the WebCGMNodeList starts at 0.

IDL Definition
interface WebCGMNodeList {
  readonly   attribute unsigned long count;
  WebCGMNode           item(in unsigned long index);
  WebCGMNode           removeItem ( in unsigned long index )
                         raises( WebCGMException );
  WebCGMNode           appendItem ( in WebCGMNode newItem )
                         raises( WebCGMException );
};
Basic example

EXAMPLE:

This simple example shows how to use a method of the WebCGMNodeList interface from HTML & ECMAScript. In a WebCGM-DOM enabled browser, execution of this HTML-ECMAScript code should count the number of planes in the fleet application structure..

<html>
<head>
<title>Example, NodeList interface</title>
<script type="text/ecmascript">
  function getNodeList() {
    try {
      var webcgm = document.getElementById('ivx1').getWebCGMDocument();
      var pic = webcgm.firstPicture;
      var mylist = pic.getAppStructureByID("fleet").childNodes;
      document.getElementById("_1").firstChild.data = "The fleet contains "+mylist.count+" planes." ;
    } catch (e) {
      alert(e.description);
    }
  }
</script>
</head>

<body onload="getNodeList()">
<table border="1" rules="cols">
  <tr style="text-align:center;">
    <th>Example, WebCGMNodeList interface</th>  </tr>
  <tr>
    <td><object id="ivx1" data="ex_NodeList.cgm" 
                  type="image/cgm;Version=4;ProfileId=WebCGM" 
                  width="480" height="360"></object>  </td>  </tr>
</table>
<p id="_1">The fleet contains xx planes.</p>
</body>
</html>
Attributes
count of type unsigned long, readonly

The number of nodes in the list. The range of valid child node indices is 0 to count-1 inclusive.

Methods
item

Returns the indexed item in the collection.

Parameters
index of type unsigned long

Index into the collection.

Return value
WebCGMNode; The node of the indexed position in the WebCGMNodeList that is not a valid index., or null if that is not a valid index.
Exceptions
No exceptions.
removeItem

Removes an existing item from the list.

Parameters
index of type unsigned long

The index of the item which is to be removed. The first item is number 0.

Return value
WebCGMNode; The removed item.
Exceptions
WebCGMException; NO_MODIFICATION_ALLOWED_ERR: Raised when the list cannot be modified.
appendItem

Inserts a new item at the end of the list. If newItem is already in a list, it is removed from its previous list before it is inserted into this list.

Parameters
newItem of type WebCGMNode

The item which is to be inserted into the list.

Return value
WebCGMNode; The inserted item.
Exceptions
WebCGMException; NO_MODIFICATION_ALLOWED_ERR: Raised when the list cannot be modified.

5.7.8 Interface WebCGMAttr

The WebCGMAttr interface represents an attribute in a XML_METADATA_NODE, a PICTURE_NODE or a APP_STRUCTURE_NODE.

Note that WebCGMAttr objects inherit the WebCGMNode interface, but since they are not actually child nodes of the element they describe, the WebCGM DOM does not consider them part of the document tree. Thus, the WebCGMNode attributes parentNode, previousSibling, and nextSibling have a null value for WebCGMAttr objects.

IDL Definition
interface WebCGMAttr: WebCGMNode {
  readonly attribute WebCGMString        name;
           attribute WebCGMString        value;
  readonly attribute WebCGMNode          ownerNode;
};
Attributes
name of type WebCGMString, readonly

Returns the name of this attribute. If WebCGMNode.localName is different from the empty string, this attribute is a qualified name.

value of type WebCGMString, readonly

On retrieval, the value of the attribute is returned as a string, see WebCGMNode.getAppStructureAttr(). On setting, is it equivalent to WebCGMNode.setAppStructureAttr().

Exceptions on setting

WebCGMException; NO_MODIFICATION_ALLOWED_ERR: Raised when the node is readonly.

ownerNode of type WebCGMNode, readonly

The Element node this attribute is attached to or null if this attribute is not in use.

Methods
No defined methods.

5.7.9 Interface WebCGMEventListener

The WebCGMEventListener interface is the primary method for handling events. Users register their listener on the WebCGMMetafile node with the addEventListener method.

IDL Definition
interface WebCGMEventListener {
  void       handleEvent(in WebCGMEvent evt);
};
Example of addEventListener

EXAMPLE:

This simple example shows how to use the addEventListener method of the WebCGMMetafile interface from HTML & ECMAScript. In a WebCGM-DOM enabled browser, execution of this HTML-ECMAScript code should display a picture with two black-framed square figures, and a mouse click anywhere on either of them should result in an alert saying "Event handler installed successfully."

<html>
<head>
<title>Example for WebCGMEventListener</title>
<script type="text/ecmascript">

var cgmDoc;

function InstallEventListener() {
  try {
    cgmDoc = document.getElementById("ivx1").getWebCGMDocument();
    cgmDoc.src = 'ex_WebCGM_Event.cgm';
    cgmDoc.addEventListener( "click", handleClick);
  }
  catch(e) {
    alert( "Failed:  " + e.description );
  }
}
function handleClick(evt) {
  alert( "Event handler installed successfully." );
}
</script>
</head>

<body onload="InstallEventListener();">
  <table border="1" rules="cols">
    <tr style="text-align:center;">
      <th>Example, WebCGMEventListener Interface</th> </tr>
    <tr>
      <td><object id="ivx1" type="image/cgm;Version=4;ProfileId=WebCGM" 
                  width="480" height="360"></object>  </td>  </tr>
  </table>
</body>
</html>
Attributes
No defined attributes.
Methods
handleEvent

This method is called whenever an event occurs of the type for which the WebCGMEventListener interface was registered.

Parameters
evt of type WebCGMEvent

The WebCGMEvent containing contextual information about the event.

Return value
No return value.
Exceptions
No exceptions.

5.7.10 Interface WebCGMEvent

The WebCGMEvent interface is used to provide contextual information about an event to the handler processing the event.

There exists three levels of interactivity in WebCGM:

This section also describes how a user agent processes the three different levels of interactivity.

When a mouse event occurs, the WebCGM user agent determines the target object of the mouse event. For the purposes of this discussion, "object" means Application Structure (APS). The target object is the topmost object whose interactive region is under the mouse at the time of the event. (Note that the definition of interactive region excludes objects that are fully transparent due to the setting of their graphical attributes.)

An application structure of type 'grnode' or 'layer' cannot be a target of a mouse event. Instead, if the mouse pointer was over a 'grnode' when the event occurred; its closest ancestor object of type 'grobject', 'para' or 'subpara' will be designated as the target element. When an object is not displayed (i.e., 'visibility' attribute is set to off) or made non-interactive (i.e., 'interactivity' attribute is set to off), that object cannot be the target of mouse events.

The event is either initially dispatched to the Metafile, or else not dispatched, depending on the following:

See also the descriptions of grobject, grnode, para or subpara, for related specifics.

The processing order for user interface events is as follows:

Since hyperlinks will in general change the context of a document it is more appropriate to allow explicit handlers to act on an event first and then process the hyperlink. The reverse order cannot guarantee that the script would get executed. Script writers should be made aware that this specification does not cover user agent event facilities such as zooming, panning or context menus. The mechanism to invoke such functionality will likely be different between vendors. Script writers are encouraged to become aware of those differences and thus, write highly interoperable WebCGM scripts.

IDL Definition
interface WebCGMEvent {
  readonly attribute WebCGMString     type;
  readonly attribute WebCGMNode       target;
  readonly attribute unsigned short   button;
  readonly attribute long             numPressed;
  readonly attribute float            clientX;
  readonly attribute float            clientY;
  readonly attribute boolean          ctrlKey;
  readonly attribute boolean          shiftKey;
  readonly attribute boolean          altKey;
  readonly attribute boolean          metaKey;

  void               preventDefault();
};
Basic example

EXAMPLE:

This simple example shows how to use the attributes of the WebCGMEvent interface from HTML & ECMAScript. In a WebCGM-DOM enabled browser, successful execution of this HTML-ECMAScript code should display a picture with two black-framed square figures. A mouse click anywhere on the left black background should report the X/Y location of the click, and a mouseover of the left ellipse should report the mouseover.

<html>
<head>
<title>Example for WebCGMEvent</title>
<script type="text/ecmascript">

  var cgmDoc;

  function handleClick(evt) {
    try {
      if( evt.target.apsId == "grobject_rect_1" ) {
          alert( "ClientX = " + evt.clientX + "  ClientY = " + evt.clientY );
      }
    }
    catch(e) {
      alert( e );
    }
  }
  function handleMOver(evt) {
    try {
      if( evt.target.apsId == "grobject_circle_1" ) {
        alert( "You have mouse-over'd grobject_circle_1" );
      }
    }
    catch(e2) {
        alert( e2 );
    }
  }
  function addHandlers() {
    try {
      cgmDoc = document.getElementById("ivx1").getWebCGMDocument();
      cgmDoc.src = 'ex_WebCGM_Event.cgm';
      cgmDoc.addEventListener ("click", handleClick);
      cgmDoc.addEventListener ("mouseover", handleMOver);
    }
    catch(e4) {
        alert( e4 );
    }      
  }
</script>
</head>
<body onload="addHandlers();">
    <table border="1" rules="cols">
        <tr style="text-align:center;">
            <th>Example, WebCGMEvent Interface</th> </tr>
        <tr> 
            <td><object id="ivx1" type="image/cgm;Version=4;ProfileId=WebCGM" 
                        width="480" height="360"></object> </td> </tr>
    </table>
</body>
</html>
Attributes
type of type WebCGMString, readonly

The name of the event (case-insensitive). The name must be an XML name.

target of type WebCGMNode, readonly

Used to indicate the WebCGMNode (Application Structure) to which the event was originally dispatched.

button of type unsigned short, readonly

During mouse events caused by the depression or release of a mouse button, button is used to indicate which mouse button changed state. The values for button range from zero to indicate the left button of the mouse, one to indicate the middle button if present, and two to indicate the right button. For mice configured for left handed use in which the button actions are reversed the values are instead read from right to left.

numPressed of type long, readonly

Indicates the number of times a mouse button has been pressed and released over the same screen location during a user action. The attribute value is 1 when the user begins this action and increments by 1 for each full sequence of pressing and releasing. If the user moves the mouse between the mousedown and mouseup the value will be set to 0, indicating that no click is occurring.

clientX of type float, readonly

The horizontal coordinate at which the event occurred expressed in Normalized VDC.

clientY of type float, readonly

The vertical coordinate at which the event occurred expressed in Normalized VDC.

ctrlKey of type boolean, readonly

Used to indicate whether the 'ctrl' key was depressed during the firing of the event.

shiftKey of type boolean, readonly

Used to indicate whether the 'shift' key was depressed during the firing of the event.

altKey of type boolean, readonly

Used to indicate whether the 'alt' key was depressed during the firing of the event. On some platforms this key may map to an alternative key name.

metaKey of type boolean, readonly

Used to indicate whether the 'meta' key was depressed during the firing of the event. On some platforms this key may map to an alternative key name.

Methods
preventDefault

Calling preventDefault has the effect of canceling the event. Any default action associated with the event will not occur.

Parameters
No parameters.
Return value
No return value.
Exceptions
No exceptions.

WebCGM supports the following types of events:

click The click event occurs when the pointing device button is clicked. A click is defined as a mousedown and mouseup over the same screen location. The sequence of these events is: mousedown, mouseup, click. If multiple clicks occur at the same screen location, the sequence repeats with the detail attribute incrementing with each repetition. The Application Structure (if any) which was under the mouse pointer when clicked is populated in the WebCGMEvent.target property.

mousedown The mousedown event occurs when the pointing device button is pressed. The Application Structure (if any) which was under the mouse pointer when it was pressed down is populated in the WebCGMEvent.target property.

mouseup The mouseup event occurs when the pointing device button is released. The Application Structure (if any) which was under the mouse pointer when it was released is populated in the WebCGMEvent.target property.

mouseover The mouseover event occurs when the pointing device is moved onto an Application Structure. The Application Structure that the mouse pointer moved over is populated in the WebCGMEvent.target property.

mouseout The mouseout event occurs when the pointing device is moved away from an Application Structure. The Application Structure that the mouse pointer moved away from is populated in the WebCGMEvent.target property.

load The load event occurs when the WebCGM DOM implementation finishes loading all content within a WebCGM metafile.

unload The unload event occurs when the WebCGM DOM implementation removes a WebCGM metafile from a window or frame.


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