7.1 Introduction

7.1.1 Name

The name of this component is "Core". This name shall be used when referring to this component in the COMPONENT statement (see 7.2.5.4 Component statement).

7.1.2 Overview

This clause describes the Core component of this part of ISO/IEC 19775. The Core component supplies the base functionality for the X3D run-time system, including the abstract base node type, field types, the event model, and routing. Table 7.1 lists the major topics in this clause.

7.2 Concepts

7.2.1 Overview of the core

The Core component provides the minimum functionality required by all X3D-compliant implementations. The Core component supplies the following abstract constructs:

1. X3D field types descended from the abstract type X3DField;
2. the base abstract node types X3DNode and X3DPrototypeInstance;
3. commonly used interfaces such as X3DBindableNode and X3DUrlObject;
4. utility types such as X3DRoute and X3DScene that are available only to application programmer interfaces for scripting and external scene access;
5. the X3D event model and routing;
6. abstract file structure; and
7. prototyping.

The Core component is a prerequisite component for all other X3D components.

The Core component may be supported at a variety of levels, allowing for a range of implementations that are conformant to the X3D architecture, object model and event model. For more information on these topics, see 4. Concepts.

7.2.2 Bindable children nodes

Several X3D nodes, such as Background, TextureBackground, Fog, NavigationInfo, and Viewpoint are bindable children nodes, inheriting from the abstract node type X3DBindableNode. These nodes have the unique behaviour that only one of each type can be bound (i.e., affect the user's experience) at any instant in time. The browser shall maintain an independent, separate stack for each type of bindable node. Each of these nodes includes a set_bind inputOnly field and an isBound outputOnly field. The set_bind inputOnly field is used to move a given node to and from its respective top of stack. A TRUE value sent to the set_bind inputOnly field moves the node to the top of the stack; sending a FALSE value removes it from the stack. The isBound event is output when a given node is:

1. moved to the top of the stack;
2. removed from the top of the stack;
3. pushed down from the top of the stack by another node being placed on top.

That is, isBound events are sent when a given node becomes, or ceases to be, the active node. The node at the top of the stack (the most recently bound node) is the active node for its type and is used by the browser to set the world state. If the stack is empty (i.e., either the X3D file has no bindable nodes for a given type or the stack has been popped until empty), the default field values for that node type are used to set world state. The results are undefined if a multiply instanced (DEF/USE) bindable node is bound.

The following rules describe the behaviour of the binding stack for a node of type <bindable node>, (Background, Fog, NavigationInfo, or Viewpoint):

4. During read, the first encountered <bindable node> is bound by pushing it to the top of the <bindable node> stack. Nodes contained within files referenced by Inline nodes, within the strings passed to the Browser.createX3DFromString() method, or within X3D files passed to the Browser.createX3DFromURL() method (see 2.[I19775-2]) are not candidates for the first encountered <bindable node>. The first node within a locally defined prototype instance is a valid candidate for the first encountered <bindable node>. The first encountered <bindable node> sends an isBound TRUE event.
5. When a set_bind TRUE event is received by a <bindable node>,
   1. If it is not on the top of the stack: the current top of stack node sends an isBound FALSE event. The new node is moved to the top of the stack and becomes the currently bound <bindable node>. The new <bindable node> (top of stack) sends an isBound TRUE event.
   2. If the node is already at the top of the stack, this event has no effect.
6. When a set_bind FALSE event is received by a <bindable node> in the stack, it is removed from the stack. If it was on the top of the stack,
   1. it sends an isBound FALSE event;
   2. the next node in the stack becomes the currently bound <bindable node> (i.e., pop) and issues an isBound TRUE event.
7. If a set_bind FALSE event is received by a node not in the stack, the event is ignored and isBound events are not sent.
8. When a node replaces another node at the top of the stack, the isBound TRUE and FALSE output events from the two nodes are sent simultaneously (i.e., with identical timestamps).
9. If a bound node is deleted, it behaves as if it received a set_bind FALSE event (see f above).

The results are undefined if a bindable node is bound and is the child of an LOD, Switch, or any node or prototype that disables its children. If a bindable node is bound that results in collision with geometry, the browser shall perform its self-defined navigation adjustments as if the user navigated to this point (see 23.3.2 Collision).

7.2.3 Sensors

Sensors are nodes that generate events based on external inputs to the scene graph, such as user input, changes to the viewing environment, messages from the network or ticks of the system clock. X3D defines the following types of
sensors:

1. Pointing device sensors (see 20 Point device sensor component),
2. Environmental sensors (see 22 Environmental sensor component),
3. Key device sensors (see 21 Key device sensor component),
4. Load sensors (see 9 Networking component), and
5. Time sensors (see 8 Time component).

Sensors are children nodes in the hierarchy and therefore may be parented by grouping nodes as described in 10.2.1 Grouping and children node types.

Each type of sensor defines when an event is generated. The state of the scene graph after several sensors have generated events shall be as if each event is processed separately, in order. If sensors generate events at the same time, the state of the scene graph will be undefined if the results depend on the ordering of the events.

It is possible to create dependencies between various types of sensors. For example, a TouchSensor may result in a change to a VisibilitySensor node's transformation, which in turn may cause the VisibilitySensor node's visibility status to change. For a detailed description of how dependencies among sensors are handled during execution, see 4.4.8.3 Execution model.

7.2.4 Metadata

7.2.4.1 Overview

Metadata is information that is associated with the objects of the X3D world but is not a direct part of the world representation. This part of ISO/IEC 19775 defines an abstract interface X3DMetadataObject that identifies a node as containing metadata and metadata nodes that specify metadata values in various data types.

7.2.4.2 Data types for metadata

This part of ISO/IEC 19775 specifies three basic representation types:  strings, single-precision and double-precision floating point values, and integers. Each piece of metadata has two additional strings that describe:

1. the metadata standard (if any) from which the metadata specification emanates, and
2. the identification for the particular piece of metadata being provided.

The MetadataSet node is provided to support cases when a specific set of metadata requires more than a single data type.

7.2.4.3 Integration with X3D worlds

The X3DNode abstract node type specifies an SFNode field metadata that may only be populated with nodes derived from X3DMetadataObject. If the metadata field is empty, no metadata is associated with the node. Since all nodes in X3D are derived from X3DNode, metadata may be placed anywhere in an X3D world.

Metadata is not included as part of the depiction of an X3D world. However, metadata nodes may have a DEF name and the values of the fields of a metadata node may be accessed by the SAI services.

The contents of the value field of a metadata node is not interpreted by the X3D browser.

Metadata may also be attached to other X3D nodes by setting the metadata field of that node to a node derived from the X3DMetadataNode abstract node type.

7.2.4.4 Assigning metadata to an entire X3D world

The META statement (see 7.2.5.5 META statement) may be used to assign metadata to the entire world.

7.2.5 Abstract X3D structure

7.2.5.1 Organization

An X3D world is conceptually defined as a sequence of statements organized conceptually as a file. The first item in the file is the Header statement. The second item in the file is the PROFILE statement. The PROFILE statement may be optionally followed by one or more COMPONENT statements. The remainder of the file consists of the other elements defined in this part of ISO/IEC 19775. Optional META statements shall follow any COMPONENT statements or, if there are no COMPONENT statements, the PROFILE statement. All other statements shall follow the statements described above.

Any additional X3D content loaded into the scene via Inline nodes or scenes loaded using createX3DFromStream, createX3DFromString, or createX3DFromUrl, shall be declared as having a profile that has an equal or smaller set of required functionality; i.e., there can be no components explicitly declared, or implied by the profile in that content, that requires functionality not declared in the original profile and component declarations for the containing scene.

Although an X3D world is described as being contained in a file, the file may be conceptual and created dynamically during run-time as described in 2.[I19775-2].

7.2.5.2 Header statement

The Header statement is an encoding-dependent statement containing the following elements:

1. identification of the standard being supported (for this standard, this is "X3D");
2. version of the standard being supported (for this version of this part of ISO/IEC 19775, the version number is "3.1");
3. identification of the character encoding being supported (for this standard, this is "UTF-8"), and
4. optional comments that may apply to the file.

While the exact representation of this information is dependent on the encoding, this information shall always be stored as human-readable text.

7.2.5.3 PROFILE statement

Every X3D application shall declare a profile at the beginning of execution. This declaration tells the browser the exact set of components and their support levels that are required for the application to run, allowing for a browser to dynamically load the appropriate components if it so desires, and providing a mechanism for strict conformance should the browser choose to enforce it. If a browser supports the combination of declared profile and components (see 7.2.5.4 COMPONENT statement), it may proceed with presenting the world; otherwise, it shall fail.

The profile is declared via a PROFILE statement immediately following the Header statement at the top of the file. The form of the PROFILE statement is:

PROFILE <name>

where name is a string that does not contain whitespace.

The following profiles are defined in this standard:

1. Core (see A Core profile),
2. Interchange (see B Interchange profile),
3. Interactive (see C Interactive profile),
4. MPEG-4 interactive (see D MPEG-4 interactive profile),
5. Immersive (see E Immersive profile),
6. Full (see F Full profile), and
7. CADInterchange (see H CAD interchange profile).

The profile name is implicitly qualified by the version number of the standard (see 7.2.5.2 Header statement). Browsers shall use both the profile name and the version number to determine the specific characteristics of the profile.

7.2.5.4 COMPONENT statement

X3D applications may explicitly declare additional components required for the application to run. This is useful for combining features that do not appear together in a predefined profile, such as adding Humanoid Animation support to the Immersive profile. If a browser supports the combination of declared profile and components, it may proceed with presenting the world; otherwise, it shall fail.

Declaring a component in a file shall only add support for nodes and functionality defined in that component at the requested support level. Nodes that are defined as part of the prerequisite components (see 4.5.3 Base Components) shall not be automatically included. A user shall declare all components and levels for the nodes and/or functionality being used either explicitly through the use of the COMPONENT statement or implicitly through the PROFILE statement.

Components are declared by COMPONENT statements at the top of the file, immediately following the PROFILE statement but preceding any other content. The form of the component statement is:

COMPONENT <name> <level>

where <name> is a string that does not contain whitespace, and <level> is a positive integer.

If support for a component at the desired level is implied by the application's declared profile, the declaration for that component is unnecessary but may be included.

7.2.5.5 META statement

X3D applications may explicitly declare metadata about the world being defined. This is done by adding one or more META statements that contain such information. Such statements do not affect the scene graph but simply provide additional information in the world.

Metadata is specified by META statements at the top of the file, immediately following the PROFILE and COMPONENT statements but preceding any other content. The form of the META statement is:

META <key> <data>

where <key> is a string that identifies the metadata and <data> is a string that defines the value for the metadata identified by <key>.

7.3 Abstract types

7.3.1 X3DBindableNode

X3DBindableNode : X3DChildNode {
  SFBool [in]     set_bind
  SFNode [in,out] metadata NULL [X3DMetadataObject]
  SFTime [out]    bindTime
  SFBool [out]    isBound
}

X3DBindableNode is the abstract base type for all bindable children nodes, including Background, TextureBackground, Fog, NavigationInfo and Viewpoint. For complete discussion of bindable behaviors, see 7.2.2 Bindable children nodes.

7.3.2 X3DChildNode

X3DChildNode : X3DNode { 
  SFNode [in,out] metadata NULL [X3DMetadataObject]
}

This abstract node type indicates that the concrete nodes which are instantiated based on it may be used in children, addChildren, and removeChildren fields.

More details on the children, addChildren, and removeChildren fields can be found in 10.2.1 Grouping and children node types.

7.3.3 X3DMetadataObject

X3DMetadataObject { 
  SFString [in,out] name      ""
  SFString [in,out] reference "" 
}

This abstract interface is the basis for all metadata nodes. The interface is inherited by all metadata nodes.

The specification of the reference field is optional. If provided, it identifies the metadata standard or other specification that defines the name field. If the reference field is not provided or is empty, the meaning of the name field is considered implicit to the characters in the string.

7.3.4 X3DNode

X3DNode {
  SFNode [in,out] metadata NULL [X3DMetadataObject]
}

This abstract node type is the base type for all nodes in the X3D system.

7.3.5 X3DPrototypeInstance

X3DPrototypeInstance : X3DNode {
  SFNode [in,out] metdata NULL [X3DMetadataObject]
}

This abstract node type is the base type for all prototype instances in the X3D system. Any user-defined nodes declared with PROTO or EXTERNPROTO are instantiated using this base type. An X3DPrototypeInstance may be place anywhere in the scene graph where it is legal to place the first node declared within the prototype instance. For example, if the base type of first node is X3DAppearanceNode, that prototype may be instantiated anywhere in the scene graph that allows for an appearance node (EXAMPLE  Shape).

7.3.6 X3DSensorNode

X3DSensorNode  : X3DChildNode {
  SFBool [in,out] enabled  TRUE
  SFNode [in,out] metadata NULL [X3DMetadataObject]
  SFBool [out]    isActive
}

This abstract node type is the base type for all sensors.

7.4 Node reference

7.4.1 MetadataDouble

MetadataDouble : X3DNode, X3DMetadataObject {
  SFNode   [in,out] metadata  NULL [X3DMetadataObject]
  SFString [in,out] name      ""
  SFString [in,out] reference ""
  MFDouble [in,out] value     []
}

The metadata provided by this node is contained in the double-precision floating point numbers of the value field.

7.4.2 MetadataFloat

MetadataFloat : X3DNode, X3DMetadataObject { 
  SFNode   [in,out] metadata  NULL [X3DMetadataObject]
  SFString [in,out] name      ""
  SFString [in,out] reference ""
  MFFloat  [in,out] value     []
}

The metadata provided by this node is contained in the single-precision floating point numbers of the value field.

7.4.3 MetadataInteger

MetadataInteger : X3DNode, X3DMetadataObject { 
  SFNode   [in,out] metadata  NULL [X3DMetadataObject]
  SFString [in,out] name      ""
  SFString [in,out] reference ""
  MFInt32  [in,out] value     []
}

The metadata provided by this node is contained in the integers of the value field.

7.4.4 MetadataSet

MetadataSet : X3DNode, X3DMetadataObject { 
  SFNode   [in,out] metadata  NULL [X3DMetadataObject]
  SFString [in,out] name      ""
  SFString [in,out] reference ""
  MFNode   [in,out] value     [] [X3DMetadataObject]
}

The metadata provided by this node is contained in the metadata nodes of the value field.

7.4.5 MetadataString

MetadataString : X3DNode, X3DMetadataObject { 
  SFNode   [in,out] metadata  NULL [X3DMetadataObject]
  SFString [in,out] name      ""
  SFString [in,out] reference ""
  MFString [in,out] value     []
}

The metadata provided by this node is contained in the strings of the value field.

7.5 Support levels

The Core component provides two levels of support as specified in Table 7.2. Level 1 provides the minimum basis for all profiles and components. Level 2 adds support for prototypes.