Chapter 1 – Introduction to VRML

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Chapter 2 – Basic VRML

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Structure of VRML File

A VRML file consists of the following majorfunctional components

the VRML header

the scene graph which includes

Shapes

Interpolators

sensors and scripts

the prototypes

event routing

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#VRML V2.0 utf8

# A brown hut

Group {

children [

# Draw the hut walls

Shape {

appearance DEF Brown Appearance {

material Material {

diffuseColor 0.6 0.4 0.0

}

geometry Cylinder {

height 2.0

radius 2.0

}

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# Draw the hut roof

Transform {

translation 0.0 2.0 0.0

children Shape {

appearance USE Brown

geometry Cone {

height 2.0

bottomRadius 2.5

}

]

}

$E:\BOOK\WIN\CH02\02FIG01.GIF$

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VRML Header

For easy identification of VRML files, every VRMLfile shall begin with

#VRML V2.0 <encoding type>

The <encoding type> is either "utf8" or any otherauthorized values defined in other parts of ISO/IEC14772

In this course, we are using "utf8" which indicates aclear text encoding that allows for internationalcharacters to be displayed in VRML

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UTF-8 File Format

# character begins a comment

Only the first comment (the file header) has semantic meaning

The only exception is within double-quoted SFString andMFString fields where the # character is defined to be part ofthe string

Commas, spaces, tabs, linefeeds, and carriage-returns areseparator characters wherever they appear outside of stringfields

One or more separator characters separate the syntacticalentities in VRML

The separator characters collectively are termed whitespace

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VRML Statements

VRML file may contain any combination of thefollowing :

Any number of PROTO or EXTERNPROTOstatements

Any number of root children node statements

Any number of USE statements

Any number of ROUTE statements

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Scene Graph

The scene graph contains nodes, which describeobjects and their properties

It contains hierarchically grouped geometry toprovide an audio-visual representation of objects

It also provides a means for nodes to participate inthe event generation and routing mechanism

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Nodes

Nodes are used to describe shapes and properties ofthe world such as:

shapes

colors

lights

viewpoints, how to position and orient shapes

animation times, sensors and interpolators

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Nodes

A node contains the following information:

type of nodes

a set of curly brackets

fields

For example, a cylinder node could be described:

cylinder

{

height 2.0

radius 2.0

}

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Fields and Field Values

Fields define the attributes of a node

In the above example, the height field defines theheight of the cylinder

Fields are optional within the nodes

If the values are not given, then a default value wouldbe assigned

For example, the default values of cylinder haveradius = 1.0 units and height = 2.0 units

Field values defines the attributes like color, size orposition

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Defining and Using Node Name

Any node can be defined in the world

The name of the node must begin with a letter,followed by any combination of characters and/ordigits

DEF node_name name_type

Once a node has a name, we could reuse the node.

The node with defined name is called original nodewhile the reuse nodes are called instances

USE node_name

Note that the node names are case-sensitive

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Describing Shapes

A VRML shape has appearance (based on material),color and surface texture

These attributes are specified by the field valueswithin a shape node

VRML supports several primitives shape geometriesthat include boxes, cylinders, cones, and shapes

VRML also supports advanced shape geometries likeextruded shapes and elevation grids

Using these primitive shapes, we could group themand build more complex shapes

Shapes can be grouped by the Group node

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Event Routing

Some VRML nodes generate events in response toenvironmental changes or user interaction

Event routing provides a mechanism, which allowevents to be propagated to effect changes in othernodes

Once generated, events are sent to their routeddestinations in time order and processed by thereceiving node

This processing can change the state of the node,generate additional events, or change the structure ofthe scene graph

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Event Routing

In order to build a dynamic world and propagateevents based on environmental changes, we need“wiring instructions”

This process involves

a pair of nodes to wire together

a wiring route or path between these two nodes

Once the nodes are wired, messages could be sentalong that route

Such message is called events which containscommunicating values

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Event Routing

For a successful routing, nodes must have eventInand the corresponding eventOut

An eventIn receives events when it is connected to aroute and a message is being sent to it

An eventOut sends the events out along the connectedroute

VRML “wiring” architecture is built by describing aroute from one node’s eventOut to another node’seventIn

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Event Routing

The receiving node reacts based on the event dependson the following:

type of node receiving the event

node input jack to which the route is wired

values contained in the event

current activities of the node

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Presentation and Interaction

The interpretation, execution, and presentation ofVRML files will typically be undertaken a browser,which displays the shapes and sounds in the scenegraph

This presentation is known as a virtual world and isnavigated in the browser by a human or mechanicalentity, known as a user

The world is displayed as if experienced from aparticular location; that position and orientation inthe world is known as the viewer

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Presentation and Interaction

The browser may define navigation paradigms (suchas walking or flying) that enables the user to move theviewer through the virtual world

The browser may provide a mechanism allowing theuser to interact with the world through sensor nodesin the scene graph hierarchy

Sensors respond to user interaction with geometricobjects in the world, the movement of the userthrough the world, or the passage of time

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Presentation and Interaction

The visual presentation of geometric objectsin a VRML world follows a conceptual modeldesigned to resemble the physicalcharacteristics of light

The VRML lighting model describes howappearance properties and lights in the worldare combined to produce displayed colours

Refer to the diagram for the conceptualmodel of the VRML browser

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Presentation and Interaction

Conceptual Model of VRML Browser

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USE Statement

The USE statement enables us to reuse all the objectsthat we have defined earlier in the scene graph

A USE statement consists of the USE keywordfollowed by a node name as follows:

USE <name>

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ROUTE Statement

A ROUTE statement provides the message passingmechanism from one node to another

A ROUTE statement consists of the ROUTE keywordfollowed in order by a node name, a period character,a field name, the TO keyword, a node name, a periodcharacter, and a field name.

Whitespace is allowed but not required before orafter the period characters:

ROUTE <name>.<field/eventName> TO<name>.<field/eventName>

All these commands will be covered in more details asthe course proceeds