© TMC Computer School HC20203 VRML 2 - 1 HIGHER DIPLOMA IN COMPUTING Chapter 2 – Basic VRML

© TMC Computer School

HC20203 VRML 2 - 1

HIGHER DIPLOMA IN COMPUTING

Chapter 2 – Basic VRML


HC20203 VRML 2 - 2


Structure of VRML File

A VRML file consists of the following major functional 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 hutGroup { 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 } } } ]}


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VRML Header

For easy identification of VRML files, every VRML file shall begin with #VRML V2.0 <encoding type>

The <encoding type> is either "utf8" or any other authorized values defined in other parts of ISO/IEC 14772

In this course, we are using "utf8" which indicates a clear text encoding that allows for international characters 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 and MFString fields where the # character is defined to be part of the string

Commas, spaces, tabs, linefeeds, and carriage-returns are separator characters wherever they appear outside of string fields

One or more separator characters separate the syntactical entities in VRML

The separator characters collectively are termed whitespace


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VRML Statements

VRML file may contain any combination of the following : Any number of PROTO or EXTERNPROTO

statements 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 describe objects and their properties

It contains hierarchically grouped geometry to provide an audio-visual representation of objects

It also provides a means for nodes to participate in the event generation and routing mechanism


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Nodes

Nodes are used to describe shapes and properties of the 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 2.0radius 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 the height of the cylinder

Fields are optional within the nodes

If the values are not given, then a default value would be assigned

For example, the default values of cylinder have radius = 1.0 units and height = 2.0 units

Field values defines the attributes like color, size or position


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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/or digits

DEF node_name name_type Once a node has a name, we could reuse the node. The node with defined name is called original node

while 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 values within a shape node

VRML supports several primitives shape geometries that include boxes, cylinders, cones, and shapes

VRML also supports advanced shape geometries like extruded shapes and elevation grids

Using these primitive shapes, we could group them and 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 to environmental changes or user interaction

Event routing provides a mechanism, which allow events to be propagated to effect changes in other nodes

Once generated, events are sent to their routed destinations in time order and processed by the receiving node

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


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Event Routing

In order to build a dynamic world and propagate events 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 sent along that route

Such message is called events which contains communicating values


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Event Routing

For a successful routing, nodes must have eventIn and the corresponding eventOut

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

An eventOut sends the events out along the connected route

VRML “wiring” architecture is built by describing a route from one node’s eventOut to another node’s eventIn


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Event Routing

The receiving node reacts based on the event depends on 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 of VRML files will typically be undertaken a browser, which displays the shapes and sounds in the scene graph

This presentation is known as a virtual world and is navigated in the browser by a human or mechanical entity, known as a user

The world is displayed as if experienced from a particular location; that position and orientation in the world is known as the viewer


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The browser may define navigation paradigms (such as walking or flying) that enables the user to move the viewer through the virtual world

The browser may provide a mechanism allowing the user to interact with the world through sensor nodes in the scene graph hierarchy

Sensors respond to user interaction with geometric objects in the world, the movement of the user through the world, or the passage of time


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The visual presentation of geometric objects in a VRML world follows a conceptual model designed to resemble the physical characteristics of light

The VRML lighting model describes how appearance properties and lights in the world are combined to produce displayed colours

Refer to the diagram for the conceptual model of the VRML browser


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Conceptual Model of VRML Browser


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USE Statement

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

A USE statement consists of the USE keyword followed by a node name as follows: USE <name>


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ROUTE Statement

A ROUTE statement provides the message passing mechanism from one node to another

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

Whitespace is allowed but not required before or after the period characters: ROUTE <name>.<field/eventName> TO

<name>.<field/eventName> All these commands will be covered in more details as

the course proceeds

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© TMC Computer School HC20203 VRML 2 - 1 HIGHER DIPLOMA IN COMPUTING Chapter 2 – Basic VRML