• VRML 2.0
• Differences between VRML 2 and Open Inventor
• Animation using VRML 2

VRML 2.0

VRML 2.0 is the current standard for displaying 3D graphics on the World Wide Web. It is loosely based on the Open Inventor .iv file format. It carries over the concepts of the scene graph, nodes with "fields", engines, and sensors. Most (all?) VRML browsers are implemented as Netscape plugins. Examples: Cosmo Player (http://vrml.sgi.com), Netscape's Live3D.

VRML programming is mostly done in the VRML file format (see the VRML 2.0 specification), as opposed to C++ or Scheme. It is designed for ease-of-use as opposed to extensibility. However, there are Script nodes and other APIs which allow Java and VRMLScript programs to affect the scene.

Differences between VRML 2 and Open Inventor

• children are stored explicitly in "children" field of group nodes.
• no traversal state (like transform and material nodes). (Why? Probably to make inter-node dependencies more explicit.)
  • no Separator nodes; Group is an implicit Separator
  • Transform is now a special type of group node. Affects all of the nodes in its children field.
  • Shape node contains SFNode fields for Appearance and Geometry nodes; Appearance node defines material and/or texture
  • Example: static geometry.

    #VRML V2.0 utf8
    
    Viewpoint {
       position 3.0 0.0 0.0
    }
    Group {
       children [
          Shape {
             appearance Appearance {
                 material Material {
                     diffuseColor 0.8 0.2 0.2
                 }
             }
             geometry Box {}
          }
          Transform {
             translation 3.0 0.0 0.0
             children [
                Shape {
                   appearance Appearance {
                      material Material {
                         diffuseColor 0.2 0.8 0.2
                      }
                   }
                   geometry Sphere {}
                }
                Transform {
                   translation 3.0 0.0 0.0
                   children [
                      Shape {
                         appearance Appearance {
                            material Material {
                               diffuseColor 0.2 0.2 0.8
                            }
                         }
                         geometry Cone {}
                      }
                   ]
                }
             ]
          }
       ]
    }
    

• (Almost) all dynamism in scene is due to field-to-field connections and event-driven model. Four types of "fields".
  • field: value is set when the file is read in, and is static from then on.
  • eventIn: a field which can only be connected TO.
  • eventOut: a field which can only be connected FROM.
  • exposedField: a combination of an eventIn and an eventOut.
• Routes connect eventOuts to eventIns (just like field-to-field connections in Inventor). Fan-out is allowed; fan-in is not.
• An "event" occurs when an eventOut "sets its value". This event propagates down the routing chain. Events are typed by the type of the eventOut from which they are generated.
• Built-in engines are interpolators (Position, Orientation, Color).
• VRML sensors combine concepts of Inventor sensors and draggers. Cylinder, Plane, Sphere sensors detect click-and-drag motion of geometry in certain directions. Time sensors generate a continuous stream of events suitable for animation.
• Example: color animation upon mouse click.

  #VRML V2.0 utf8
  
  Group {
     children [
        DEF SENSOR TouchSensor {}
        Shape {
           appearance Appearance {
              material DEF MAT Material {
                 diffuseColor 0.2 0.2 0.8
              }
           }
           geometry Sphere {}
        }
        DEF INTERP ColorInterpolator {
           key [ 0.0, 0.5, 1.0]
           keyValue [ 0.2 0.2 0.8,
                      0.8 0.2 0.2,
                      0.2 0.2 0.8 ]
        }
        DEF TIMER TimeSensor {}
     ]
  }
  
  ROUTE SENSOR.touchTime TO TIMER.startTime
  ROUTE TIMER.fraction_changed TO INTERP.set_fraction
  ROUTE INTERP.value_changed TO MAT.diffuseColor
  

• Script node allows arbitrary engines to be constructed (using VRMLScript or Java).
• Extending VRML: PROTOs. Can define new node types which are compositions of basic node types; eventIns/Outs of PROTO maps to fields in the containing nodes.

Animation using VRML 2

How animation works:

• Trigger is usually sensor (TouchSensor, ProximitySensor)
• Logic: can be Script node. Can store internal state (light switch example), or be eliminated.
• Timer generates time events which are fed into engine or engines (typically interpolator).
• Target geometry moves, changes color, etc.

Example: moving doors in Doom.

• TouchSensor -> TimeSensor's startTime -> PositionInterpolator -> Transform.

$Id: index.html,v 1.1 1997/01/24 17:50:06 kbrussel Exp $