Sensing the Viewer

Motivation

Sensing the viewer enables you to trigger animations:

• when a region is visible to the viewer
• when the viewer is within a region
• when the viewer collides with a shape

The LOD and Billboard nodes are special-purpose viewer sensors with built-in responses.

Sensing the viewer

There are three types of viewer sensors:

• A VisibilitySensor node senses if the viewer can see a region
• A ProximitySensor node senses if the viewer is within a region
• A Collision node senses if the viewer has collided with shapes

Using visibility and proximity sensors

VisibilitySensor and ProximitySensor nodes sense a box-shaped region:

• center - region center
• size - region dimensions

Both nodes have similar outputs:

• enterTime - sends time on visible or region entry
• exitTime - sends time on not visible or region exit
• isActive - sends true on entry, false on exit

Syntax: VisibilitySensor

A VisibilitySensor node senses if the viewer sees or stops seeing a region:

• center and size - the region’s location and size
• enterTime and exitTime - sends time on entry/exit
• isActive - sends true/false on entry/exit

XML Encoding

<VisibilitySensor DEF='Sensor'
    center='0.0 0.0 0.0'
    size='14.0 14.0 14.0'/>

<ROUTE fromNode='Sensor' fromField='enterTime' toNode='Clock' toField='set_startTime'/>

Classic VRML Encoding

DEF Sensor VisibilitySensor {
  center 0.0 0.0 0.0
  size   14.0 14.0 14.0
}

ROUTE Sensor.enterTime TO Clock.set_startTime

Syntax: ProximitySensor

A ProximitySensor node senses if the viewer enters or leaves a region:

• center and size - the region’s location and size
• enterTime and exitTime - sends time on entry/exit
• isActive - sends true/false on entry/exit

XML Encoding

<ProximitySensor DEF='Sensor'
    center='0.0 0.0 0.0'
    size='14.0 14.0 14.0'/>

<ROUTE fromNode='Sensor' fromField='enterTime' toNode='Clock' toField='set_startTime'/>

Classic VRML Encoding

DEF Sensor ProximitySensor {
  center 0.0 0.0 0.0
  size   14.0 14.0 14.0
}

ROUTE Sensor.enterTime TO Clock.set_startTime

A ProximitySensor node senses the viewer while in a region:

• position and orientation - sends position and orientation while viewer is in the region

XML Encoding

<ProximitySensor DEF='Sensor' ... />

<ROUTE fromNode='Sensor' fromField='position_changed' toNode='PetRobotFollower' toField='set_translation'/>

Classic VRML Encoding

DEF Sensor ProximitySensor { ... }

ROUTE Sensor.position_changed TO PetRobotFollower.set_translation

Detecting viewer-shape collision

A Collision grouping node senses shapes within the group:

• Detects if the viewer collides with any shape in the group
• Automatically stops the viewer from going through the shape

Collision occurs when the viewer’s avatar gets close to a shape:

• Collision distance is controlled by the avatar size in the NavigationInfo node

Creating collision groups

Collision checking is expensive so, check for collision with a proxy shape instead:

• Proxy shapes are typically extremely simplified versions of the actual shapes
• Proxy shapes are never drawn

A collision group with a proxy shape, but no children, creates an invisible collidable shape:

• Windows and invisible railings
• Invisible world limits

Syntax: Collision

A Collision grouping node senses if the viewer collides with group shapes:

• enabled - enable/disable sensor
• proxy - simple shape to sense instead of children
• children - children to sense
• collideTime - sends time on collision

XML Encoding

<Collision DEF='Collide'
    enabled='true'>
  <Shape containerField='proxy'>
    <Box ... />
  </Shape>
  <!-- children ... -->
</Collision>

<ROUTE fromNode='Collide' fromField='collideTime' toNode='OuchSound' toField='set_startTime'/>

Classic VRML Encoding

DEF Collide Collision {
  enabled TRUE
  proxy Shape {
    geometry Box { ... }
  }
  children [ ... ]
}

ROUTE Collide.collideTime TO OuchSound.set_startTime

Optimizing collision detection

• Collision is on by default
  • Turn it off whenever possible!
• However, once a parent turns off collision, a child can’t turn it back on!
• Collision results from viewer colliding with a shape, but not from a shape colliding with a viewer

Using multiple sensors

Any number of sensors can sense at the same time:

• You can have multiple visibility, proximity, and collision sensors
• Sensor areas can overlap
• If multiple sensors should trigger, they do

Summary

A VisibilitySensor node checks if a region is visible to the viewer:

• The region is described by a center and a size
• Time is sent on entry and exit of visibility
• True/false is sent on entry and exit of visibility

A ProximitySensor node checks if the viewer is within a region:

• The region is described by a center and a size
• Time is sent on viewer entry and exit
• True/false is sent on viewer entry and exit
• Position and orientation of the viewer is sent while within the sensed region

A Collision grouping node checks if the viewer has run into a shape:

• The shapes are defined by the group’s children or a proxy
• Collision time is sent on contact