Scene Graphs in Ygdrasil

Messages

• Elements in a Ygdrasil scene graph are initialized with messages called "initial messages"
• a ygTransform node takes the following messages
• position ( float, float, float )
• orientation ( float, float, float )
• size ( float | float, float, float )
• a ygObject node takes, among others, the following messages
• file ( string )
• cache ( [bool] )
• a ygSwitch node takes the following messages
• on
• off
• toggle
• Elements in a Ygdrasil scene graph are changed during runtime with events
• events are "generated" by nodes
• the ygCAVEWand, derived from ygWand, node generates button related events
• button1
• buttonUp1
• button2
• buttonUp2
• button3
• buttonUp3
• we can used the when message to send messages to other nodes in response to events
• when( eventName, message )
• for example when( button1, mySwitch.on )
• only the node that generates the event can respond to the event 
• nodes that receive messages from other nodes need to have a unique name (i.e. mySwitch)

example

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Establishing Space

• Often we will want to establish spaces
• for establishing environmental effects
• fog
• clipping planes
• background colors
• for detecting the presence of users and wands
• for limiting the locations within which events can be generated
• the ygSpace node can establish a space with the following volumes
• infinite
• sphere
• box
• cylinder
• point
• the userTrigger node is a ygSpace node that generates events in the presence of a ygHead node
• enter
• exit
• empty
• firstenter
• the wandTrigger node is a ygSpace node that generates events in the presence of a ygWand node
• enter
• exit
• any events generated by the ygWand node (i.e. button1)

example

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Establishing User Presence

• the user plays a significant role in the virtual world
• we care about much more than rendering the scene from the head perspective
• we care about determining where the user is in the scene
• the userTrigger node detects the presence of a ygHead node
• we care about determining the location of the user wand and the direction it is pointing
• the wandTrigger node detects the presence of a ygWand node
• the pointAtTrigger node detects if a ygWand node is pointing at something
• we may also want to represent the user in the scene graph just like we represent the rest of the scene
• a user avatar requires the following minimum parts
• head
• hand
• the following additional elements add to the avatar
• body - located below the head in a consistent orientation
• articulated arms
• articulated legs
• we can use a ygCAVETracker node to position the ygWand or ygHead nodes
• a transform node that gets position and orientation from a sensor
• usually the head sensor is sensor number 0
• usually the wand sensor is sensor number 1
• the ygUser node
• establishes the CAVE frame of reference
• allows a unique name to be assigned to each user
• head and wand trackers are always relative to the CAVE frame of reference
• the ygNavigator node navigates the CAVE frame of reference to different locations and orientations within the scene
• the ygCAVENavigator node implements the traditional "point and move" technique

example