10.1.5 Blinker Engines

Blinker engines directly modify the traversal state of a scene graph by causing particular pieces of the graph to be cycled through. The most obvious use of this is to cause objects to repeatedly appear and disappear from view, hence the name "Blinker". This class of engine is derived from the Switch group node, which controls which of several pieces of a scene graph can be traversed. The Blinker adds an animation effect by continually cycling traverals of its children, one child at a time. Even if the Blinker engine only has one child, the effect of that child on the scene will alternately appear and disappear.

The important fields are:

• whichChild: the index number of the child to begin the traverse cycle with. Child nodes are indexed by the integers 0, 1, 2, etc. The default value is 0, the first child node.
• speed: the number of cycles per second. The default value is 1.
• on: a boolean switch that decides if the engine is currently running or not. The default value is TRUE.

Example

Here is a complete Inventor .iv file for an example that uses a Blinker engine node to alternate between displaying a cube and a cone. Just cut and paste the text into a file named test.iv and then do ivview -q test.iv:

#Inventor V2.0 ascii

Separator {
    PerspectiveCamera {
	position	0 0.5 10
    }
    Blinker {
	whichChild	0
	speed	0.5
	Separator {
	    Translation {
                translation  -2 0 0
	    }
	    Cone {
	    }
	}
	Separator {
	    Translation {
                translation  +2 0 0
	    }
	    Cube {
	    }
	}

    }
}

You should note several things about this example:

• The Blinker engine is also a grouping node just like Separator, so the children are contained within the engine description.
• The whichChild field of the Blinker engine is initially set to 0, which means that the first object to be displayed is the Cone object.
• The speed field is set to 0.5, which means that each cycle will take 2 seconds.
• The Cone node is translated two units to the left, while the Cube node is translated two units to the right, so that they will display side-by-side instead of being centred at the same location.
• The Separator nodes are used to group each translation with the object that it is meant to translate, otherwise the effect of each of those translations would continue to propagate down through the rest of the scene description until the end of any enclosing grouping node (the end of the Blinker node in this case).

More Information

More information about Blinker engines is available in chapter 13 of Inventor Mentor and in the man page for SoBlinker.