10. Engines

Engines are what make things change in an image that you have created with Open Inventor. Animation is one obvious use of an engine, but not the only one. Engines can be used to respond to external events, such as mouse or keyboard activity. To realize the full power of engines you would need to do some C++ programming, but there are some simple animation effects that you can create in an IV file.

The basic ideas of using an engine in a .iv file are:

• It has one or more input fields. You connect an input to some other field in your scene graph. When that other field changes, the input to the engine changes. The engine then takes some action in response to the input change. Each input field can only be connected to one thing.
• It has one or more output fields. This is where the actions of the engine are visible. You connect the output fields to other portions of your scene graph, so that the current state of the scene graph will change whenever the output of the engine changes. Each output field can be connected to more than one thing.
• It is an instance of one of Inventor's engine classes. While it is possible to specify engines other than those that come with Inventor, this is only useful if you are doing some C++ programming to extend Inventor in some way. In practice there are a small number of engine types that you will want to use for animation, and they will usually be driven (either directly or indirectly) by the computer's own internal clock. In other words, for animations you will often connect an input field of an engine to some other field in your scene graph that responds in some way to changes in time.

10.1 Types of Engines

1. ElapsedTime
2. TimeCounter
3. Calculator
4. Rotor
5. Blinker

More Information

If you want detailed information about engines, you can browse through chapter 13 of Inventor Mentor. It is oriented towards C++ programming, but gives a good overview of some of the concepts for what engines can do and how they work.