5. The Order of Effect

5.1 Determining the Order of Effect

In the example of the scene with the boat and the lake, why does only the lake get coloured and not the boat? It has to do with the order in which property nodes take effect in an Open Inventor scene graph, a very important concept to understand when using Open Inventor. Property nodes include both geometric transformations (like RotationXYZ), and nodes that control the appearance of objects (like SoMaterial). There are a few simple rules about ordering to remember:

• Once you declare an effect with a property node, the effect will act on everything below it in the file. Nothing described above the node will be effected.
• To limit the range of an effect, enclose the property node and the nodes you want it to act upon in a Separator node. That way the effect only lasts until you reach the bottom of the enclosing Separator.
• Geometric transformations accumulate in effect. In other words, if you have two RotationXYZ nodes in your file positioned one right after the other, both rotations take effect. The first rotation in the file takes first, the second rotation takes effect after the first one.
• Non-geometric properties that control object appearance override each other. In other words, if you had two Material nodes in your file positioned one right after the other, any colour specified in the second node would override the colour specified in the first node.

5.2 Why is there an "Order of Effect"?

Why does this issue of order of effect come up? It is an artifact of the process by which Open Inventor uses the scene graph to construct the image you see in a viewer. Open Inventor traverses the scene graph, which means that it starts at the root node and follows the links to the various child nodes. As it visits each node, it gets more information about what the scene should look like. Open Inventor doesn't have to finish traversing the entire scene graph in order to begin displaying the picture. The action of visiting each node is used to render more and more of the image.

5.3 When Do Traversals Happen?

Scene graph traversals happen whenever something changes in the scene. The things that initiate or modify traversal are:

• the creation or deletion of a node;
• a change in the value of a field;
• the actions of nodes that modify the traversal state directly, like Blinker engines.

Understanding the traversal process provides you with a key to creating animations in Open Inventor. The way animation works is by causing a traversal. The person viewing the animation will perceive change or motion when the rendered image after the traversal looks different from the rendered image before the traversal.

More Information

More information about scene graph traversal in Open Inventor is available in chapter 3 of Inventor Mentor.