SCENE GRAPH

Scene Graphs

Nodes

Basic building block used to create three-dimensional scene databases in Inventor
Each node is an object that holds a piece of information, such as a surface material, shape description, geometric transformation, light,or camera
All 3D shapes, attributes, cameras, and light sources present in a scene are represented as nodes
Each node is composed of a set of data elements, known as fields, that describe the parameters of the node

Nodes are divided into three basic categories

Shape nodes, which represent 3D geometric objects
Property nodes, which represent appearance and other qualitative characteristics of the scene
Group nodes, which are containers that collect nodes into graphs

Nodes may contain data elements within fields. Each field within a node is also a class, rather than a basic data type; for example, if a node needed to contain a single floating point value, it would have an SoSFFloat field data member rather than just a float, as in a C data structure.

There are two main reasons for doing this:

All field types have consistent set and get functions; the interface to get the value stored in a field is consistent over all field types.
Because of these set and get functions, a node can tell when the value of a field stored within it has changed, and can automatically notify the application; for example, it could (and usually does) automatically rerender the scene.

Basic Data Types and Naming Conventions

Upon first inspection, the names of Inventor classes seem strange: they all have either an "So" or "Sb" prefix. The difference between them is important. "So" stands for scene object, and data types with this prefix are usually related to the scene graph; for example, all nodes have an "So" prefix. "Sb" stands for scene basic, and refers to basic data types such as vectors, matrices, and colors.

Some of the more important Sb types are as follows:

SbBool: defines a boolean value. In Scheme this type is an integer which is 1 when true and 0 when false.
SbColor: defines a RGB (red, green, blue) color.
SbMatrix: 4x4 matrix.
SbVecnx: 2D or 3D vector. n is the number of elements in the vector (2, 3, or 4); x is the type of element (f for float, s for short integer).

Groups

A container for collecting child objects
Collect property, shape, and other group nodes into graphs

Scene Graph

Consists of one or more nodes
Hierarchical scenes are created by adding nodes as children of grouping nodes

Scene Database

Consists of information representing one or more 3D scenes
Can contain several scene graphs

Scene Graph Showing Nodes Used to Create a Robot

Rendered Image of the Robot


#Inventor V2.0 ascii
Separator {
    Separator { # Body
        Transform { translation 0 3 0 }
        Material { # A bronze color:
            ambientColor .33 .22 .27
            diffuseColor .78 .57 .11
            specularColor .99 .94 .81
            shininess .28
        }
        Cylinder { radius 2.5 height 6 }
    }
    Separator { # Head
        Transform { translation 0 7.5 0 }
        Material { # A silver color:
            ambientColor .2 .2 .2
            diffuseColor .6 .6 .6
            specularColor .5 .5 .5
            shininess .5
        }
        Sphere { }
    }
}