Chapter 2: An Inventor Sampler

This chapter introduces Inventor programming and some key Inventor concepts. Most of the chapter are variations on the "Hello Cone" program, a simple program that displays a cone on the screen.

Up to Summary of The Inventor Mentor
Back to Chapter 1 - Overview
On to Chapter 3 - Nodes and Groups

"Hello Cone"

The "Hello Cone" program is a 20-line program that opens an XtRenderArea in a window, creates a 3D scene consisting of a camera, light, a red material, adds the scene to the XtRenderArea, and then enters the Xt main event loop, which takes care of rendering the cone in response to window system events. Bonus information you won't find in the book: several people in the Inventor group (myself included) believe that these examples should have been expressed in the ASCII file format, instead of being presented as programming exercises. However, since Inventor is a programming library first and a file format second, we lost that argument. However, I'll present the file format for the examples here; if you have an SGI machine, you can use the '/usr/sbin/ivview' program to view them.

Here is HelloCone: 

#Inventor V2.0 ascii
Separator {
    PerspectiveCamera {
	position 0 0 4.18154
	nearDistance 2.44949
	farDistance 5.91359
	focalDistance 4.18154
    }
    DirectionalLight {}
    Material {
	diffuseColor [ 1 0 0 ]
    }
    Cone { }
}

Using Engines to Make the Cone Spin

The first variation presented adds an engine to make the cone spin. A rotation transformation (RotationXYZ) is added to the scene, and an ElapsedTime engine is connected to the rotation's angle so that the angle of rotation in radians equals the time in seconds since the program started (the axis of rotation is set to the X axis). In the ASCII file format, this scene looks like: 
#Inventor V2.0 ascii
Separator {
    PerspectiveCamera {
	position 0 0 4.18154
	nearDistance 2.44949
	farDistance	5.91359
	focalDistance 4.18154
    }
    DirectionalLight {}
    RotationXYZ {
	axis X
	angle = ElapsedTime { }.timeOut
    }
    Material {
	diffuseColor [ 1 0 0 ]
    }
    Cone { }
}

Adding a Trackball Manipulator

Next, a trackball manipulator is used to allow the user to interactively manipulate the rotation of the cone. This is done by replacing the RotationXYZ node of the previous example with a TrackballManip: 
#Inventor V2.0 ascii
Separator {
    PerspectiveCamera {
	position 0 0 4.18154
	nearDistance 2.44949
	farDistance	5.91359
	focalDistance 4.18154
    }
    DirectionalLight {}
    TrackballManip {}
    Material {
	diffuseColor [ 1 0 0 ]
    }
    Cone { }
}

Adding the Examiner Viewer

Finally, an XtExaminerViewer is used instead of a simple XtRenderArea. The XtExaminerViewer adds user interface controls to allow the user to manipulate their view of the scene; for example, a zoom slider that controls the field of view (like a zoom lens on a camera), thumbwheel widgets that control rotation about the screen X and Y axes, etc. The particular viewing paradigm used (ExaminerViewer, WalkViewer, FlyViewer, etc) is not considered part of the scene and is not saved in the file format, so there is no corresponding file format example for this variant on "Hello Cone".

Naming Conventions, Scene Basics

The chapter ends with a description of the conventions used by Inventor. For example, most Inventor C++ classes are prefixed with "So", which stands for "Scene Object" (at least, that's the official explanation-- insiders know that it really stands for "Scenario", the internal name of the project before it had a name), all enumerated values are all uppercase, etc. The Inventor library also contains some useful low level classes that are prefixed with "Sb", such as SbLine, SbColor and SbRotation.

Coordinate Systems in Inventor

The convention for the coordinate system used by Inventor is also described. It is a right-handed coordinate system with +z coming out of the screen. Angles are specified in radians, and objects may be defined in their own local coordinate system. The world coordinate space is the default coordinate space; and object can be transformed into world coordinates by applying all of the transformation affecting it. Side note: the book implies that cameras and lights are special and aren't affected by transformations (that they are defined directly in world coordinates). That isn't true -- transformations are applied to cameras and lights, just like other objects.

Glossary

Camera
Defines the eyepoint from which the scene is viewed. The Inventor SoCamera class is an abstract base class that encapsulates functionality for the two types of cameras Inventor supports, SoOrthographicCamera and SoPerspectiveCamera.

DirectionalLight
Defines light shining in a given direction.

ElapsedTime
An engine that outputs how much time has elapsed since a given time (by default, how much time has elapsed since it was created).

Engine
An object that reads one or more values, performs some operation on those values, and then writes the results into some part of a scene. Engines can be to program animations, or to create "smart" 3D objects with behavior.

ExaminerViewer
A component with user interface for interacting with a 3D scene. The ExaminerViewer uses a "hold an object in your hand and rotate it" paradigm for examining a 3D scene. It maps mouse gestures to 3D rotations of the camera, to rotate around an object to look at it from all sides.

Light
An object in the scene that casts light into all or part of the scene. The Inventor SoLight class is an abstract base class; Inventor supports Directional, Point, and Spot lights.

Manipulator
An object that is part of the scene that responds to input device events and does something. For example, the TrackballManip maps mouse events into 3D rotations.

Material
An object that specifies how a shape's surface will respond to light. The lighting model used by Inventor for materials is fairly simple, and includes diffuse color, ambient color, specular color, specular exponent, emissive color, and transparency.

Node
The generic name for any object that is part of the scene, such as lights, cameras, shapes.

Rotation
A 3-dimensional orientation. Rotations are generally specified as an axis to rotation about and an angle of rotation about that axis.

Scene, Scene Graph
A set of nodes grouped together that represent a virtual environment or 3D world.

Translation
A 3-dimensional position, specified by three numbers (X, Y and Z).

Xt
The X toolkit. Inventor is separated into two libraries; libInventor is the core library containing all of the window-system-independent code. On Unix machines running the X window system, libInventorXt contains window-system-specific components like the XtRenderArea or the XtMaterialEditor. Inventor is being ported to Windows NT, which will have similar (but not identical) components as libInventorXt.

Author/Summarizer:
Gavin Bell

Up to Summary of The Inventor Mentor
Back to Chapter 1 - Overview
On to Chapter 3 - Nodes and Groups