Open Inventor

Mr. Hartley shows how to do interactive 3-D programming using Open Inventor, Release 2, which he used to create the images on our cover.
Inventor Manipulators

This program can be enhanced to allow more than the viewpoint to change by adding a manipulator. Using SoTrackballManip provides the ability to interactively rotate an object about its own center on any of the three axes, as shown in Figure 2.

Figure 2. Rotating the Cone

Only two lines need to be added to the source code. At the end of the includes section of the program above, add the line:

#include <Inventor/manips/SoTrackballManip.h>

and immediately after the root->ref call, add the line:

root->addChild(new SoTrackballManip);
To be sure I was making the cone rotate, I moved it off to the side using mouse button two, and turned on the origin's three axes display using the pop-up menu of mouse button three.

Clicking the top button on the right of the main window changes the mouse to selection mode. Moving the cursor to any intersection of the double rings and selecting it with mouse button one causes it to be highlighted in yellow, allowing us to rotate the cone about its axis. If the mouse is still moving when released, it will continue spinning. Clicking on the hand icon will revert to viewpoint manipulation mode, so that both the cone and its manipulator can be rotated about the viewpoint axis. The cone is now spinning on its own axis and orbiting around the center of the scene.

Other manipulators will perform rotations, scaling and translation (movement) tasks and any combination of these functions.

Editing Nodes

Inventor comes with a number of standard node editors for modifying the scene. Figure 3 is a typical model (without any alterations) in SceneViewer, a demonstration program that comes with the Inventor distribution. For a little more information about the figure, see the sidebar “A Bit About Figure 3”.

Figure 3. A model of Deep Flight One, the single person submarine designed and built by Hawkes Ocean Technologies of San Raphael, California

Selecting Editors from the main menu, we can select “Material Editor” and get the dialog shown in Figure 4.

Figure 4. Material Editor Dialog

The blue dome can be selected on the front of the craft and made semi-transparent; a few lights can be added and the viewer's built-in head lamp turned off. Inventor has three types of lights which can be added to a scene:

  • SoPointLight acts like a light bulb, radiating light equally in all directions.

  • SoDirectionalLight emits light along a single direction as though it were nearly an infinite distance away, like the sun.

  • SoSpotLight emits light in a cone, allowing it to be pointed and focused.

Figure 5 is an example of a green SoDirectionalLight and red and white SoSpotLights. Their respective icons have been left on so the manipulators are visible—they would normally be left off during rendering. With the mouse, I can select and move them as well as change their direction. The SoSpotLights have the added cone to allow focusing the light by changing the radius of projection.

Figure 5. Results of Intersecting Color Spotlights

In Figure 5, we see how light behaves when pools of light intersect. Having changed the dome's material properties, we see how the green light appears as a reflection. If fog, other atmospheric effects and texture mapping were added, we would have a highly realistic scene, ready for imaginative animation.

This submarine model is part of a simulator on which I am working. It includes a digital elevation map viewer that reads and renders topographical data to display terrain in which to drive. I used some of John Beale's ideas about making use of government data in rendering landscape images. On his site at, he provides a few useful converters, which he has adapted from others, to turn the one minute elevation maps into PGM file pixmaps.

Figure 6. Terrain Display

The terrain-handling section of the program is shown in Figure 6. It allows control of terrain resolution and color. The viewer on the right side is standard, so the image can be zoomed, panned and rotated as desired. Depending on machine speed and terrain complexity, the rendering is more or less interactive. Personally, I have found a value of 24 to be an ideal terrain-complexity value.

The elevation data is kept in a 1204 by 1204 integer array representing the heights surveyed in the map. This would be a bit much to try manipulating interactively on a PC, so the terrain-complexity thumbwheel allows us to choose the amount of reduction we want. A very simple algorithm is used. The value of the thumbwheel, two in Figure 6, is used as the length of a square from which we take the maximum height, and then added as a coordinate point to an Inventor SoQuadMesh, basically a grid composed of quadrilaterals. With a value of two, the maximum height is four elevation points. A value of ten provides a maximum height of 100 points, eliminating 99% of the map for rendering purposes. As this works interactively, the value can be changed until a balance is found that combines the amount of needed detail with the desired rendering speed. Within this, the area of interest can be located and only the needed detail rendered.

Each SoQuadMesh node in Figure 6 is defined by a set of four vertices, each shared with its adjacent neighbors. These vertices have an associated color component which is smoothed out over the area of the SoQuadMesh. To give some visual indication of the height and to generally make things more interesting, I have set each elevation to have a different color based on a simple formula specified by the user.

The six vertical wheels handle the assignment of maximum and minimum depths for each color component, so overlapping blue and red results in a purple area of overlap. The amount of color is calculated as follows: from the maximum and minimum ranges the midpoint is found, and the percentage of distance from the color border to this midpoint is the percentage of that color to use. Thus, a vertex at the midpoint has a value of 1.0 for that color component and a vertex three quarters of the way to the edge will have a value of 0.75 of the color.

The file format read in is basically an ASCII PGM file containing three values representing the maximum X, Y and Z values, followed by all elevations. This is handy because the file can be read with the xv command and saved in any other graphical file format needed.

Originally, the terrain represented in Figure 6 was the pixmap shown in Figure 7, with lighter areas having more height.

Figure 7. Original Pixmap Terrain Image


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