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.

Open Inventor is a powerful 3-D graphics library that allows the user to create interactive 3-D applications quickly and easily. It adds object-oriented programming to OpenGL, the most widely available standard 3-D API. This layer of object-oriented abstraction does not come at the expense of losing control of our applications—all the power of programming directly with OpenGL is still available.

One measure of how much detail is needed in order to get things done is the size of the standard reference books. My personal top five essential books on graphics programming are listed in Resources. To be functional in Inventor programming, the most essential one is Josie Wernecke's The Inventor Mentor.

What is in it?

Inventor organizes its data into a scene graph, a structured collection of graphical objects stored as nodes. These nodes can represent many things, from geometric primitives, engines, lights and material properties to transformation nodes that can include scaling, rotation and translation properties.

Inventor efficiently handles many of the graphic operations which would otherwise have to be coded by the user. It has facilities for scene graph management, picking, viewing and user interaction. The standard viewers come in five basic flavors: fly, walk, plane, examiner and render area.

Editors for materials, directional lights, transformations and other node properties can be attached to the scene graph, and changes rendered interactively in any of the standard editor viewers. These editors come in source form, so they can be customized to suit the user's specific needs.

Where did it come from?

Open Inventor was developed by Silicon Graphics (SGI), a company that builds graphical workstations. It is the second version of Iris Inventor, which encapsulated IrisGL, from which OpenGL is derived. The Inventor/GL API is richly featured and has been maturing for about a decade now. It has proven worthy of the time and effort needed to learn it. Although there is a lot to learn, much can easily be done even by a newbie, as will be demonstrated shortly.

Template Graphics Software (TGS) has a source license to Open Inventor and OpenGL. TGS takes care of porting and distributing Inventor to Linux and other UNIX platforms and Windows NT/95, enhancing and enriching it along the way. In September 1997, the first version of Inventor for Linux appeared on the TGS Internet site (see Resources).

What is the VRML connection?

The Inventor file format was chosen as the basis for the VRML version 1.0 file format, commonly distinguished by its .WRL extension. Many of the Inventor nodes were used directly in VRML, and it has often been referred to as “Inventor with all of the good stuff ripped out”. With the new definition of VRML 2.0, the recent releases of Inventor, including the latest Linux version, have been updated to be able to read, write and process VRML files. Actually, I have found the standard Inventor viewers make better VRML model viewers than some of the ones available for Windows and UNIX/Linux. They provide better performance and a better rendering appearance.

What can I do with it?

Inventor is an ideal environment for creating animations, simulations, data visualizations, VR work and CAD.

At Pratt & Whitney, we use Catia on high-end UNIX workstations for our design/manufacturing process in the development of gas turbine engines for aircraft, industrial and marine applications. Catia is a very powerful 3-D CAD/CAM system, and has its own API for querying drawing models to retrieve geometry and other information.

Using this API, an Open Inventor program xmtriag was written to convert Catia solid models to stereolithography (STL) format, ready for input into an STL machine for rapid prototyping. In addition to the STL format, the program generates an Inventor file for us to visually verify that the part was translated correctly.

My company uses this program as a utility in our design process to take a 3-D snapshot of an engine part, convert it to STL format and send it via FTP to a supplier, who then sends us back a quote and, eventually, a part. These parts can be used for design reviews or for casting purposes to make precision molds. This method saves both money and time in the design/review/manufacture cycle and increases quality. In fact, this process has been so successful we are in the process of installing our own in-house rapid prototyping facility.

Besides those in engineering, many others are interested in seeing and querying our drawing database. To avoid tying up Catia licenses, a viewer was developed to extract the 3-D geometry from the model database and convert it to Inventor format. These new Inventor models can be kept as a light version of the parts instead of storing them in Catia. Using an SQL database, we can select parts from a hierarchical drawing tree regardless of the platform being used and view associated engineering and other data of the selected parts. In addition to Catia, this converter/viewer will process Unigraphics and other CAD drawing formats.

As a result, we have not had to buy new Catia licenses or other expensive third-party viewers. The bulk of the graphics development was done by one person, with some ancillary help from database-oriented personnel over a period of six months. Excluding the price of the Inventor licenses, which are free on SGI workstations, the ratio of prices for third-party viewer licenses to this person's salary was over 100 to 1.