IGES is an acronym for the Initial Graphics Exchange Specification, a computer graphics and geometry exachange standard started by the American National Standards Institute in 1979. It is by far the most widely used CAD model exchange specification. IGES is under control of the NCGA (National Computer Graphics Association) and is part of the U.S. Product Data Association (USPRO) and the IGES/PDES Organisation (IGO).

The aim of IGES is a good one: document a detailed format for all possible geometric entities. By doing so, all CAD/CAM products would be able to perfectly share data created on other platforms and software. In pursuit of this goal, IGES has undergone constant improvement and revision. It has currently grown to version 5.3 (approved by ANSI, September 23, 1996), with 621 pages of mathematical detail on the formats to use in reading and writing IGES computer files. Any CAD user should be able to read an IGES file and begin using the intended parts, with only manufacturing related additional CAD work.

Unfortunately, IGES sucks. Here is why:

The IGES specification is just that, a specification. It can be interpreted differently by the CAD/CAM coder and as such contains properties that are difficuly to understand and implement consistently. Over the years this has led to many companies implementation of the IGES format differing to a great extent, rendering many of them incompatible.

Modern CAD systems have a large number of different ways they can write IGES data. These choices can make the resulting IGES file better or worse for its intended reader, depending on compatibility issues that are often poorly understood. For example, a Catia user can export analytic surfaces such as cones and planes, or change them into spline surfaces before exporting. Some CAM systems would prefer the first format, others the second.

The CAD realm is based on parametric surface mathematics, including Bezier, B-Spline and NURB (Non-Uniform Rational B-spline) surfaces. All surface calculations are performed to within the specific CAD system's accuracy. The IGES problem this creates is when IGES files are moved between two CAD/CAM products using different accuracies. Moving a coarse toleranced IGES file to a fine toleranced system produces curves that don't close and surfaces that have gaps and overlaps. Moving a fine toleranced IGES to a coarse toleranced system loses detail for the opposite reason.

The ugly part of IGES is that the most advanced CAD technology today, solid modeling, is even more sensitive to IGES data problems, than the old-style surface modeling was. The goal of a solid modeler is to produce a part definition as a mathematically precise, valid solid. This means no gaps, cracks, or overlaps of surfaces. It means the surfaces themselves must not self-intersect or fold on themselves. There are a lot of IGES surface files out there that are not quite ready for prime time as input to a solid modeler.

All of these problems can be eliminated by working from original solid model data formats. For example, Unigraphics and SolidWorks, can use the Parasolid solid model format in modeling engine without translation. Another popular solid modeling format among CAD systems, is the SAT format from the ACIS solid modeling engine. Many products can also exchange SAT models directly, without using IGES. There are hundreds of CAD systems sharing either the Parasolid or SAT solid formats. As more CAM systems become solids-based, more programmers will benefit from the advantages of working directly from the original solid part data.

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