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get igs


Purpose
To convert third-party models in IGES format to files that can be read in GIG and used as GIG models.

Introduction
This converter allows GIG users to import CAD model files (in IGES 5.2 format) directly into GIG. The converted data can be rendered in GIG using ELECTROGIG's proprietary raytracer and then output to transparency, print, or video for high quality presentations. The converted files can also be animated in GIG to produce high quality animations.

Preparation
Some knowledge of GIG, UNIX, and the configuration of your workstation is required before using the IGES file converter. The file name must have the extension .igs (in lower case) and must be moved or copied to the 'intlib' directory of the project chosen by the GIG user.

The intlib directory of the currently active project could also be accessed using the environment variable LD:

Usage
Now a copy of the IGES file has been placed in the correct directory, select the get igs button from the channels menu in the GIG interface. The selection submenu will then be displayed where you can make your selection.

The IGES file will be converted to a GIG model consisting of GIG Nurbs objects, which can be saved and used as a normal GIG model. In particular, it is possible to select the individual surfaces (ie. through get solid tag) and enter the Nurbs modeler by pressing edit nurbs in the edit menu.

Supported IGES 5.2 entities
The following IGES 5.2 entities are recognized by the GIG IGES converter and converted to GIG geometry:

Troubleshooting:

Checking supported entities
The great variety of IGES entities and the various subsets supported by the IGES writing routines in CAD systems are a source of trouble for IGES reading routines. If you suspect that your IGES files are not converted correctly, you can call the converter from the UNIX commandline.

Boundary entities versus trimmed parametric surfaces
If, during conversion, these error messages appear:

then the CAD system that wrote the IGES file chose the wrong option to write trimmed surfaces. In short, there are two ways for IGES to describe trimmed surfaces: through boundary entities and through trimmed (parametric) surfaces. GIG supports the latter only. Luckily, most CAD systems allow you the option to choose either one of these methods.

Note that boundary entities are considered 'untested' in the official IGES 5.2 specifications.

Unexpected appearance of Nurbs models in GIG
Many CAD systems use B-spline or Nurbs entities (118, 120, 122, 128, 142) to write their data to the IGES format. The default representation of Nurbs in the GIG interface is the Nurbs control polygon. This polygon often does not resemble the actual shape of the object. Choosing another display mode will show a better representation of the Nurbs.

In some cases, a Nurbs object will be regarded as 'solid' when it should be 'hollow' or the other way around. For example, a hollow cylinder in a CAD system might appear as a solid cylinder, with end planes, in GIG. In this case you can toggle the solid/hollow box in the solid construct menu for each individual Nurbs object.

Large CAD files
Sometimes a CAD model is too large for the standard configuration of GIG. In these cases, stack overflow error messages can appear during the conversion. Usually, this can be solved by copying a configuration file with larger configuration values to your home directory. This will cause GIG to require more swapspace. The large configuration file should not be necessary for CAD models of less than 10 MB.

Faulty IGES syntax
The IGES file format uses a strict formatting scheme and is rather complicated. Sometimes errors will occur in the IGES files because the CAD system that wrote it did not completely follow the IGES 5.2 specifications, or because file transfer messed up the file lay-out. If error messages appear of the following type:

then it might be that your IGES file is corrupt. Please check it against the IGES 5.2 specificiations or contact GIG.

Viewpoint IGES files
There have been some cases of IGES files created by Viewpoint Labs where there was an error in the systax of AIE (402) entities. The converter will still accept these files if the word 'Viewpoint' appears in the first line of the IGES file.

Some Viewpoint IGES files describe a model with small, triangular, planar Nurbs surfaces for all the polygons that make up the model. Although GIG can convert and render these models, for efficiency it is better to convert the equivalent .obj file with the get obj converter.

Comments
The trim curves that are part of trimmed surfaces in IGES format have a slightly different definition from the trim curves generated in the GIG Nurbs modeler. Sometimes this will cause the Nurbs modeler to complain about invalid trim curves. These warnings can be ignored; the trimmed surfaces will still be rendered as intended.

Known bugs
GIG can handle Nurbs surfaces and trim curves up to (polynomial) degree 10. Entities with a higher degree will cause an error message.

See also
confile