Polylines and poor quality is what many users see in CGM files. In most cases the wrong version of CGM has been used.

The Computer Graphics Metafile (CGM) standard is defined in ISO/IEC 8632:1992. This standard was first issued in 1987, and it has been amended and extended several times since then. The current version has been released as ISO 8632:1999 in the year 2000. As a result several versions of CGM exist that have a significant impact on the quality that can be achieved when using CGM.

There are 4 versions of CGM: 1, 2, 3, and 4. Sometimes they are referred to as "levels", however, the correct term is versions. All CGM versions are upward compatible, i.e. a version 1 file is also a version 2 file, a version 2 file is also a version 3 file, and a version 3 file is also a version 4 file. The important differences between the CGM versions are described below.

Version 1
This version defines the basic graphic primitives and their attributes. Examples are lines, ellipses, circles, rectangles, and text elements. There is no curve definition (e.g. for Bezier curves), and no efficient raster compression. Use this version only if you have no other choice. If you save a high-quality illustration to a CGM file using Version 1, all curves will be smashed into polylines. It is also very likely that you will lose precision on text elements.

Version 2
This version was a minor revision. It added segments and some other details. Most of these details are not used in practice.

Version 3
With version 3 a lot of new elements were introduced. Important for technical illustrations are the curve definitions, improved text elements, line style definitions, and tiled raster arrays including compression schemes. Today CGM version 3 is the reference version for high-quality illustrations.
Ensure that you use at least version 3 to maintain the best graphical quality.

Version 4
This version does not define any new graphic elements or attributes. It introduced the concept of the Application Structures, which provide a way to store non-graphical information in a CGM file. The standard defines the syntax of these application structures; it does not define the semantics. These are usually defined in a profile.

Application structures are used to identify objects in a CGM file, and to associate additional attributes with them. These may be hotspot regions, names, links, screen tips, or other attributes. Version 4 is the most recommended version of CGM in use today. On top of the graphical quality of a version 3 file it allows for the definition of graphical objects and other non-graphical data that is used to structure the file and make it more versatile.

Conclusions
The difference in quality between a CGM version 1 file and a version 3 or 4 file can be significant. Especially if curves, precision text, and raster portions are used, the file should be stored in CGM version 3 or 4. Once a file has been stored as a version 1 file there is almost no way to regain the quality of a version 3 file. At any time you can derive a version 1 file from a version 3 or 4 file if needed.

Version 4 is also helpful if you want to work in native CGM mode as offered by IsoDraw. It allows for storage of certain settings like pen or color names, or other preferences in the CGM file. This facilitates the editing process a lot.