Vector File Size Issues

Not counting palette and attribute information, the size of a vector file is directly proportional to the number of objects it contains. Contrast this with a complex bitmap file, which stays the same size no matter how complex the image described within. The only impact complexity has on bitmap files is on the degree of compression available to the file creator.

Vector files thus can vary greatly in size. A format can store images efficiently by using some form of shorthand notation to allow the compact definition of complex elements. A vector format rich in objects might be able to represent a single complex element using a Bezier curve, for instance. Another format not supporting Bezier curves would need to represent the same curve inefficiently, perhaps using a series of lines. Each line, in this case, would be a separate element, producing a file much larger than one supporting Bezier curves directly.

A format creator was probably addressing the problem of file size when he or she decided to support the creation and naming of complex elements. Great size savings come about when elements are repeated in the image; all that needs to be stored after the original element definition is a pointer to that definition, as well as attribute and position information specific to each individual repeated element.

Size savings may also come about from the way in which a format stores information. Different formats may support identical information in widely varying ways. For example, in the CGM format a hatch pattern is represented as a single object. In the PIC and Autodesk DXF formats, however, each line in the hatch pattern is stored as a separate element.

Because vector data is stored as numbers, it can be scaled, rotated, and otherwise manipulated easily and quickly, at least compared to bitmap data. Also, because scaling is so easy, vector files are not subject to image size limitations in the same way as bitmap files.

Vector formats normally do not support data compression as most bitmap formats do. Some formats, however, support an alternate encoding method that produces smaller data files, but contains the same information. CGM, for instance, normally stores vector information in a clear-text ASCII format that is human-readable, as does the example we presented earlier in this chapter. It also allows the storage of information in a binary format, however, which results in smaller files at the cost of readability and cross-platform portability. The DXF format also has a binary analog called DXB (Data eXchange Binary) which is not only smaller, but faster to load into its parent application (AutoCAD). It is, however, not portable to other applications.

Copyright © 1996, 1994 O'Reilly & Associates, Inc. All Rights Reserved.

Hosted by uCoz