Chapter 5. Metafiles

Metafiles can contain both bitmap and vector data.

Contents:
Platform Independence?
How Metafiles Are Organized
Pros and Cons of Metafiles

When the term metafile first appeared, it was used in discussions of device- and machine-independent interchange formats. In the mid-1970s, the National Center for Atmospheric Research (NCAR), along with several other research institutions, reportedly used a format called metacode, which was device- and platform-independent to a certain degree. What is known for certain is that in 1979, the SIGGRAPH Graphics Standards and Planning Committee used the term, referring to a part of their published standards recommendations. These early attempts at defining device- and platform-independent formats mainly concerned themselves with vector data. Although work has continued along this line, we will refer to formats that can accommodate both bitmap and vector data as metafiles, because for all practical purposes the interchange formats in widespread use in the marketplace handle both types of data.

Although metafile formats may be used to store only bitmap or only vector information, it is more likely that they will contain both types of data. From a programmer's point of view, bitmap and vector data are two very different problems. Because of this, supporting both bitmap and vector data types adds to the complexity of a format. Thus, programmers find themselves avoiding the use of metafile formats unless the added complexity is warranted--either because they need to support multiple data types or for external reasons.

The simplest metafiles resemble vector format files. Historically, limitations of vector formats were exceeded when the data that needed to be stored became complex and diverse. Vector formats were extended conceptually, allowing the definition of vector data elements in terms of a language or grammar, and also by allowing the storage of bitmap data. In a certain sense, the resulting formats went beyond the capabilities of both bitmap and vector formats--hence the term metafile.

Platform Independence?

Metafiles are widely used to transport bitmap or vector data between hardware platforms. The character-oriented nature of ASCII metafiles, in particular, eliminates problems due to byte ordering. It also eliminates problems encountered when transferring files across networks where the eighth bit of each byte is stripped off, which can leave binary files damaged beyond repair. Also, because a metafile supports both bitmap and vector data, an application designer can kill two birds with one stone by providing support for one metafile rather than two separate bitmap and vector formats.

Metafiles are also used to transfer image data between software platforms. A creator application, for instance, can save an image in both bitmap and vector form in a metafile. This file may then be read by any bitmap-capable or vector-capable application supporting the particular metafile format. Many desktop publishing programs, for instance, can manipulate and print vector data, but are unable to display that same data on the screen. To accommodate this limitation, a bitmap representation of the image is often included along with the vector data in a metafile. The application can read the bitmap representation of the image from the metafile, which serves as a reduced-quality visual representation of the image that will eventually appear on the printed page. When the page is actually printed, however, the vector data from the metafile is used to produce the image on the printer. Display PostScript files are an example of this type of arrangement.