Text in Vector Files

Vector formats that allow the storage of text strings do so in one of two ways. The simplest approach is to store the text as a literal ASCII string along with font, position, color, and attribute information. Although the text is provided in a compact form, this scheme requires the rendering application to have a knowledge of the font to be used, which is always problematic. Because font names are for the most part vendor-controlled, it is sometimes difficult to even specify the font to be drawn. The CGM format addresses this problem through the use of an international registry of font names and associated descriptive data. Any rendering application supporting CGM must have access to this data, or it must use the font metric data supplied in the CGM file's header. Text, however, because it is stored in human-readable format, can be edited.

The second approach, and by far the most flexible, is to store the characters making up the text string as outlines constructed from a series of primitive vector data elements. Under this scheme each creator application must have access to font outline data; because it is stored like any other vector data, font outline data can be scaled at will, rotated, and otherwise manipulated. Until recently, access to outline data has been a problem, but vendors have realized the importance of support for outline fonts and are now routinely supplying this capability at the operating system level.

Because the graphics industry at large and the font industry have grown up in parallel, and only lately have begun to merge, there are naturally some incompatibilities between data storage models. Most fonts, for instance, are stored as a series of splines joined end-to-end, and a particular spline type may not be supported by the file format in use. In this case, the creator application may choose to convert the splines to arcs or lines and store these instead. This may or may not have an effect on the appearance of the text.

Creator applications may even incorporate vector or stroke fonts, which are usually primitive sets of character outlines with an angular or mechanical look, designed to be drawn with a minimum of fuss. Although many vendors have chosen to make their own, one widely available source is the Hershey fonts. Hershey data is available commercially, but is no longer considered adequate for general use.

The use of vector, stroke, or outline fonts usually increases the size of a file dramatically, but this may be offset by an increase in visual quality in the case of spline-based outline fonts. Although there are a number of older font formats still in use, spline-based outline font data in the TrueType and Adobe Type 1 formats is easily available on all the major platforms. There is seldom any need to use stroke fonts now. Unfortunately, the reconstruction of characters from spline outline data is no trivial task, and the higher quality afforded by the ready availability of TrueType and Adobe Type 1 fonts comes at a price in rendering time and program development costs.