Outline fonts or vector fonts are collections of vector images, i.e. a set of lines and curves to define the border of glyphs. Early vector fonts were used by vector monitors and vector plotters using their own internal fonts, usually with thin single strokes instead of thick outlined glyphs.
The advent of desktop publishing brought the need for a universal standard to integrate the graphical user interface of the first Macintosh and laser printers. The term to describe the integration technology was WYSIWYG (What You See Is What You Get). The universal standard was (and still is) Adobe PostScript. Examples are PostScript Type 1 and Type 3 fonts, TrueType and Open Type.
The primary advantage of outline fonts is that they can be easily transformed by applying a mathematical function to each vector point, scaling them without causing pixellation. Outline font characters can be scaled to any size and otherwise transformed with more attractive results than bitmap fonts, but requires considerably more processing and may yield undesirable rendering, depending on the font, rendering software, and output size.
Outline fonts have a major problem, in that Bezier curves cannot be rendered accurately onto a raster display (such as most computer monitors and printers), and their rendering can change shape depending on the desired size and position. Measures such as font hinting have to be used to reduce the visual impact of this problem, which require sophisticated software that is difficult to implement correctly.
Many modern desktop computer systems include software to do this, but they use considerably more processing power than bitmap fonts, and there can be minor rendering defects, particularly at small font sizes. Despite this, they are frequently used because people often consider the processing time and defects to be acceptable when compared to the ability to scale fonts freely.