Representation of characters

An important concept to understand is how fonts are represented. For the fonts printed by page printers using AFP licensed programs, characters are represented by data describing each dot to be printed (raster fonts) or by mathematical formulas (outline fonts).

Raster fonts

A raster font is created by a sequence of dots, called picture elements (pels), that form a character called a raster pattern. The number of dots per inch that a printer generates is called the print resolution, or density. A resolution of 240 pels means that a printer prints 240 pels per inch both vertically and horizontally, or 57,600 pels per square inch (240 × 240).

The next figure shows two images of different print resolutions. The image with many small dots has more pels per inch and greater print resolution than the image with fewer large dots.

Print resolution examples

The type of printer determines the printed pel density. Because raster fonts can have 240-pel or 300-pel formats, different fonts are available for printers with different resolutions (for example, 240-pel and 300-pel printers).

Outline fonts

Characters in outline fonts are described by mathematical formulas rather than by pels. These formulas are used by rasterizing software to create bitmap characters based on two variables: resolution and point size. This means that a single outline font can offer many print resolutions and point sizes. Hints are also contained in the outline fonts to make sure that typographic characteristics of the typeface are maintained in a consistent manner throughout all printed characters. Some of these characteristics include horizontal and vertical stroke widths, serifs, and curve radii.

TrueType and OpenType fonts are outline fonts that consist of tables for identifying the formatting information used to support Unicode encoding.

Rotation of characters

The ability to print in different directions and with different character rotations is also determined by the type of printer. Print direction shows the direction in which characters are added to a line of text. Character rotation is the clockwise rotation of a character with respect to the character baseline. The character baseline is a reference on which characters are aligned as they are added to the page in the print direction. The character baseline is always parallel to the print direction.

The next table shows how print direction and character rotation can be combined to print in many orientations.

Print direction and character rotation combinations for print orientations