DLI Implementation and Reference Guide - Datalogics

4.2 DLI Implementation and Reference Guide
PDF Font Overview
Concepts and Facilities: Guide to the DL Pager Composition System
It is important to start by understanding the difference between the terms character
and glyph. A character is an abstract, named entity, such as "hyphen." A glyph is the
representation of a character contained within a font: the mark in the PDF file which
visually represents the character, what you see on-screen or on paper. This is an
important distinction, since some font types use character identifiers to typeset text;
others use glyph identifiers to typeset text. For this chapter, the terms character and
glyph will be used interchangeably unless the technical distinction is important, in
which case this will be noted.
Characters and glyphs in a PDF font can be addressed using either one-byte encodings
(such as WinAnsiEncoding), multibyte encodings (such as Shift-JIS), or 2-byte
glyph identifier (GID) encodings for some TrueType fonts. Some fonts support more
than one method of accessing characters.
There are two categories of fonts in the PDF file format:
• Simple fonts
• Composite fonts
Simple Fonts
Simple fonts are those fonts which are addressed using one byte per glyph or
character. PostScript Type1 fonts and most TrueType fonts fall into this category. With
these fonts, no more than 255 characters are addressable. DLI offers the flexibility to
easily encode which characters are accessible by accepting an encoding array. The
widths of these characters can also be controlled by providing an array of character
widths.
PDF TrueType fonts which have a post table can use encoding vectors to access fonts
outside the Adobe standard one-byte encodings.