CHAPTER 5
388
Text
5.1 Organization and Use of Fonts
A
character
is an abstract symbol, whereas a
glyph
is a specific graphical render-
ing of a character. For example, the glyphs
A
,
A
, and
A
are renderings of the ab-
stract “A” character. Historically these two terms have often been used
interchangeably in computer typography (as evidenced by the names chosen for
some PDF dictionary keys and PostScript operators), but advances in this area
have made the distinction more meaningful. Consequently, this book distin-
guishes between characters and glyphs, though with some residual names that are
inconsistent.
Glyphs are organized into fonts. A
font
defines glyphs for a particular character
set; for example, the Helvetica and Times fonts define glyphs for a set of standard
Latin characters. A font for use with a PDF consumer application is prepared in
the form of a program. Such a
font program
is written in a special-purpose lan-
guage, such as the
Type 1
or
TrueType
font format, that is understood by a special-
ized font interpreter.
In PDF, the term
font
refers to a
font dictionary,
a PDF object that identifies the
font program and contains additional information about it. There are several dif-
ferent font types, identified by the
Subtype
entry of the font dictionary.
For most font types, the font program is defined in a separate
font file,
which may
be either embedded in a PDF stream object or obtained from an external source.
The font program contains
glyph descriptions
that generate glyphs.
A content stream paints glyphs on the page by specifying a font dictionary and a
string object that is interpreted as a sequence of one or more character codes
identifying glyphs in the font. This operation is called
showing
the text string; the
text strings drawn in this way are called
show strings.
The glyph description con-
sists of a sequence of graphics operators that produce the specific shape for that
character in this font. To render a glyph, the application executes the glyph de-
scription.
Programmers who have experience with scan conversion of general shapes may
be concerned about the amount of computation that this description seems to
imply. However, this is only the abstract behavior of glyph descriptions and font
programs, not how they are implemented. In fact, an efficient implementation
can be achieved through careful caching and reuse of previously rendered glyphs.