SECTION 5.6
433
Composite Fonts
5.6 Composite Fonts
A
composite font,
also called a Type 0 font, is one whose glyphs are obtained from
a fontlike object called a
CIDFont.
A composite font is represented by a font dic-
tionary whose
Subtype
value is
Type0
. The Type 0 font is known as the
root font,
and its associated CIDFont is called its
descendant.
Note:
Composite fonts in PDF are analogous to composite fonts in PostScript but
with some limitations. In particular, PDF requires that the character encoding be
defined by a CMap (described below), which is only one of several encoding methods
available in PostScript.Also, PostScript allows a Type 0 font to have multiple descen-
dants, which might also be Type 0 fonts. PDF supports only a single descendant,
which must be a CIDFont.
When the current font is composite, the text-showing operators behave different-
ly than with simple fonts. For simple fonts, each byte of a string to be shown se-
lects one glyph, whereas for composite fonts, a sequence of one or more bytes can
be decoded to select a glyph from the descendant CIDFont. This facility supports
the use of very large character sets, such as those for the Chinese, Japanese, and
Korean languages. It also simplifies the organization of fonts that have complex
encoding requirements.
This section first introduces the architecture of
CID-keyed fonts,
which are the
only kind of composite font supported in PDF. Then it describes the
CIDFont
and
CMap
dictionaries, which are the PDF objects that represent the correspondingly
named components of a CID-keyed font. Finally, it describes the Type 0 font dic-
tionary, which combines a CIDFont and a CMap to produce a font whose glyphs
can be accessed by means of variable-length character codes in a string to be
shown.
5.6.1 CID-Keyed Fonts Overview
CID-keyed fonts provide a convenient and efficient method for defining
multiple-byte character encodings, fonts with a large number of glyphs, and fonts
that incorporate glyphs obtained from other fonts. These capabilities provide
great flexibility for representing text in writing systems for languages with large
character sets, such as Chinese, Japanese, and Korean (CJK).
The CID-keyed font architecture specifies the external representation of certain
font programs, called
CMap
and
CIDFont
files, along with some conventions for