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CHAPTER 5                                                                       Text



combining and using those files. As mentioned earlier, PDF does not support the
entire CID-keyed font architecture, which is independent of PDF; CID-keyed
fonts can be used in other environments. For complete documentation on the ar-
chitecture and the file formats, see Adobe Technical Notes #5092, CID-Keyed
Font Technology Overview, and #5014, Adobe CMap and CIDFont Files Specifica-
tion. This section describes only the PDF objects that represent these font pro-
grams.

The term CID-keyed font reflects the fact that CID (character identifier) numbers
are used to index and access the glyph descriptions in the font. This method is
more efficient for large fonts than the method of accessing by character name, as
is used for some simple fonts. CIDs range from 0 to a maximum value that is sub-
ject to an implementation limit (see Table C.1 on page 992).

A character collection is an ordered set of all glyphs needed to support one or
more popular character sets for a particular language. The order of the glyphs in
the character collection determines the CID number for each glyph. Each CID-
keyed font must explicitly reference the character collection on which its CID
numbers are based; see Section 5.6.2, “CIDSystemInfo Dictionaries.”

A CMap (character map) file specifies the correspondence between character
codes and the CID numbers used to identify glyphs. It is equivalent to the con-
cept of an encoding in simple fonts. Whereas a simple font allows a maximum of
256 glyphs to be encoded and accessible at one time, a CMap can describe a map-
ping from multiple-byte codes to thousands of glyphs in a large CID-keyed font.
For example, it can describe Shift-JIS, one of several widely used encodings for
Japanese.

A CMap can reference an entire character collection, a subset, or multiple charac-
ter collections. It can also reference characters in other fonts by character code or
character name. The CMap mapping yields a font number (which in PDF is al-
ways 0) and a character selector (which in PDF is always a CID). Furthermore, a
CMap can incorporate another CMap by reference, without having to duplicate it.
These features enable character collections to be combined or supplemented and
make all the constituent characters accessible to text-showing operations through
a single encoding.

A CIDFont file contains the glyph descriptions for a character collection. The
glyph descriptions themselves are typically in a format similar to those used in
simple fonts, such as Type 1. However, they are identified by CIDs rather than by
names, and they are organized differently.

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