PDFlib 8 Windows COM/.NET Tutorial

4 Unicode and Legacy Encodings
This chapter provides basic information about Unicode and other encoding schemes.
Text handling in PDFlib heavily relies on the Unicode standard, but also supports various
legacy and special encodings.
4.1 Important Unicode Concepts
Characters and glyphs. When dealing with text it is important to clearly distinguish
the following concepts:
> Characters are the smallest units which convey information in a language. Common
examples are the letters in the Latin alphabet, Chinese ideographs, and Japanese syllables.
Characters have a meaning: they are semantic entities.
> Glyphs are different graphical variants which represent one or more particular characters.
Glyphs have an appearance: they are representational entities.
There is no one-to-one relationship between characters and glyphs. For example, a ligature
is a single glyph which is represented by two or more separate characters. On the
other hand, a specific glyph may be used to represent different characters depending on
the context (some characters look identical, see Figure 4.1).
BMP and PUA. The following terms will occur frequently in Unicode-based environments:
> The Basic Multilingual Plane (BMP) comprises the code points in the Unicode range
U+0000...U+FFFF. The Unicode standard contains many more code points in the supplementary
planes, i.e. in the range U+10000...U+10FFFF.
> A Private Use Area (PUA) is one of several ranges which are reserved for private use.
PUA code points cannot be used for general interchange since the Unicode standard
does not specify any characters in this range. The Basic Multilingual Plane includes a
PUA in the range U+E000...U+F8FF. Plane fifteen (U+F0000... U+FFFFD) and plane sixteen
(U+100000...U+10FFFD) are completely reserved for private use.
Characters
Glyphs
U+0067 LATIN SMALL LETTER G
U+0066 LATIN SMALL LETTER F +
U+0069 LATIN SMALL LETTER I
U+2126 OHM SIGN or
U+03A9 GREEK CAPITAL LETTER OMEGA
U+2167 ROMAN NUMERAL EIGHT or
U+0056 V U+0049 I U+0049 I U+0049 I
Fig. 4.1
Relationship of glyphs
and characters
4.1 Important Unicode Concepts 99