PDFlib 8 Windows COM/.NET Tutorial

6.4.3 Complex Script Shaping
The shaping process selects appropriate glyph forms depending on whether a character
is located at the start, middle, or end of a word, or in a standalone position. Shaping is a
crucial component of Arabic and Hindi text formatting. Shaping may also replace a sequence
of two or more characters with a suitable ligature. Since the shaping process determines
the appropriate character forms automatically, explicit ligatures and Unicode
presentation forms (e.g. Arabic Presentation Forms-A U+FB50) must not be used as input
characters.
Since complex scripts require multiple different glyph forms per character and additional
rules for selecting and placing these glyphs, shaping for complex scripts does not
work with all kinds of fonts, but requires suitable fonts which contain the necessary information.
Shaping works for TrueType and OpenType fonts which contain the required
feature tables (see »Requirements for shaping«, page 166, for detailed requirements).
Shaping can only be done for characters in the same font because the shaping information
is specific to a particular font. As it doesn’t make sense, for example, to form ligatures
across different fonts, complex script shaping cannot be applied to a word which
contains characters from different fonts.
Override shaping behavior. In some cases users may want to override the default
shaping behavior. PDFlib supports several Unicode formatting characters for this purpose.
For convenience, these formatting characters can also be specified with entity
names (see Table 6.4).
Table 6.4 Unicode control characters for overriding the default shaping behavior
formatting
character entity name Unicode name function
U+200C ZWNJ ZERO WIDTH NON-JOINER prevent the two adjacent characters from
forming a cursive connection
U+200D ZWJ ZERO WIDTH JOINER force the two adjacent characters to form a
cursive connection
6.4.4 Bidirectional Formatting
Cookbook A full code sample can be found in the Cookbook topic complex_scripts/bidi_formatting.
For right-to-left text (especially Arabic and Hebrew, but also some other scripts) it is
very common to have nested sequences of left-to-right Latin text, e.g. an address or a
quote in another language. These mixed sequences of text require bidirectional (Bidi)
formatting. Since numerals are always written from left to right, the Bidi problem affects
even text which is completely written in Arabic or Hebrew. PDFlib implements bidirectional
text reordering according to the Unicode Bidi algorithm as specified in Unicode
Standard Annex #9 1 . Bidi processing does not have to be enabled with an option,
but will automatically be applied as part of the shaping process if text in a right-to-left
script with an appropriate script option is encountered.
1. See www.unicode.org/unicode/reports/tr9/
6.4 Complex Script Output 167