PdfASymbolicFontEncodingStrategy

PdfASymbolicFontEncodingStrategy class

This class describes rules which can be used to tune process of copy encoding data for cases
when TrueType symbolic font has more than one encoding.
Some PDF documents after conversion into PDF/A format could have error
“More than one encoding in symbolic TrueType font’s cmap”.
What is a reason of this error? All TrueType symbolic fonts have special table “cmap”
in it’s internal data. This table maps character codes to glyph indices.
And this table could contain different encoding subtables which
describe encodings used. See advanced info about cmap tables at
https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6cmap.html.
Usually cmap table contains several encoding subtables, but PDF/A standard requires
that only one encoding subtable (3,0) must be leaved for this font in PDF/A document.
And key question here - what data must be taken from another subtables to copy into
destination encoding table (3,0)? Majority of fonts have ‘well-formed’ cmap tables where
every encoding subtable is fully consistent with another subtable. But some fonts
have cmap tables with collisions - where for example one subtable has glyph index
100 for unicode 100, but another subtable has glyph index 200 for the same unicode 100.
To solve this problems special strategy needed.
By default following strategy used:
mac subtable(1,0) is looked for. If this table is found, only this data used to fill destination
table (3,0). If mac subtable is not found then all subtables except (3,0) are iterated
and used to copy data into destination (3,0) subtable. Also mapping for every unicode(unicode, glyph index)
is copied into destination table only if destination table has no this unicode at current moment.
So, for example if first subtabe has glyph index 100 for unicode 100, and next subtable has glyph
index 200 for the same unicode 100, only data from first subtable (unicode=100, glyph index = 100) will be copied.
So each previous subtable takes precedence over the next.
Properties of this class PdfASymbolicFontEncodingStrategy help tune default behaviour.
If property preferred_cmap_encoding_table of type CMapEncodingTableType
is set, then relevant subtable will be used in precedence to mac subtable(1,0). Value ‘MacTable’ from
enumeration CMapEncodingTableType has no sense in this case, cause it
points on the same mac subtable (1,0) which will be used by default.
Property None discards all priorities for any subtable.
If this property is set, then only subtables from declared queue will be used in specified order.
If subtables specified are not found then default iteration of all subtables and copy strategy described above
will be used.
Object QueueItem specifies encoding subtable used. This subtable can be set
via combination of members(PlatformID, PlatformSpecificId) or via CMapEncodingTableType
enumeration.

The PdfASymbolicFontEncodingStrategy type exposes the following members:

Constructors

Name	Description
PdfASymbolicFontEncodingStrategy()	Constructor. Sets default subtable (mac 1,0)
PdfASymbolicFontEncodingStrategy(preferred_encoding_table)	Initializes a new instance of the PdfASymbolicFontEncodingStrategy class

Properties

Name	Description
preferred_cmap_encoding_table	Specifies subtable which will be used in precedence to mac subtable(1,0). Value ‘MacTable’ from enumeration CMapEncodingTableType has no sense in this case.

PdfASymbolicFontEncodingStrategy