DLI Implementation and Reference Guide - Datalogics

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4.2 DLI Implementation and Reference Guide PDF Font Overview It is important to start by understanding the difference between the terms character and glyph. A character is an abstract, named entity, such as "hyphen." A glyph is the representation of a character contained within a font: the mark in the PDF file which visually represents the character, what you see onscreen or on paper. This is an important distinction, since some font types use character identifiers to typeset text; others use glyph identifiers to typeset text. For this chapter, the terms character and glyph will be used interchangeably unless the technical distinction is important, in which case this will be noted. Characters and glyphs in a PDF font can be addressed using either one-byte encodings (such as WinAnsiEncoding), multibyte encodings (such as Shift-JIS), or 2-byte glyph identifier (GID) encodings for some TrueType fonts. Some fonts support more than one method of accessing characters. There are two categories of fonts in the PDF file format: • Simple fonts • Composite fonts Simple Fonts Simple fonts are those fonts which are addressed using one byte per glyph or character. PostScript Type1 fonts and most TrueType fonts fall into this category. With these fonts, no more than 255 characters are addressable. DLI offers the flexibility to easily encode which characters are accessible by accepting an encoding array. The widths of these characters can also be controlled by providing an array of character widths. PDF TrueType fonts which have a post table can use encoding vectors to access fonts outside the Adobe standard one-byte encodings. Composite Fonts Composite fonts are fonts which use multibyte encodings or GID values. These are known as CID fonts (for "Character ID") or Type0 fonts, even though many are not accessed using a CID value. There are two varieties of Type0 fonts: • CIDType0 • CIDType2
Fonts 4.3 CIDType0 Fonts CIDType0 fonts use the CMap file format as specified by Adobe, and will accept a number of different encodings, though these CMap files are typically somewhat narrow in their range of addressable characters. A CIDType0 font is usable with a set of CMap files; this is the only way to access characters. CIDType2 Fonts CIDType2 fonts are also TrueType font streams, but are addressed using 2-byte glyph identifiers (glyph IDs). These should not be created using CMaps, since the fonts are not keyed by character but by glyph. Instead, a CIDToGID map is used to convert the 2-byte character in the PDF text strings to glyph IDs for the font. For font files loaded into a DLPDFINSTANCE, DLI uses the International Components for Unicode (ICU) to translate Unicode inputs and a large array of vendor-defined multibyte encodings to output text. The DLPDFTEXT structure is used to properly combine characters and support right-to-left reading order. For CIDType2 fonts, a mapping from the DLPDFTEXT input to glyph IDs for output is generated. Whenever possible, the output text in the PDF file is in UTF16 big-endian format. The Adobe PDF Library and DLI also allow CIDType2 fonts to be used for a number of single-byte vendor encodings. Output is in two-byte glyph IDs which correspond to glyph identifiers in the embedded font stream. Text is input using the DLPDFCONTENT text calls as if using a single-byte font. Most TrueType and OpenType font files can be used to create either TrueType (single-byte) or CIDType2 PDF fonts. In all cases, TrueType and CIDType2 fonts should be both embedded and subset to ensure reliable results when viewing and printing. In this chapter, the term "TrueType font" means a PDF TrueType font using a one-byte encoding, and the term "CIDType2 font" means a TrueType or OpenType font using GID values to access characters in the PDF file. Structure of a DLI Font A font is reflected in the structure DLPDFFONT. These structures are created by the font creation calls on a per-document basis. They may be used only within the document they were created with. Fonts are tracked within the document, and destroyed when the document is destroyed. Pointers to fonts are invalid after the destruction of their document. All DLPDFFONT structures for a given document will be released when that document is destroyed. Therefore, methods that use DLPDFFONT as a parameter (i.e. dlpdfcontenttext, dlpdfcontentwidetext, dlpdfcontenttextwidth, dlpdfcontentwidetextwidth) must be called before the document is destroyed
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