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Fonts

Types of Font



------	Composite Fonts -------

	Type0 (Composed of glyphs from a CIDFont)

		Children:
		
		CIDFont 	CIDFontType0	(Type 1 font glyph descriptions)
				CIDFontType2	(TrueType font glyph descriptions)
	
------	Simple Fonts Below -------

	Type 1	Type 1 (defines gylphs using type 1 font technology)
		MMType1 (multiple master font - extends type 1 fonts to support many typefaces for a single font)

	Type 3	(defines glyphs with streams of PDF graphics operations)
	
	TrueType (from the TrueType font format)

Terminology

  • Font dictionary: PDF dictionary with information about the font
  • Font program: Glyph information in specialized font format

Composite Fonts

  • Glyphs are selected from a font-like CIDFont.
  • Has a single CIDFont descendant.
  • Multiple-byte sequences select a single glyph.

Used for multiple-byte character encodings and large numbers of glyphs.

Well suited to Chinese, Japanese and Korean (CJK).

CID stands for character identifier. This is a number used to access glyph descriptions.

The CMap maps between character codes and CID numbers for the glyphs.

A CIDFont file provides the glyph descriptions for a character collection. The glyph descriptions are identified by CIDs.

CID keyed font combines a CMap with a CIDFont.

The Encoding contains the CMap. The DescendantFonts contains the CIDFont to use with the CMap.

CIDFont

A Type0 font descendant (CIDFont) must be either a CIDFontType0 (Adobe Type 1) or CIDFontType2 (TrueType).

For Type 2 CIDFonts (TrueType) the glyphs are identified by a glyph index (GID).

  • If the font program is embedded as a stream the CIDFont dictionary must contain a CIDToGIDMap which maps from CIDs to Glyph Indexes.

  • If the font program is a predefined external font the CIDFont must not contain a CIDToGIDMap. It may only use a predefined CMap.

Though a CID may not be used to select the glyph as in the predefined case, it is always used to select glyph metrics. Every CIDFont must describe CID 0 which is the .notdef character for missing characters.

Glyph Metrics in CIDFonts

Widths for CIDFonts are defined in the DW and W entries in the CIDFont dictionary.

  • DW provides the default width for glyphs which are not specified individually.
  • W defines widths for individual CIDs.

Vertical writing has other stuff, see the spec.

CMap

The CMap maps from character codes to character selectors (CIDs).

The CMap defines the writing mode horizontal or vertical.

Type 0 Fonts

The Font dictionary has the following entries:

  • Type (name): /Font
  • Subtype (name): /Type0
  • BaseFont (name): The PostScript name of the font.
  • Encoding (name/stream R): Name of a predefined CMap or a stream for an embedded CMap.
  • DescendantFonts (array): Single element pointing to the CIDFont.
  • ToUnicode (stream R)?: Stream containing a CMap file to map codes to Unicode.

Simple Fonts

  • Glyphs are selected by single-byte character codes. Index into a 256 entry glyph table.
  • Only supports horizontal writing mode.

Further Description

Type 1 Fonts

The Font program is a PostScript program describing glyph shape. See the Adobe Type 1 Font Format specification.

The Font dictionary has the following entries:

  • Type (name): /Font
  • Subtype (name): /Type1
  • Name (name?): Font name
  • BaseFont (name): The PostScript name of the font. Equivalent to the FontName value in the Font program.
  • FirstChar (int): The first character code in the Widths array.
  • LastChar (int) The last character code in the Widths array.
  • Widths (numeric[] R): An array defining the glyph width in units of 1000 == 1 text space unit.
  • FontDescriptor (Dict<> R): Describes font metrics other than widths.
  • Encoding (name/Dict<> R): Specifies the character encoding if different from default.
  • ToUnicode (stream R): CMap mapping character code to Unicode.