Text and Fonts - PDF Explained - page 85

1. Various details from the font file are extracted—a process that varies depending
upon the font format in question. These details (metrics, encodings etc.) are used
to fill out a font dictionary, the font metrics, and the font encoding dictionary.
2. These details can now be stripped from the font file in question, if that is allowed
by the font format, leaving just the glyph descriptions—all this information is now
in the font dictionary. This reduces the size of the embedded font.
3. The font may be subsetted , removing whole glyph descriptions, reducing the font
file to one which holds only the characters which are actually used. For example,
a font only used for the title of a document may only actually use ten characters.
Depending on the font format, the encoding may have to be altered to place all
these characters in the first few character positions in the font so they are numbered
1,2,3…. Subset fonts may be identified by a prefix formed of six uppercase letters
followed by a + , such as RTFGRF+ . This unique code is generated when the subset is
created to allow different subsets to be distinguished from one another.
An example of an embedded font is given in Example 6-2 .
Example 6-2. An embedded font, including encoding and font descriptor
9 0 obj
<</Type /Font
/Subtype /TrueType It's a TrueType font
/BaseFont /GCCBBY+TT8Et00 Font is TT8Et00. GCCBBY+ prefix identifies as a subset font.
/FontDescriptor 8 0 R
/FirstChar 1 There are 41 characters in this font .
/LastChar 41
/Widths
[603 603 603 603 603 603 603 603 603 603 603 603 603 603 The widths. It's a fixed-width font.
603 603 603 603 603 603 603 603 603 603 603 603 603 603
603 603 603 603 603 603 603 603 603 603 603 603 603]
/Encoding 14 0 R
>>
14 0 obj The font encoding.
<< /Type /Encoding
/BaseEncoding /WinAnsiEncoding The base encoding
/Differences The changes. In this case, it's a subset font with the characters at position 1 onward.
[1 /w /i /d /g /e /t /s /T /h /space /r /u /l /a /x /bracketleft
/underscore /J /o /n /S /m /quotesingle /A /p /c /bracketright
/one /colon /braceleft /b /k /braceright /v /period /parenleft
/two /parenright /asterisk /y /P]
>>
endobj
8 0 obj The font descriptor, giving the remaining metrics .
<< /Type /FontDescriptor
/FontName /GCCBBY+TT8Et00
/FontBBox [0 -205 602 770]
/Flags 4
/Ascent 770
/CapHeight 770