dvips: PostScript typesetting
6.1.6 How PostScript typesets a character
-----------------------------------------
The output of Dvips is a program in the PostScript language that
instructs your (presumably PostScript-capable) printer how to typeset
your document by transforming it into toner on paper. Your printer, in
turn, contains a PostScript interpreter that carries out the
instructions in this typesetting program.
The program must include the definition of any PostScript fonts that
you use in your document. Fonts built into your printer (probably the
standard 35: 'Times-Roman', 'ZapfDingbats', ...) are defined within the
interpreter itself. Other fonts must be downloaded as pfa or pfb files
(⇒Glyph files) from your host (the computer on which you're
running Dvips).
You may be wondering exactly how a PostScript interpreter figures out
what character to typeset, with this mass of metrics, glyphs, encodings,
and other information. (If you're not wondering, skip this section ...)
The basic PostScript operator for imaging characters is 'show'.
Suppose you've asked TeX to typeset an 'S'. This will eventually wind up
in the Dvips output as the equivalent of this PostScript operation:
(S) show
Here is how PostScript typesets the 'S':
1. PostScript interpreters use ASCII; therefore 'S' is represented as
the integer 83. (Any of the 256 possible characters representable
in a standard 8-bit byte can be typeset.)
2. A PostScript "dictionary" is a mapping of names to arbitrary
values. A font, to the interpreter, is a dictionary which contains
entries for certain names. (If these entries are missing, the
interpreter refuses to do anything with that font.)
PostScript has a notion of "the current font"--whatever font is
currently being typeset in.
3. One of the mandatory entries in a font dictionary is 'Encoding',
which defines the encoding vector (⇒Encodings) for that
font. This vector of 256 names maps each possible input character
to a name.
4. The interpreter retrieves the entry at position 83 of the encoding
vector. This value is a PostScript name: '/S'.
5. For Type 1 fonts (we're not going to discuss anything else), the
interpreter now looks up '/S' as a key in a dictionary named
'CharStrings', another mandatory entry in a font dictionary.
6. The value of 'S' in 'CharStrings' is the equivalent of a series of
standard PostScript commands like 'curveto', 'lineto', 'fill', and
so on. These commands are executed to draw the character. There
can also be "hint information" that helps adapt the character to
low-resolution rasters. (⇒Glyph files.) The commands are
actually represented in a more compact way than standard PostScript
source; see the Type 1 book for details.
This method for typesetting characters is used in both Level 1 and
Level 2 PostScript. See the PostScript reference manuals for more
information.
Info Catalog
dvips: Bitmap font encodings
dvips: Font concepts