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(dvips.info)PostScript typesetting

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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
(Note: 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 (Note: 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.  (Note: 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.