(web2c.info)TCX files

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4.4.2 TCX files: Character translations
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TCX (TeX character translation) files help TeX support direct input of
8-bit international characters if fonts containing those characters are
being used.  Specifically, they map an input (keyboard) character code
to the internal TeX character code (a superset of ASCII).

   Of the various proposals for handling more than one input encoding,
TCX files were chosen because they follow Knuth's original ideas for the
use of the 'xchr' and 'xord' tables.  He ventured that these would be
changed in the WEB source in order to adjust the actual version to a
given environment.  It turns out, however, that recompiling the WEB
sources is not as simple a task as Knuth may have imagined; therefore,
TCX files, providing the possibility of changing of the conversion
tables on on-the-fly, have been implemented instead.

   This approach limits the portability of TeX documents, as some
implementations do not support it (or use a different method for
input-internal reencoding).  It may also be problematic to determine the
encoding to use for a TeX document of unknown provenance; in the worst
case, failure to do so correctly may result in subtle errors in the
typeset output.  But we feel the benefits outweigh these disadvantages.

   This is entirely independent of the MLTeX extension (Note: MLTeX):
whereas a TCX file defines how an input keyboard character is mapped to
TeX's internal code, MLTeX defines substitutions for a non-existing
character glyph in a font with a '\accent' construction made out of two
separate character glyphs.  TCX files involve no new primitives; it is
not possible to specify that an input (keyboard) character maps to more
than one character.

   Information on specifying TCX files:

   * The best way to specify a TCX file is to list it explicitly in the
     first line of the main document:
          %& -translate-file=TCXFILE

   * You can also specify a TCX file to be used on a particular TeX run
     with the command-line option '-translate-file=TCXFILE'.

   * TCX files are searched for along the 'WEB2C' path.

   * Initial TeX (Note: Initial TeX.) ignores TCX files.

   The Web2c distribution comes with a number of TCX files.  Two
important ones are 'il1-t1.tcx' and 'il2-t1.tcx', which support ISO
Latin 1 and ISO Latin 2, respectively, with Cork-encoded fonts
(a.k.a. the LaTeX T1 encoding).  TCX files for Czech, Polish, and Slovak
are also provided.

   One other notable TCX file is 'empty.tcx', which is, well, empty.
Its purpose is to reset Web2C's behavior to the default (only visible
ASCII being printable, as described below) when a format was dumped with
another TCX being active--which is in fact the case for everything but
plain TeX in the TeX Live and other distributions.  Thus:

     latex somefile8.tex
     => terminal etc. output with 8-bit chars
     latex --translate-file=empty.tcx somefile8.tex
     => terminal etc. output with ^^ notation

   Syntax of TCX files:
  1. Line-oriented.  Blank lines are ignored.

  2. Whitespace is ignored except as a separator.

  3. Comments start with '%' and continue to the end of the line.

  4. Otherwise, a line consists of one or two character codes,
     optionally followed by 0 or 1.  The last number indicates whether
     DEST is considered printable.
          SRC [DEST [PRNT]]

  5. Each character code may be specified in octal with a leading '0',
     hexadecimal with a leading '0x', or decimal otherwise.  Values must
     be between 0 and 255, inclusive (decimal).

  6. If the DEST code is not specified, it is taken to be the same as
     SRC.

  7. If the same SRC code is specified more than once, it is the last
     definition that counts.

   Finally, here's what happens: when TeX sees an input character with
code SRC, it 1) changes SRC to DEST; and 2) makes the DEST code
"printable", i.e., printed as-is in diagnostics and the log file rather
than in '^^' notation.

   By default, no characters are translated, and character codes between
32 and 126 inclusive (decimal) are printable.

   Specifying translations for the printable ASCII characters (codes
32-127) will yield unpredictable results.  Additionally you shouldn't
make the following characters printable: '^^I' (TAB), '^^J' (line feed),
'^^M' (carriage return), and '^^?' (delete), since TeX uses them in
various ways.

   Thus, the idea is to specify the input (keyboard) character code for
SRC, and the output (font) character code for DEST.

   By default, only the printable ASCII characters are considered
printable by TeX.  If you specify the '-8bit' option, all characters are
considered printable by default.  If you specify both the '-8bit' option
and a TCX file, then the TCX can set specific characters to be
non-printable.

   Both the specified TCX encoding and whether characters are printable
are saved in the dump files (like 'tex.fmt').  So by giving these
options in combination with '-ini', you control the defaults seen by
anyone who uses the resulting dump file.

   When loading a dump, if the '-8bit' option was given, then all
characters become printable by default.

   When loading a dump, if a TCX file was specified, then the TCX data
from the dump is ignored and the data from the file used instead.