Table of Contents
*****************
Liblouis User's and Programmer's Manual
1 Introduction
2 Test Programs
2.1 lou_debug
2.2 lou_checktable
2.3 lou_allround
2.4 lou_translate
2.5 lou_checkhyphens
3 How to Write Translation Tables
3.1 Hyphenation Tables
3.2 Character-Definition Opcodes
3.3 Braille Indicator Opcodes
3.4 Emphasis Opcodes
3.5 Special Symbol Opcodes
3.6 Special Processing Opcodes
3.7 Translation Opcodes
3.8 Character-Class Opcodes
3.9 Swap Opcodes
3.10 The Context and Multipass Opcodes
3.11 The correct Opcode
3.12 Miscellaneous Opcodes
3.13 Deprecated Opcodes
4 Notes on Back-Translation
5 Programming with liblouis
5.1 License
5.2 Overview
5.3 Data structure of liblouis tables
5.4 lou_version
5.5 lou_translateString
5.6 lou_translate
5.7 lou_backTranslateString
5.8 lou_backTranslate
5.9 lou_hyphenate
5.10 lou_compileString
5.11 lou_dotsToChar
5.12 lou_charToDots
5.13 lou_logFile
5.14 lou_logPrint
5.15 lou_logEnd
5.16 lou_setDataPath
5.17 lou_getDataPath
5.18 lou_getTable
5.19 lou_readCharFromFile
5.20 lou_free
5.21 Python bindings
Opcode Index
Function Index
Program Index
Liblouis User's and Programmer's Manual
***************************************
This manual is for liblouis (version 2.3.0, 9 May 2011), a Braille
Translation and Back-Translation Library derived from the Linux screen
reader BRLTTY.
Copyright (C) 1999-2006 by the BRLTTY Team.
Copyright (C) 2004-2007 ViewPlus Technologies, Inc. `www.viewplus.com'.
Copyright (C) 2007,2009 Abilitiessoft, Inc. `www.abilitiessoft.com'.
This file is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser (or library) General Public
License (LGPL) as published by the Free Software Foundation;
either version 3, or (at your option) any later version.
This file is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser (or Library) General Public License LGPL for more details.
You should have received a copy of the GNU Lesser (or Library)
General Public License (LGPL) along with this program; see the
file COPYING. If not, write to the Free Software Foundation, 51
Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
1 Introduction
**************
Liblouis is an open-source braille translator and back-translator
derived from the translation routines in the BRLTTY screen reader for
Linux. It has, however, gone far beyond these routines. It is named in
honor of Louis Braille. In Linux and Mac OSX it is a shared library,
and in Windows it is a DLL. For installation instructions see the
README file. Please report bugs and oddities to the maintainer,
<john.boyer@abilitiessoft.com>
This documentation is derived from Chapter 7 of the BRLTTY manual,
but it has been extensively rewritten to cover new features.
Please read the following copyright and warranty information. Note
that this information also applies to all source code, tables and other
files in this distribution of liblouis. It applies similarly to the
sister library liblouisxml.
This file is maintained by John J. Boyer
<john.boyer@abilitiessoft.com>.
Persons who wish to program with liblouis but will not be writing
translation tables may want to skip ahead to *note Programming with
liblouis::.
2 Test Programs
***************
Five test programs are provided as part of the liblouis package. They
are intended for testing liblouis and for debugging tables. None of
them is suitable for braille transcription. An application that can be
used for transcription is `xml2brl', which is part of the liblouisxml
package (*note Introduction: (liblouisxml)Top.). The source code of the
test programs can be studied to learn how to use the liblouis library
and they can be used to perform the following functions.
All of these programs recognize the `--help' and `--version' options.
`--help'
`-h'
Print a usage message listing all available options, then exit
successfully.
`--version'
`-v'
Print the version number, then exit successfully.
2.1 lou_debug
=============
The lou_debug tool is intended for debugging liblouis translation
tables. The command line for lou_debug is:
lou_debug [OPTIONS] TABLE[,TABLE,...]
The command line options that are accepted by lou_debug are described
in *note common options::.
The table (or comma-separated list of tables) is compiled. If no
errors are found a brief command summary is printed, then the prompt
`Command:'. You can then input one of the command letters and get
output, as described below.
Most of the commands print information in the various arrays of
`TranslationTableHeader'. Since these arrays are pointers to chains of
hashed items, the commands first print the hash number, then the first
item, then the next item chained to it, and so on. After each item
there is a prompt indicated by `=>'. You can then press enter (`<RET>')
to see the next item in the chain or the first item in the next chain.
Or you can press `h' (for next-(h)ash) to skip to the next hash chain.
You can also press `e' to exit the command and go back to the
`command:' prompt.
`h'
Brings up a screen of somewhat more extensive help.
`f'
Display the first forward-translation rule in the first non-empty
hash bucket. The number of the bucket is displayed at the
beginning of the chain. Each rule is identified by the word
`Rule:'. The fields are displayed by phrases consisting of the
name of the field, an equal sign, and its value. The before and
after fields are displayed only if they are nonzero. Special
opcodes such as the `correct' opcode (*note correct: correct
opcode.) and the multipass opcodes are shown with the code that
instructs the virtual machine that interprets them. If you want to
see only the rules for a particular character string you can type
`p' at the `command:' prompt. This will take you to the
`particular:' prompt, where you can press `f' and then type in the
string. The whole hash chain containing the string will be
displayed.
`b'
Display back-translation rules. This display is very similar to
that of forward translation rules except that the dot pattern is
displayed before the character string.
`c'
Display character definitions, again within their hash chains.
`d'
Displays single-cell dot definitions. If a character-definition
opcode gives a multi-cell dot pattern, it is displayed among the
back-translation rules.
`C'
Display the character-to-dots map. This is set up by the
character-definition opcodes and can also be influenced by the
`display' opcode (*note display: display opcode.).
`D'
Display the dot to character map, which shows which single-cell dot
patterns map to which characters.
`z'
Show the multi-cell dot patterns which have been assigned to the
characters from 0 to 255 to comply with computer braille codes
such as a 6-dot code. Note that the character-definition opcodes
should use 8-dot computer braille.
`p'
Bring up a secondary (`particular:') prompt from which you can
examine particular character strings, dot patterns, etc. The
commands (given in its own command summary) are very similar to
those of the main `command:' prompt, but you can type a character
string or dot pattern. They include `h', `f', `b', `c', `d', `C',
`D', `z' and `x' (to exit this prompt), but not `p', `i' and `m'.
`i'
Show braille indicators. This shows the dot patterns for various
opcodes such as the `capsign' opcode (*note capsign: capsign
opcode.) and the `numsign' opcode (*note numsign: numsign opcode.).
It also shows emphasis dot patterns, such as those for the
`italword', the `firstletterbold' opcode (*note firstletterbold:
firstletterbold opcode.), etc. If a given opcode has not been used
nothing is printed for it.
`m'
Display various miscellaneous information about the table, such as
the number of passes, whether certain opcodes have been used, and
whether there is a hyphenation table.
`q'
Exit the program.
2.2 lou_checktable
==================
To use this program type the following:
lou_checktable [OPTIONS] TABLE
Aside from the standard options (*note common options::)
lou_checktable also accepts the following options:
`--quiet'
`-q'
Do not write to standard error if there are no errors.
If the table contains errors, appropriate messages will be displayed.
If there are no errors the message `no errors found.' will be shown.
2.3 lou_allround
================
This program tests every capability of the liblouis library. It is
completely interactive. Invoke it as follows:
lou_allround [OPTIONS]
The command line options that are accepted by lou_debug are described
in *note common options::.
You will see a few lines telling you how to use the program. Pressing
one of the letters in parentheses and then enter will take you to a
message asking for more information or for the answer to a yes/no
question. Typing the letter `r' and then <RET> will take you to a
screen where you can enter a line to be processed by the library and
then view the results.
2.4 lou_translate
=================
This program translates whatever is on the standard input unit and
prints it on the standard output unit. It is intended for large-scale
testing of the accuracy of translation and back-translation. The
command line for lou_translate is:
lou_translate [OPTION] TABLE
Aside from the standard options (*note common options::) this program
also accepts the following options:
`--forward'
`-f'
Do a forward translation.
`--backward'
`-b'
Do a backward translation.
To use it to translate or back-translate a file use a line like
lou_translate --forward en-us-g2.ctb <liblouis.txt >testtrans
2.5 lou_checkhyphens
====================
This program checks the accuracy of hyphenation in Braille translation
for both translated and untranslated words. It is completely
interactive. Invoke it as follows:
lou_checkhyphens [OPTIONS]
The command line options that are accepted by lou_checkhyphens are
described in *note common options::.
You will see a few lines telling you how to use the program.
3 How to Write Translation Tables
*********************************
Many translation (contraction) tables have already been made up. They
are included in this distribution in the tables directory and should be
studied as part of the documentation. The most helpful (and normative)
are listed in the following table:
`chardefs.cti'
Character definitions for U.S. tables
`compress.ctb'
Remove excessive whitespace
`en-us-g1.ctb'
Uncontracted American English
`en-us-g2.ctb'
Contracted or Grade 2 American English
`en-us-brf.dis'
Make liblouis output conform to BRF standard
`en-us-comp8.ctb'
8-dot computer braille for use in coding examples
`en-us-comp6.ctb'
6-dot computer braille
`nemeth.ctb'
Nemeth Code translation for use with liblouisxml
`nemeth_edit.ctb'
Fixes errors at the boundaries of math and text
The names used for files containing translation tables are completely
arbitrary. They are not interpreted in any way by the translator.
Contraction tables may be 8-bit ASCII files, 16-bit big-endian Unicode
files or 16-bit little-endian Unicode files. Blank lines are ignored.
Any leading and trailing whitespace (any number of blanks and/or tabs)
is ignored. Lines which begin with a number sign or hatch mark (`#')
are ignored, i.e. they are comments. If the number sign is not the
first non-blank character in the line, it is treated as an ordinary
character. If the first non-blank character is less-than (`<') the line
is also treated as a comment. This makes it possible to mark up tables
as xhtml documents. Lines which are not blank or comments define table
entries. The general format of a table entry is:
opcode operands comments
Table entries may not be split between lines. The opcode is a
mnemonic that specifies what the entry does. The operands may be
character sequences, braille dot patterns or occasionally something
else. They are described for each opcode, please *note Opcode Index::.
With some exceptions, opcodes expect a certain number of operands. Any
text on the line after the last operand is ignored, and may be a
comment. A few opcodes accept a variable number of operands. In this
case a number sign begins a comment unless it is preceded by a backslash
(`\').
Here are some examples of table entries.
# This is a comment.
always world 456-2456 A word and the dot pattern of its contraction
Most opcodes have both a "characters" operand and a "dots" operand,
though some have only one and a few have other types.
The characters operand consists of any combination of characters and
escape sequences proceeded and followed by whitespace. Escape sequences
are used to represent difficult characters. They begin with a backslash
(`\`). They are:
`\'
backslash
`\f'
form feed
`\n'
new line
`\r'
carriage return
`\s'
blank (space)
`\t'
horizontal tab
`\v'
vertical tab
`\e'
"escape" character (hex 1b, dec 27)
`\xhhhh'
4-digit hexadecimal value of a character
If liblouis has been compiled for 32-bit Unicode the following are
also recognized.
`\yhhhhh'
5-digit (20 bit) character
`\zhhhhhhhh'
Full 32-bit value.
The dots operand is a braille dot pattern. The real braille dots, 1
through 8, must be specified with their standard numbers. liblouis
recognizes "virtual dots," which are used for special purposes, such as
distinguishing accent marks. There are seven virtual dots. They are
specified by the number 9 and the letters `a' through `f'. For a
multi-cell dot pattern, the cell specifications must be separated from
one another by a dash (`-'). For example, the contraction for the
English word `lord' (the letter `l' preceded by dot 5) would be
specified as 5-123. A space may be specified with the special dot
number 0.
An opcode which is helpful in writing translation tables is
`include'. Its format is:
include filename
It reads the file indicated by `filename' and incorporates or
includes its entries into the table. Included files can include other
files, which can include other files, etc. For an example, see what
files are included by the entry `include en-us-g1.ctb' in the table
`en-us-g2.ctb'. If the included file is not in the same directory as
the main table, use a full path name for filename. Tables can also be
specified in a table list, in which the table names are separated by
commas and given as a single table name in calls to the translation
functions.
The order of the various types of opcodes or table entries is
important. Character-definition opcodes should come first. However, if
the optional `display' opcode (*note display: display opcode.) is used
it should precede character-definition opcodes. Braille-indicator
opcodes should come next. Translation opcodes should follow. The
`context' opcode (*note context: context opcode.) is a translation
opcode, even though it is considered along with the multipass opcodes.
These latter should follow the translation opcodes. The `correct'
opcode (*note correct: correct opcode.) can be used anywhere after the
character-definition opcodes, but it is probably a good idea to group
all `correct' opcodes together. The `include' opcode (*note include:
include opcode.) can be used anywhere, but the order of entries in the
combined table must conform to the order given above. Within each type
of opcode, the order of entries is generally unimportant. Thus the
translation entries can be grouped alphabetically or in any other order
that is convenient. Hyphenation tables may be specified either with an
`include' opcode or as part of a table list. They should come after
everything else. Character-definition opcodes are necessary for
hyphenation tables to work.
3.1 Hyphenation Tables
======================
Hyphenation tables are necessary to make opcodes such as the `nocross'
opcode (*note nocross: nocross opcode.) function properly. There are no
opcodes for hyphenation table entries because these tables have a
special format. Therefore, they cannot be specified as part of an
ordinary table. Rather, they must be included using the `include'
opcode (*note include: include opcode.) or as part of a table list. The
liblouis hyphenation algorithm was adopted from the one used by
OpenOffice. Note that Hyphenation tables must follow character
definitions and should preferably be the last. For an example of a
hyphenation table, see `hyph_en_US.dic'.
3.2 Character-Definition Opcodes
================================
These opcodes are needed to define attributes such as digit,
punctuation, letter, etc. for all characters and their dot patterns.
liblouis has no built-in character definitions, but such definitions
are essential to the operation of the `context' opcode (*note context:
context opcode.), the `correct' opcode (*note correct: correct
opcode.), the multipass opcodes and the back-translator. If the dot
pattern is a single cell, it is used to define the mapping between dot
patterns and characters, unless a `display' opcode (*note display:
display opcode.) for that character-dot-pattern pair has been used
previously. If only a single-cell dot pattern has been given for a
character, that dot pattern is defined with the character's own
attributes. If more than one cell is given and some of them have not
previously been defined as single cells, the undefined cells are
entered into the dots table with the space attribute. This is done for
backward compatibility with old tables, but it may cause problems with
the above opcodes or back-translation. For this reason, every
single-cell dot pattern should be defined before it is used in a
multi-cell character representation. The best way to do this is to use
the 8-dot computer braille representation for the particular braille
code. If a character or dot pattern used in any rule, except those with
the `display' opcode, the `repeated' opcode (*note repeated: repeated
opcode.) or the `replace' opcode (*note replace: replace opcode.), is
not defined by one of the character-definition opcodes, liblouis will
give an error message and refuse to continue until the problem is
fixed. If the translator or back-translator encounters an undefined
character in its input it produces a succinct error indication in its
output, and the character is treated as a space.
`space character dots'
Defines a character as a space and also defines the dot pattern as
such. for example:
space \s 0 \s is the escape sequence for blank; 0 means no dots.
`punctuation character dots'
Associates a punctuation mark in the particular language with a
braille representation and defines the character and dot pattern as
punctuation. For example:
punctuation . 46 dot pattern for period in NAB computer braille
`digit character dots'
Associates a digit with a dot pattern and defines the character as
a digit. For example:
digit 0 356 NAB computer braille
`uplow characters dots [,dots]'
The characters operand must be a pair of letters, of which the
first is uppercase and the second lowercase. The first dots
suboperand indicates the dot pattern for the upper-case letter. It
may have more than one cell. The second dots suboperand must be
separated from the first by a comma and is optional, as indicated
by the square brackets. If present, it indicates the dot pattern
for the lower-case letter. It may also have more than one cell. If
the second dots suboperand is not present the first is used for
the lower-case letter as well as the upper-case letter. This
opcode is needed because not all languages follow a consistent
pattern in assigning Unicode codes to upper and lower case
letters. It should be used even for languages that do. The
distinction is important in the forward translator. for example:
uplow Aa 17,1
`grouping name characters dots ,dots'
This opcode is used to indicate pairs of grouping symbols used in
processing mathematical expressions. These symbols are usually
generated by the MathML interpreter in liblouisxml. They are used
in multipass opcodes. The name operand must contain only letters,
but they may be upper- or lower-case. The characters operand must
contain exactly two Unicode characters. The dots operand must
contain exactly two braille cells, separated by a comma. Note that
grouping dot patterns also need to be declared with the
`exactdots' opcode (*note exactdots: exactdots opcode.). The
characters may need to be declared with the `math' opcode (*note
math: math opcode.).
grouping mrow \x0001\x0002 1e,2e
grouping mfrac \x0003\x0004 3e,4e
`letter character dots'
Associates a letter in the language with a braille representation
and defines the character as a letter. This is intended for
letters which are neither uppercase nor lowercase.
`lowercase character dots'
Associates a character with a dot pattern and defines the
character as a lowercase letter. Both the character and the dot
pattern have the attributes lowercase and letter.
`uppercase character dots'
Associates a character with a dot pattern and defines the
character as an uppercase letter. Both the character and the dot
pattern have the attributes uppercase and letter. `lowercase' and
`uppercase' should be used when a letter has only one case.
Otherwise use the `uplow' opcode (*note uplow: uplow opcode.).
`litdigit digit dots'
Associates a digit with the dot pattern which should be used to
represent it in literary texts. For example:
litdigit 0 245
litdigit 1 1
`sign character dots'
Associates a character with a dot pattern and defines both as a
sign. This opcode should be used for things like at sign (`@'),
percent (`%'), dollar sign (`$'), etc. Do not use it to define
ordinary punctuation such as period and comma. For example:
sign % 4-25-1234 literary percent sign
`math character dots'
Associates a character and a dot pattern and defines them as a
mathematical symbol. It should be used for less than (`<'),
greater than(`>'), equals(`='), plus(`+'), etc. For example:
math + 346 plus
3.3 Braille Indicator Opcodes
=============================
Braille indicators are dot patterns which are inserted into the braille
text to indicate such things as capitalization, italic type, computer
braille, etc. The opcodes which define them are followed only by a dot
pattern, which may be one or more cells.
`capsign dots'
The dot pattern which indicates capitalization of a single letter.
In English, this is dot 6. For example:
capsign 6
`begcaps dots'
The dot pattern which begins a block of capital letters. For
example:
begcaps 6-6
`endcaps dots'
The dot pattern which ends a block of capital letters within a
word. For example:
endcaps 6-3
`letsign dots'
This indicator is needed in Grade 2 to show that a single letter is
not a contraction. It is also used when an abbreviation happens to
be a sequence of letters that is the same as a contraction. For
example:
letsign 56
`noletsign letters'
The letters in the operand will not be proceeded by a letter sign.
More than one `noletsign' opcode can be used. This is equivalent
to a single entry containing all the letters. In addition, if a
single letter, such as `a' in English, is defined as a `word'
(*note word: word opcode.) or `largesign' (*note largesign:
largesign opcode.), it will be treated as though it had also been
specified in a `noletsign' entry.
`noletsignbefore characters'
If any of the characters proceeds a single letter without a space a
letter sign is not used. By default the characters apostrophe
(`'') and period (`.') have this property. Use of a
`noletsignbefore' entry cancels the defaults. If more than one
`noletsignbefore' entry is used, the characters in all entries are
combined.
`noletsignafter characters'
If any of the characters follows a single letter without a space a
letter sign is not used. By default the characters apostrophe
(`'') and period (`.') have this property. Use of a
`noletsignafter' entry cancels the defaults. If more than one
`noletsignafter' entry is used the characters in all entries are
combined.
`numsign dots'
The translator inserts this indicator before numbers made up of
digits defined with the `litdigit' opcode (*note litdigit:
litdigit opcode.) to show that they are a number and not letters
or some other symbols. For example:
numsign 3456
3.4 Emphasis Opcodes
====================
These also define braille indicators, but they require more
explanation. There are four sets, for italic, bold, underline and
computer braille. In each of the first three sets there are seven
opcodes, for use before the first word of a phrase, for use before the
last word, for use after the last word, for use before the first letter
(or character) if emphasis starts in the middle of a word, for use
after the last letter (or character) if emphasis ends in the middle of
a word, before a single letter (or character), and to specify the
length of a phrase to which the first-word and last-word-before
indicators apply. This rather elaborate set of emphasis opcodes was
devised to try to meet all contingencies. It is unlikely that a
translation table will contain all of them. The translator checks for
their presence. If they are present, it first looks to see if the
single-letter indicator should be used. Then it looks at the word (or
phrase) indicators and finally at the multi-letter indicators.
The translator will apply up to two emphasis indicators to each
phrase or string of characters, depending on what the `typeform'
parameter in its calling sequence indicates (*note Programming with
liblouis::).
For computer braille there are only two braille indicators, for the
beginning and end of a sequence of characters to be rendered in
computer braille. Such a sequence may also have other emphasis. The
computer braille indicators are applied not only when computer braille
is indicated in the `typeform' parameter, but also when a sequence of
characters is determined to be computer braille because it contains a
subsequence defined by the `compbrl' opcode (*note compbrl: compbrl
opcode.) or the `literal' opcode (*note literal: literal opcode.).
Here are the various emphasis opcodes.
`firstwordital dots'
This is the braille indicator to be placed before the first word
of an italicized phrase that is longer than the value given in the
`lenitalphrase' opcode (*note lenitalphrase: lenitalphrase
opcode.). For example:
firstwordital 46-46 English indicator
`lastworditalbefore dots'
This is the braille indicator to be placed before the last word of
an italicized phrase. In addition, if `firstwordital' is not used,
this braille indicator is doubled and placed before the first
word. Do not use `lastworditalbefore' and `lastworditalafter' in
the same table. For example:
lastworditalbefore 4-6
`lastworditalafter dots'
This is the braille indicator to be placed after the last word of
an italicized phrase. Do not use `lastworditalbefore' and
`lastworditalafter' in the same table. See also the
`lenitalphrase' opcode (*note lenitalphrase: lenitalphrase
opcode.) for more information.
`firstletterital dots'
This is the braille indicator to be placed before the first letter
(or character) if italicization begins in the middle of a word.
`lastletterital dots'
This is the braille indicator to be placed after the last letter
(or character) when italicization ends in the middle of a word.
`singleletterital dots'
This braille indicator is used if only a single letter (or
character) is italicized.
`lenitalphrase number'
If `lastworditalbefore' is used, an italicized phrase is checked
to see how many words it contains. If this number is less than or
equal to the number given in the `lenitalphrase' opcode, the
`lastworditalbefore' sign is placed in front of each word. If it
is greater, the `firstwordital' indicator is placed before the
first word and the `lastworditalbefore' indicator is placed after
the last word. Note that if the `firstwordital' opcode is not used
its indicator is made up by doubling the dot pattern given in the
`lastworditalbefore' entry. For example:
lenitalphrase 4
`firstwordbold dots'
This is the braille indicator to be placed before the first word
of a bold phrase. For example:
firstwordbold 456-456
`lastwordboldbefore dots'
This is the braille indicator to be placed before the last word of
a bold phrase. In addition, if `firstwordbold' is not used, this
braille indicator is doubled and placed before the first word. Do
not use `lastwordboldbefore' and `lastwordboldafter' in the same
table. For example:
lastwordboldbefore 456
`lastwordboldafter dots'
This is the braille indicator to be placed after the last word of a
bold phrase. Do not use `lastwordboldbefore' and
`lastwordboldafter' in the same table.
`firstletterbold dots'
This is the braille indicator to be placed before the first letter
(or character) if bold emphasis begins in the middle of a word.
`lastletterbold dots'
This is the braille indicator to be placed after the last letter
(or character) when bold emphasis ends in the middle of a word.
`singleletterbold dots'
This braille indicator is used if only a single letter (or
character) is in boldface.
`lenboldphrase number'
If `lastwordboldbefore' is used, a bold phrase is checked to see
how many words it contains. If this number is less than or equal to
the number given in the `lenboldphrase' opcode, the
`lastwordboldbefore' sign is placed in front of each word. If it
is greater, the `firstwordbold' indicator is placed before the
first word and the `lastwordboldbefore' indicator is placed after
the last word. Note that if the `firstwordbold' opcode is not used
its indicator is made up by doubling the dot pattern given in the
`lastwordboldbefore' entry.
`firstwordunder dots'
This is the braille indicator to be placed before the first word
of an underlined phrase.
`lastwordunderbefore dots'
This is the braille indicator to be placed before the last word of
an underlined phrase. In addition, if `firstwordunder' is not used,
this braille indicator is doubled and placed before the first word.
`lastwordunderafter dots'
This is the braille indicator to be placed after the last word of
an underlined phrase.
`firstletterunder dots'
This is the braille indicator to be placed before the first letter
(or character) if underline emphasis begins in the middle of a
word.
`lastletterunder dots'
This is the braille indicator to be placed after the last letter
(or character) when underline emphasis ends in the middle of a
word.
`singleletterunder dots'
This braille indicator is used if only a single letter (or
character) is underlined.
`lenunderphrase number'
If `lastwordunderbefore' is used, an underlined phrase is checked
to see how many words it contains. If this number is less than or
equal to the number given in the `lenunderphrase' opcode, the
`lastwordunderbefore' sign is placed in front of each word. If it
is greater, the `firstwordunder' indicator is placed before the
first word and the `lastwordunderbefore' indicator is placed after
the last word. Note that if the `firstwordunder' opcode is not
used its indicator is made up by doubling the dot pattern given in
the `lastwordunderbefore' entry.
`begcomp dots'
This braille indicator is placed before a sequence of characters
translated in computer braille, whether this sequence is indicated
in the `typeform' parameter (*note Programming with liblouis::) or
inferred because it contains a subsequence specified by the
`compbrl' opcode (*note compbrl: compbrl opcode.).
`endcomp dots'
This braille indicator is placed after a sequence of characters
translated in computer braille, whether this sequence is indicated
in the `typeform' parameter (*note Programming with liblouis::) or
inferred because it contains a subsequence specified by the
`compbrl' opcode (*note compbrl: compbrl opcode.).
3.5 Special Symbol Opcodes
==========================
These opcodes define certain symbols, such as the decimal point, which
require special treatment.
`decpoint character dots'
This opcode defines the decimal point. The character operand must
have only one character. For example, in `en-us-g1.ctb' we have:
decpoint . 46
`hyphen character dots'
This opcode defines the hyphen, that is, the character used in
compound words such as have-nots. The back-translator uses it to
determine the end of individual words.
3.6 Special Processing Opcodes
==============================
These opcodes cause special processing to be carried out.
`capsnocont'
This opcode has no operands. If it is specified, words or parts of
words in all caps are not contracted. This is needed for languages
such as Norwegian.
3.7 Translation Opcodes
=======================
These opcodes define the braille representations for character
sequences. Each of them defines an entry within the contraction table.
These entries may be defined in any order except, as noted below, when
they define alternate representations for the same character sequence.
Each of these opcodes specifies a condition under which the
translation is legal, and each also has a characters operand and a dots
operand. The text being translated is processed strictly from left to
right, character by character, with the most eligible entry for each
position being used. If there is more than one eligible entry for a
given position in the text, then the one with the longest character
string is used. If there is more than one eligible entry for the same
character string, then the one defined first is is tested for legality
first. (This is the only case in which the order of the entries makes a
difference.)
The characters operand is a sequence or string of characters preceded
and followed by whitespace. Each character can be entered in the normal
way, or it can be defined as a four-digit hexadecimal number preceded
by `\x'.
The dots operand defines the braille representation for the
characters operand. It may also be specified as an equals sign (`=').
This means that the the default representation for each character
(*note Character-Definition Opcodes::) within the sequence is to be
used.
In what follows the word `characters' means a sequence of one or
more consecutive letters between spaces and/or punctuation marks.
`noback opcode ...'
This is an opcode prefix, that is to say, it modifies the
operation of the opcode that follows it on the same line. noback
specifies that no back-translation is to be done using this line.
noback always ;\s; 0
`nofor opcode ...'
This is an opcode prefix which modifies the operation of the opcode
following it on the same line. nofor specifies that forward
translation is not to use the information on this line.
`compbrl characters'
If the characters are found within a block of text surrounded by
whitespace the entire block is translated according to the default
braille representations defined by the *note Character-Definition
Opcodes::, if 8-dot computer braille is enabled or according to
the dot patterns given in the `comp6' opcode (*note comp6: comp6
opcode.), if 6-dot computer braille is enabled. For example:
compbrl www translate URLs in computer braille
`comp6 character dots'
This opcode specifies the translation of characters in 6-dot
computer braille. It is necessary because the translation of a
single character may require more than one cell. The first operand
must be a character with a decimal representation from 0 to 255
inclusive. The second operand may specify as many cells as
necessary. The opcode is somewhat of a misnomer, since any dots,
not just dots 1 through 6, can be specified. This even includes
virtual dots.
`nocont characters'
Like `compbrl', except that the string is uncontracted. `prepunc'
opcode (*note prepunc: prepunc opcode.) and `postpunc' opcode
(*note postpunc: postpunc opcode.) rules are applied, however.
This is useful for specifying that foreign words should not be
contracted in an entire document.
`replace characters {characters}'
Replace the first set of characters, no matter where they appear,
with the second. Note that the second operand is _NOT_ a dot
pattern. It is also optional. If it is omitted the character(s)
in the first operand will be discarded. This is useful for
ignoring characters. It is possible that the "ignored" characters
may still affect the translation indirectly. Therefore, it is
preferable to use `correct' opcode (*note correct: correct
opcode.).
`always characters dots'
Replace the characters with the dot pattern no matter where they
appear. Do _NOT_ use an entry such as `always a 1'. Use the
`uplow', `letter', etc. character definition opcodes instead. For
example:
always world 456-2456 unconditional translation
`repeated characters dots'
Replace the characters with the dot pattern no matter where they
appear. Ignore any consecutive repetitions of the same character
sequence. This is useful for shortening long strings of spaces or
hyphens or periods. For example:
repeated --- 36-36-36 shorten separator lines made with hyphens
`repword characters dots'
When characters are encountered check to see if the word before
this string matches the word after it. If so, replace characters
with dots and eliminate the second word and any word following
another occurrence of characters that is the same. This opcode is
used in Malaysian braille. In this case the rule is:
repword - 123456
`largesign characters dots'
Replace the characters with the dot pattern no matter where they
appear. In addition, if two words defined as large signs follow
each other, remove the space between them. For example, in
`en-us-g2.ctb' the words `and' and `the' are both defined as large
signs. Thus, in the phrase `the cat and the dog' the space would
be deleted between `and' and `the', with the result `the cat
andthe dog'. Of course, `and' and `the' would be properly
contracted. The term `largesign' is a bit of braille jargon that
pleases braille experts.
`word characters dots'
Replace the characters with the dot pattern if they are a word,
that is, are surrounded by whitespace and/or punctuation.
`syllable characters dots'
As its name indicates, this opcode defines a "syllable" which must
be represented by exactly the dot patterns given. Contractions may
not cross the boundaries of this "syllable" either from left or
right. The character string defined by this opcode need not be a
lexical syllable, though it usually will be. The equal sign in the
following example means that the the default representation for
each character within the sequence is to be used (*note
Translation Opcodes::):
syllable horse = sawhorse, horseradish
`nocross characters dots'
Replace the characters with the dot pattern if the characters are
all in one syllable (do not cross a syllable boundary). For this
opcode to work, a hyphenation table must be included. If this is
not done, `nocross' behaves like the `always' opcode (*note
always: always opcode.). For example, if the English Grade 2 table
is being used and the appropriate hyphenation table has been
included `nocross sh 146' will cause the `sh' in `monkshood' not
to be contracted.
`joinword characters dots'
Replace the characters with the dot pattern if they are a word
which is followed by whitespace and a letter. In addition remove
the whitespace. For example, `en-us-g2.ctb' has `joinword to 235'.
This means that if the word `to' is followed by another word the
contraction is to be used and the space is to be omitted. If these
conditions are not met, the word is translated according to any
other opcodes that may apply to it.
`lowword characters dots'
Replace the characters with the dot pattern if they are a word
preceded and followed by whitespace. No punctuation either before
or after the word is allowed. The term `lowword' derives from the
fact that in English these contractions are written in the lower
part of the cell. For example:
lowword were 2356
`contraction characters'
If you look at `en-us-g2.ctb' you will see that some words are
actually contracted into some of their own letters. A famous
example among braille transcribers is `also', which is contracted
as `al'. But this is also the name of a person. To take another
example, `altogether' is contracted as `alt', but this is the
abbreviation for the alternate key on a computer keyboard.
Similarly `could' is contracted into `cd', but this is the
abbreviation for compact disk. To prevent confusion in such cases,
the letter sign (see `letsign' opcode (*note letsign: letsign
opcode.)) is placed before such letter combinations when they
actually are abbreviations, not contractions. The `contraction'
opcode tells the translator to do this.
`sufword characters dots'
Replace the characters with the dot pattern if they are either a
word or at the beginning of a word.
`prfword characters dots'
Replace the characters with the dot pattern if they are either a
word or at the end of a word.
`begword characters dots'
Replace the characters with the dot pattern if they are at the
beginning of a word.
`begmidword characters dots'
Replace the characters with the dot pattern if they are either at
the beginning or in the middle of a word.
`midword characters dots'
Replace the characters with the dot pattern if they are in the
middle of a word.
`midendword characters dots'
Replace the characters with the dot pattern if they are either in
the middle or at the end of a word.
`endword characters dots'
Replace the characters with the dot pattern if they are at the end
of a word.
`partword characters dots'
Replace the characters with the dot pattern if the characters are
anywhere in a word, that is, if they are proceeded or followed by a
letter.
`exactdots @dots'
Note that the operand must begin with an at sign (`@'). The dot
pattern following it is evaluated for validity. If it is valid,
whenever an at sign followed by this dot pattern appears in the
source document it is replaced by the characters corresponding to
the dot pattern in the output. This opcode is intended for use in
liblouisxml semantic-action files to specify exact dot patterns,
as in mathematical codes. For example:
exactdots @4-46-12356
will produce the characters with these dot patterns in the output.
`prepunc characters dots'
Replace the characters with the dot pattern if they are part of
punctuation at the beginning of a word.
`postpunc characters dots'
Replace the characters with the dot pattern if they are part of
punctuation at the end of a word.
`begnum characters dots'
Replace the characters with the dot pattern if they are at the
beginning of a number, that is, before all its digits. For
example, in `en-us-g1.ctb' we have `begnum # 4'.
`midnum characters dots'
Replace the characters with the dot pattern if they are in the
middle of a number. For example, `en-us-g1.ctb' has `midnum . 46'.
This is because the decimal point has a different dot pattern than
the period.
`endnum characters dots'
Replace the characters with the dot pattern if they are at the end
of a number. For example `en-us-g1.ctb' has `endnum th 1456'.
This handles things like `4th'. A letter sign is _NOT_ inserted.
`joinnum characters dots'
Replace the characters with the dot pattern. In addition, if
whitespace and a number follows omit the whitespace. This opcode
can be used to join currency symbols to numbers for example:
joinnum \x20AC 15 (EURO SIGN)
joinnum \x0024 145 (DOLLAR SIGN)
joinnum \x00A3 1234 (POUND SIGN)
joinnum \x00A5 13456 (YEN SIGN)
3.8 Character-Class Opcodes
===========================
These opcodes define and use character classes. A character class
associates a set of characters with a name. The name then refers to any
character within the class. A character may belong to more than one
class.
The basic character classes correspond to the character definition
opcodes, with the exception of the `uplow' opcode (*note uplow: uplow
opcode.), which defines characters belonging to the two classes
`uppercase' and `lowercase'. These classes are:
`space'
Whitespace characters such as blank and tab
`digit'
Numeric characters
`letter'
Both uppercase and lowercase alphabetic characters
`lowercase'
Lowercase alphabetic characters
`uppercase'
Uppercase alphabetic characters
`punctuation'
Punctuation marks
`sign'
Signs such as percent (`%')
`math'
Mathematical symbols
`litdigit'
Literary digit
`undefined'
Not properly defined
The opcodes which define and use character classes are shown below.
For examples see `fr-abrege.ctb'.
`class name characters'
Define a new character class. The characters operand must be
specified as a string. A character class may not be used until it
has been defined.
`after class opcode ...'
The specified opcode is further constrained in that the matched
character sequence must be immediately preceded by a character
belonging to the specified class. If this opcode is used more than
once on the same line then the union of the characters in all the
classes is used.
`before class opcode ...'
The specified opcode is further constrained in that the matched
character sequence must be immediately followed by a character
belonging to the specified class. If this opcode is used more than
once on the same line then the union of the characters in all the
classes is used.
3.9 Swap Opcodes
================
The swap opcodes are needed to tell the `context' opcode (*note
context: context opcode.), the `correct' opcode (*note correct: correct
opcode.) and multipass opcodes which dot patterns to swap for which
characters. There are three, `swapcd', `swapdd' and `swapcc'. The first
swaps dot patterns for characters. The second swaps dot patterns for
dot patterns and the third swaps characters for characters. The first
is used in the `context' opcode and the second is used in the multipass
opcodes. Dot patterns are separated by commas and may contain more than
one cell.
`swapcd name characters dots, dots, dots, ...'
See above paragraph for explanation. For example:
swapcd dropped 0123456789 356,2,23,...
`swapdd name dots, dots, dots ... dotpattern1, dotpattern2, dotpattern3, ...'
The `swapdd' opcode defines substitutions for the multipass
opcodes. In the second operand the dot patterns must be single
cells, but in the third operand multi-cell dot patterns are
allowed. This is because multi-cell patterns in the second operand
would lead to ambiguities.
`swapcc name characters characters'
The `swapcc' opcode swaps characters in its second operand for
characters in the corresponding places in its third operand. It is
intended for use with `correct' opcodes and can solve problems
such as formatting phone numbers.
3.10 The Context and Multipass Opcodes
======================================
The `context' and multipass opcodes (`pass2', `pass3' and `pass4')
provide translation capabilities beyond those of the basic translation
opcodes (*note Translation Opcodes::) discussed previously. The
multipass opcodes cause additional passes to be made over the string to
be translated. The number after the word `pass' indicates in which pass
the entry is to be applied. If no multipass opcodes are given, only the
first translation pass is made. The `context' opcode is basically a
multipass opcode for the first pass. It differs slightly from the
multipass opcodes per se. The format of all these opcodes is `opcode
test action'. The specific opcodes are invoked as follows:
`context test action'
`pass2 test action'
`pass3 test action'
`pass4 test action'
The `test' and `action' operands have suboperands. Each suboperand
begins with a non-alphanumeric character and ends when another
non-alphanumeric character is encountered. The suboperands and their
initial characters are as follows.
`" (double quote)'
a string of characters. This string must be terminated by another
double quote. It may contain any characters. If a double quote is
needed within the string, it must be preceded by a backslash
(`\'). If a space is needed, it must be represented by the escape
sequence \s. This suboperand is valid only in the test part of the
`context' opcode.
`@ (at sign)'
a sequence of dot patterns. Cells are separated by hyphens as
usual. This suboperand is not valid in the test part of the
context and correct opcodes.
`` (accent mark)'
If this is the beginning of the string being translated this
suboperand is true. It is valid only in the test part and must be
the first thing in this operand.
`~ (tilde)'
If this is the end of the string being translated this suboperand
is true. It is valid only in the test part and must be the last
thing in this operand.
`$ (dollar sign)'
a string of attributes, such as `d' for digit, `l' for letter,
etc. More than one attribute can be given. If you wish to check
characters with any attribute, use the letter `a'. Input
characters are checked to see if they have at least one of the
attributes. The attribute string can be followed by numbers
specifying how many characters are to be checked. If no numbers
are given, 1 is assumed. If two numbers separated by a hyphen are
given, the input is checked to make sure that at least the first
number of characters with the attributes are present, but no more
than the second number. If only one number is present, then
exactly that many characters must have the attributes. A period
instead of the numbers indicates an indefinite number of
characters. This suboperand is valid in all test parts but not in
action parts. For the characters which can be used in attribute
strings, see the following table.
`! (exclamation point)'
reverses the logical meaning of the suboperand which follows. For
example, !$d is true only if the character is _NOT_ a digit. This
suboperand is valid in test parts only.
`% (percent sign)'
the name of a class defined by the `class' opcode (*note class:
class opcode.) or the name of a swap set defined by the swap
opcodes (*note Swap Opcodes::). Names may contain only letters.
The letters may be upper or lower-case. The case matters. Class
names may be used in test parts only. Swap names are valid
everywhere.
`{ (left brace)'
Name: the name of a grouping pair. The left brace indicates that
the first (or left) member of the pair is to be used in matching.
If this is between replacement brackets it must be the only item.
This is also valid in the action part.
`} (right brace)'
Name: the name of a grouping pair. The right brace indicates that
the second (or right) member is to be used in matching. See the
remarks on the left brace immediately above.
`/ (slash)'
Search the input for the expression following the slash and return
true if found. This can be used to set a variable.
`_ (underscore)'
Move backward. If a number follows, move backward that number of
characters. The program never moves backward beyond the beginning
of the input string. This suboperand is valid only in test parts.
`[ (left bracket)'
start replacement here. This suboperand must always be paired with
a right bracket and is valid only in test parts. Multiple pairs of
square brackets in a single expression are not allowed.
`] (right bracket)'
end replacement here. This suboperand must always be paired with a
left bracket and is valid only in test parts.
`# (number sign or crosshatch)'
test or set a variable. Variables are referred to by numbers 1 to
50, for example, `#1', `#2', `#25'. Variables may be set by one
`context' or multipass opcode and tested by another. Thus, an
operation that occurs at one place in a translation can tell an
operation that occurs later about itself. This feature will be
used in math translation, and it may also help to alleviate the
need for new opcodes. This suboperand is valid everywhere.
Variables are set in the action part. To set a variable use an
expression like `#1=1', `#2=5', etc. Variables are also
incremented and decremented in the action part with expressions
like `#1+', `#3-', etc. These operators increment or decrement the
variable by 1.
Variables are tested in the test part with expressions like
`#1=2', `#3<4', `#5>6', etc.
`* (asterisk)'
Copy the characters or dot patterns in the input within the
replacement brackets into the output and discard anything else that
may match. This feature is used, for example, for handling numeric
subscripts in Nemeth. This suboperand is valid only in action
parts.
`? (question mark)'
Valid only in the action part. The characters to be replaced are
simply ignored. That is, they are replaced with nothing. If either
member of a grouping pair is in the replace brackets the other
member at the same level is also removed.
The characters which can be used in attribute strings are as follows:
`a'
any attribute
`d'
digit
`D'
literary digit
`l'
letter
`m'
math
`p'
punctuation
`S'
sign
`s'
space
`U'
uppercase
`u'
lowercase
`w'
first user-defined class
`x'
second user-defined class
`y'
third user-defined class
`z'
fourth user-defined class
The following illustrates the algorithm how text is evaluated with
multipass expressions:
Loop over context, pass2, pass3 and pass4 and do the following for each
pass:
a. Match the text following the cursor against all expressions in the
current pass
b. If there is no match: shift the cursor one position to the right
and continue the loop
c. If there is a match: choose the longest match
d. Do the replacement (everything between square brackets)
e. Place the cursor after the replaced text
f. continue loop
Note that if any multipass opcode or the `correct' opcode (*note
correct: correct opcode.) is used and the `pass1Only' mode bit (*note
lou_translateString::) is not set, input and output positions may be
incorrect.
3.11 The correct Opcode
=======================
`correct test action'
Because some input (such as that from an OCR program) may contain
systematic errors, it is sometimes advantageous to use a
pre-translation pass to remove them. The errors and their
corrections are specified by the `correct' opcode. If there are no
`correct' opcodes in a table, the pre-translation pass is not
used. The format of the `correct' opcode is very similar to that
of the `context' opcode (*note context: context opcode.). The only
difference is that in the action part strings may be used and dot
patterns may not be used. Some examples of `correct' opcode
entries are:
correct "\\" ? Eliminate backslashes
correct "cornf" "comf" fix a common "scano"
correct "cornm" "comm"
correct "cornp" "comp"
correct "*" ? Get rid of stray asterisks
correct "|" ? ditto for vertical bars
correct "\s?" "?" drop space before question mark
Note that if the `correct' opcode is used and the `pass1Only' mode
bit (*note lou_translateString::) is not set, input and output
positions may be incorrect.
3.12 Miscellaneous Opcodes
==========================
`include filename'
Read the file indicated by `filename' and incorporate or include
its entries into the table. Included files can include other files,
which can include other files, etc. For an example, see what files
are included by the entry include `en-us-g1.ctb' in the table
`en-us-g2.ctb'. If the included file is not in the same directory
as the main table, use a full path name for filename.
`locale characters'
Not implemented, but recognized and ignored for backward
compatibility.
`undefined dots'
If this opcode is used in a table any characters which have not
been defined in the table but are encountered in the text will be
replaced by the dot pattern. If this opcode is not used, any
undefined characters are replaced by `'\xhhhh'', where the h's are
hexadecimal digits.
`display character dots'
Associates dot patterns with the characters which will be sent to a
braille embosser, display or screen font. The character must be in
the range 0-255 and the dots must specify a single cell. Here are
some examples:
display a 1 When the character a is sent to the embosser or display,
it # will produce a dot 1.
display L 123 When the character L is sent to the display or embosser
# produces dots 1-2-3.
The `display' opcode is optional. It is used when the embosser or
display has a different mapping of characters to dot patterns than
that given in *note Character-Definition Opcodes::. If used,
display entries must proceed character-definition entries.
`multind dots opcode opcode ...'
The `multind' opcode tells the back-translator that a sequence of
braille cells represents more than one braille indicator. For
example, in `en-us-g1.ctb' we have `multind 56-6 letsign capsign'.
The back-translator can generally handle single braille indicators,
but it cannot apply them when they immediately follow each other.
It recognizes the letter sign if it is followed by a letter and
takes appropriate action. It also recognizes the capital sign if
it is followed by a letter. But when there is a letter sign
followed by a capital sign it fails to recognize the letter sign
unless the sequence has been defined with `multind'. A `multind'
entry may not contain a comment because liblouis would attempt to
interpret it as an opcode.
3.13 Deprecated Opcodes
=======================
The following opcodes are an early attempt to handle emphasis. They
have been deprecated by more specific opcodes, but are kept for
backward compatibility.
`italsign dots'
This opcode is deprecated. Use the `lastworditalbefore' opcode
(*note lastworditalbefore: lastworditalbefore opcode.) instead.
`begital dots'
This opcode is deprecated. Use the `firstletterital' opcode (*note
firstletterital: firstletterital opcode.) instead.
`endital dots'
This opcode is deprecated. Use the `lastletterital' opcode (*note
lastletterital: lastletterital opcode.) instead.
`boldsign dots'
This opcode is deprecated. Use the `lastwordboldbefore' opcode
(*note lastwordboldbefore: lastwordboldbefore opcode.) instead.
`begbold dots'
This opcode is deprecated. Use the `firstletterbold' opcode (*note
firstletterbold: firstletterbold opcode.) instead.
`endbold dots'
This opcode is deprecated. Use the `lastletterbold' opcode (*note
lastletterbold: lastletterbold opcode.) instead.
`undersign dots'
This opcode is deprecated. Use the `lastwordunderbefore' opcode
(*note lastwordunderbefore: lastwordunderbefore opcode.) instead.
`begunder dots'
This opcode is deprecated. Use the `firstletterunder' opcode
(*note firstletterunder: firstletterunder opcode.) instead.
`endunder dots'
This opcode is deprecated. Use the `lastletterunder' opcode (*note
lastletterunder: lastletterunder opcode.) instead.
`literal characters'
This opcode is deprecated. Use the `compbrl' opcode (*note
compbrl: compbrl opcode.) instead.
4 Notes on Back-Translation
***************************
Back-translation is carried out by the function
`lou_backTranslateString'. Its calling sequence is described in *note
Programming with liblouis::. Tables containing no `context' opcode
(*note context: context opcode.), `correct' opcode (*note correct:
correct opcode.) or multipass opcodes can be used for both forward and
backward translation. If these opcodes are needed different tables will
be required. `lou_backTranslateString' first performs `pass4', if
present, then `pass3', then `pass2', then the backtranslation, then
corrections. Note that this is exactly the inverse of forward
translation.
5 Programming with liblouis
***************************
5.1 License
===========
Liblouis may contain code borrowed from the Linux screen reader BRLTTY,
Copyright (C) 1999-2006 by the BRLTTY Team.
Copyright (C) 2004-2007 ViewPlus Technologies, Inc. `www.viewplus.com'.
Copyright (C) 2007,2009 Abilitiessoft, Inc. `www.abilitiessoft.com'.
Liblouis is free software: you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published
by the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Liblouis is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser
General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with Liblouis. If not, see `http://www.gnu.org/licenses/'.
5.2 Overview
============
You use the liblouis library by calling the following functions,
`lou_translateString', `lou_backTranslateString', `lou_logFile',
`lou_logPrint', `lou_endLog', `lou_getTable', `lou_translate',
`lou_backTranslate', `lou_hyphenate', `lou_charToDots',
`lou_dotsToChar', `lou_compileString', `lou_readCharFromFile',
`lou_version' and `lou_free'. These are described below. The header
file, `liblouis.h', also contains brief descriptions. Liblouis is
written in straight C. It has just three code modules,
`compileTranslationTable.c', `lou_translateString.c' and
`lou_backTranslateString.c'. In addition, there are two header files,
`liblouis.h', which defines the API, and `louis.h', used only
internally and by liblouisxml. The latter includes `liblouis.h'.
Persons who wish to use liblouis from Python may want to skip ahead
to *note Python bindings::.
`compileTranslationTable.c' keeps track of all translation tables
which an application has used. It is called by the translation,
hyphenation and checking functions when they start. If a table has not
yet been compiled `compileTranslationTable.c' checks it for correctness
and compiles it into an efficient internal representation. The main
entry point is `lou_getTable'. Since it is the module that keeps track
of memory usage, it also contains the `lou_free' function. In addition,
it contains the `lou_logFile', `lou_logPrint' and `lou_endLog'
functions, plus some utility functions which are used by the other
modules.
By default, liblouis handles all characters internally as 16-bit
unsigned integers. It can be compiled for 32-bit characters as
explained below. The meanings of these integers are not hard-coded.
Rather they are defined by the character-definition opcodes. However,
the standard printable characters, from decimal 32 to 126 are
recognized for the purpose of processing the opcodes. Hence, the
following definition is included in `liblouis.h'. It is correct for
computers with at least 32-bit processors.
#define widechar unsigned short int
To make liblouis handle 32-bit Unicode simply remove the word
`short' in the above `define'. This will cause the translate and
back-translate functions to expect input in 32-bit form and to deliver
their output in this form. The input to the compiler (tables) is
unaffected except that two new escape sequences for 20-bit and 32-bit
characters are recognized.
Here are the definitions of the eleven liblouis functions and their
parameters. They are given in terms of 16-bit Unicode. If liblouis has
been compiled for 32-bit Unicode simply read 32 instead of 16.
5.3 Data structure of liblouis tables
=====================================
The data structure `TranslationTableHeader' is defined by a `typedef'
statement in `louis.h'. To find the beginning, search for the word
`header'. As its name implies, this is actually the table header. Data
are placed in the `ruleArea' array, which is the last item defined in
this structure. This array is declared with a length of 1 and is
expanded as needed. The table header consists mostly of arrays of
pointers of size `HASHNUM'. These pointers are actually offsets into
`ruleArea' and point to chains of items which have been placed in the
same hash bucket by a simple hashing algorithm. `HASHNUM' should be a
prime and is currently 1123. The structure of the table was chosen to
optimize speed rather than memory usage.
The first part of the table contains miscellaneous information, such
as the number of passes and whether various opcodes have been used. It
also contains the amount of memory allocated to the table and the
amount actually used.
The next section contains pointers to various braille indicators and
begins with `capitalSign'. The rules pointed to contain the dot pattern
for the indicator and an opcode which is used by the back-translator
but does not appear in the list of opcodes. The braille indicators also
include various kinds of emphasis, such as italic and bold and
information about the length of emphasized phrases. The latter is
contained directly in the table item instead of in a rule.
After the braille indicators comes information about when a letter
sign should be used.
Next is an array of size `HASHNUM' which points to character
definitions. These are created by the character-definition opcodes.
Following this is a similar array pointing to definitions of
single-cell dot patterns. This is also created from the
character-definition opcodes. If a character definition contains a
multi-cell dot pattern this is compiled into ordinary forward and
backward rules. If such a multi-cell dot pattern contains a single cell
which has not previously been defined that cell is placed in this
array, but is given the attribute `space'.
Next come arrays that map characters to single-cell dot patterns and
dots to characters. These are created from both character-definition
opcodes and display opcodes.
Next is an array of size 256 which maps characters in this range to
dot patterns which may consist of multiple cells. It is used, for
example, to map `{' to dots 456-246. These mappings are created by the
`compdots' or the `comp6' opcode (*note comp6: comp6 opcode.).
Next are two small arrays that held pointers to chains of rules
produced by the `swapcd' opcode (*note swapcd: swapcd opcode.) and the
`swapdd' opcode (*note swapdd: swapdd opcode.) and by some multipass,
`context' and `correct' opcodes.
Now we get to an array of size `HASHNUM' which points to chains of
rules for forward translation.
Following this is a similar array for back-translation.
Finally is the `ruleArea', an array of variable size to which
various structures are mapped and to which almost everything else
points.
5.4 lou_version
===============
char *lou_version ()
This function returns a pointer to a character string containing the
version of liblouis, plus other information, such as the release date
and perhaps notable changes.
5.5 lou_translateString
=======================
int lou_translateString (
const char * tableList,
const widechar * inbuf,
int *inlen,
widechar *outbuf,
int *outlen,
char *typeform,
char *spacing,
int mode);
This function takes a string of 16-bit Unicode characters in `inbuf'
and translates it into a string of 16-bit characters in `outbuf'. Each
16-bit character produces a particular dot pattern in one braille cell
when sent to an embosser or braille display or to a screen type font.
Which 16-bit character represents which dot pattern is indicated by the
character-definition and display opcodes in the translation table.
The `tableList' parameter points to a list of translation tables
separated by commas. If only one table is given, no comma should be
used after it. It is these tables which control just how the
translation is made, whether in Grade 2, Grade 1, or something else.
liblouis knows where to find all the tables that have been
distributed with it. So you can just give a table name such as
`en-us-g2.ctb' and liblouis will load it. You can also give a table
name which includes a path. If this is the first table in a list, all
the tables in the list must be on the same path. You can specify a path
on which liblouis will look for table names by setting the environment
variable `LOUIS_TABLEPATH'. This environment variable can contain one or
more paths separated by commas. On receiving a table name liblouis
first checks to see if it can be found on any of these paths. If not,
it then checks to see if it can be found in the current directory, or,
if the first (or only) name in a table list, if it contains a path
name, can be found on that path. If not, it checks to see if it can be
found on the path where the distributed tables have been installed. If
a table has already been loaded and compiled this path-checking is
skipped.
The tables in a list are all compiled into the same internal table.
The list is then regarded as the name of this table. As explained in
*note How to Write Translation Tables::, each table is a file which may
be plain text, big-endian Unicode or little-endian Unicode. A table (or
list of tables) is compiled into an internal representation the first
time it is used. Liblouis keeps track of which tables have been
compiled. For this reason, it is essential to call the `lou_free'
function at the end of your application to avoid memory leaks. Do _NOT_
call `lou_free' after each translation. This will force liblouis to
compile the translation tables each time they are used, leading to
great inefficiency.
Note that both the `*inlen' and `*outlen' parameters are pointers to
integers. When the function is called, these integers contain the
maximum input and output lengths, respectively. When it returns, they
are set to the actual lengths used.
The `typeform' parameter is used to indicate italic type, boldface
type, computer braille, etc. It is a string of characters with the same
length as the input buffer pointed to by `*inbuf'. However, it is used
to pass back character-by-character results, so enough space must be
provided to match the `*outlen' parameter. Each character indicates
the typeform of the corresponding character in the input buffer. The
values are as follows: 0 plain-text; 1 italic; 2 bold; 4 underline; 8
computer braille. These values can be added for multiple emphasis. If
this parameter is `NULL', no checking for type forms is done. In
addition, if this parameter is not `NULL', it is set on return to have
an 8 at every position corresponding to a character in `outbuf' which
was defined to have a dot representation containing dot 7, dot 8 or
both, and to 0 otherwise.
The `spacing' parameter is used to indicate differences in spacing
between the input string and the translated output string. It is also
of the same length as the string pointed to by `*inbuf'. If this
parameter is `NULL', no spacing information is computed.
The `mode' parameter specifies how the translation should be done.
The valid values of mode are listed in `liblouis.h'. They are all
powers of 2, so that a combined mode can be specified by adding up
different values.
The function returns 1 if no errors were encountered and 0 if a
complete translation could not be done.
5.6 lou_translate
=================
int lou_translate (
const char * tableList,
const widechar * const inbuf,
int *inlen,
widechar * outbuf,
int *outlen,
char *typeform,
char *spacing,
int *outputPos,
int *inputPos,
int *cursorPos,
int mode);
This function adds the parameters `outputPos', `inputPos' and
`cursorPos', to facilitate use in screen reader programs. The
`outputPos' parameter must point to an array of integers with at least
`inlen' elements. On return, this array will contain the position in
`outbuf' corresponding to each input position. Similarly, `inputPos'
must point to an array of integers of at least `outlen' elements. On
return, this array will contain the position in `inbuf' corresponding
to each position in `inbuf'. `cursorPos' must point to an integer
containing the position of the cursor in the input. On return, it will
contain the cursor position in the output. Any parameter after `outlen'
may be `NULL'. In this case, the actions corresponding to it will not
be carried out. The `mode' parameter, however, must be present and must
be an integer, not a pointer to an integer. If the `compbrlAtCursor'
bit is set in the `mode' parameter the space-bounded characters
containing the cursor will be translated in computer braille. If the
`compbrlLeftCursor' bit is set only the characters to the left of the
cursor will be in computer braille. This bit overrides
`compbrlAtCursor'. The `ucBrl' (Unicode Braille) bit is used by
`lou_charToDots' and `lou_translate'. It causes the dot patterns to be
Unicode Braille rather than the liblouis representation.
`lou_dotsToChar' and `lou_backTranslate' recognize Unicode braille
automatically.
The `otherTrans' mode needs special description. If it is set
liblouis will attempt to call a wrapper for another translator. These
other translators are usually for Asian languages. The calling sequence
is the same as for liblouis itself except that the `trantab' parameter
gives the name of the other translator, possibly abbreviated, followed
by a colon, followed by whatever other information the other translator
needs. This is specific for each translator. If no such information is
needed the colon should be omitted. The result of calling either the
translate or back-translate functions with this mode bit set will be
the same as calling without it set. That is, the wrapper for the other
translator simulates a call to liblouis. Note that the wrappers are not
implemented at this time. Setting this mode bit will result in failure
(return value of 0).
5.7 lou_backTranslateString
===========================
int lou_backTranslateString (
const char * tableList,
const widechar * inbuf,
int *inlen,
widechar *outbuf,
int *outlen,
char *typeform,
char *spacing,
int mode);
This is exactly the opposite of `lou_translateString'. `inbuf' is a
string of 16-bit Unicode characters representing braille. `outbuf' will
contain a string of 16-bit Unicode characters. `typeform' will indicate
any emphasis found in the input string, while `spacing' will indicate
any differences in spacing between the input and output strings. The
`typeform' and `spacing' parameters may be `NULL' if this information is
not needed. `mode' again specifies how the back-translation should be
done.
5.8 lou_backTranslate
=====================
int lou_backTranslate (
const char * tableList,
const widechar * inbufx,
int *inlen,
widechar * outbuf,
int *outlen,
char *typeform,
char *spacing,
int *outputPos,
int *inputPos,
int *cursorPos,
int mode);
This function is exactly the inverse of `lou_translate'.
5.9 lou_hyphenate
=================
int lou_hyphenate (
const char * tableList,
const widechar * const inbuf,
int inlen,
char *hyphens,
int mode);
This function looks at the characters in `inbuf' and if it finds a
sequence of letters attempts to hyphenate it as a word. Note that
lou_hyphenate operates on single words only, and spaces or punctuation
marks between letters are not allowed. Leading and trailing punctuation
marks are ignored. The table named by the `tableList' parameter must
contain a hyphenation table. If it does not, the function does nothing.
`inlen' is the length of the character string in `inbuf'. `hyphens' is
an array of characters and must be of size `inlen'. If hyphenation is
successful it will have a 1 at the beginning of each syllable and a 0
elsewhere. If the `mode' parameter is 0 `inbuf' is assumed to contain
untranslated characters. Any nonzero value means that `inbuf' contains
a translation. In this case, it is back-translated, hyphenation is
performed, and it is re-translated so that the hyphens can be placed
correctly. The `lou_translate' and `lou_backTranslate' functions are
used in this process. `lou_hyphenate' returns 1 if hyphenation was
successful and 0 otherwise. In the latter case, the contents of the
`hyphens' parameter are undefined. This function was provided for use in
liblouisxml.
5.10 lou_compileString
======================
int lou_compileString (const char *tableList, const char *inString)
This function enables you to compile a table entry on the fly at
run-time. The new entry is added to `tableList' and remains in force
until `lou_free' is called. If `tableList' has not previously been
loaded it is loaded and compiled. `inString' contains the table entry
to be added. It may be anything valid. Error messages will be produced
if it is invalid. The function returns 1 on success and 0 on failure.
5.11 lou_dotsToChar
===================
int lou_dotsToChar (const char *tableList, const widechar *inbuf, widechar
*outbuf, int length, int)
This function takes a widechar string in `inbuf' consisting of dot
patterns and converts it to a widechar string in `outbuf' consisting of
characters according to the specifications in `tableList'. `length' is
the length of both `inbuf' and `outbuf'. The dot patterns in `inbuf'
can be in either liblouis format or Unicode braille. The function
returns 1 on success and 0 on failure.
5.12 lou_charToDots
===================
int lou_charToDots (const char *tableList, const widechar *inbuf, widechar
*outbuf, int length, int mode)
This function is the inverse of `lou_dotsToChar'. It takes a
widechar string in `inbuf' consisting of characters and converts it to
a widechar string in `outbuf' consisting of dot patterns according to
the specifications in `tableList'. `length' is the length of both
`inbuf' and `outbuf'. The dot patterns in `outbufbuf' are in liblouis
format if the mode bit `ucBrl' is not set and in Unicode format if it
is set. The function returns 1 on success and 0 on failure.
5.13 lou_logFile
================
void lou_logFile (char *fileName);
This function is used when it is not convenient either to let
messages be printed on stderr or to use redirection, as when liblouis
is used in a GUI application or in liblouisxml. Any error messages
generated will be printed to the file given in this call. The entire
path name of the file must be given.
5.14 lou_logPrint
=================
void lou_logPrint (char *format, ...);
This function is called like `fprint'. It can be used by other
libraries to print messages to the file specified by the call to
`lou_logFile'. In particular, it is used by the companion library
liblouisxml.
5.15 lou_logEnd
===============
lou_logEnd ();
This function is used at the end of processing a document to close
the log file, so that it can be read by the rest of the program.
5.16 lou_setDataPath
====================
char * lou_setDataPath (char *path);
This function is used to tell liblouis and liblouisutdml where tables
and files are located. It thus makes them completely relocatable, even
on Linux. The `path' is the directory where the subdirectories
`liblouis/tables' and `liblouisutdml/lbu_files' are rooted or located.
The function returns a pointer to the `path'.
5.17 lou_getDataPath
====================
char * lou_getDataPath ();
This function returns a pointer to the path set by
`lou_setDataPath'. If no path has been set it returns `NULL'.
5.18 lou_getTable
=================
void *lou_getTable (char *tablelist);
`tablelist' is a list of names of table files separated by commas,
as explained previously (*note `tableList' parameter in
`lou_translateString': translation-tables.). If no errors are found
this function returns a pointer to the compiled table. If errors are
found messages are printed to the log file, which is stderr unless a
different filename has been given using the `lou_logFile' function.
Errors result in a `NULL' pointer being returned.
5.19 lou_readCharFromFile
=========================
int lou_readCharFromFile (const char *fileName, int *mode);
This function is provided for situations where it is necessary to
read a file which may contain little-endian or big-endian 16-bit Unicode
characters or ASCII8 characters. The return value is a little-endian
character, encoded as an integer. The `fileName' parameter is the name
of the file to be read. The `mode' parameter is a pointer to an integer
which must be set to 1 on the first call. After that, the function
takes care of it. On end-of-file the function returns `EOF'.
5.20 lou_free
=============
void lou_free ();
This function should be called at the end of the application to free
all memory allocated by liblouis. Failure to do so will result in
memory leaks. Do _NOT_ call `lou_free' after each translation. This
will force liblouis to compile the translation tables every time they
are used, resulting in great inefficiency.
5.21 Python bindings
====================
There are Python bindings for `lou_translateString', `lou_translate'
and `lou_version'. For installation instructions see the the `README'
file in the `python' directory. Usage information is included in the
Python module itself.
Opcode Index
************
after: See 3.8. (line 1185)
always: See 3.7. (line 959)
before: See 3.8. (line 1192)
begbold: See 3.13. (line 1544)
begcaps: See 3.3. (line 614)
begcomp: See 3.4. (line 833)
begital: See 3.13. (line 1532)
begmidword: See 3.7. (line 1065)
begnum: See 3.7. (line 1106)
begunder: See 3.13. (line 1556)
begword: See 3.7. (line 1061)
boldsign: See 3.13. (line 1540)
capsign: See 3.3. (line 608)
capsnocont: See 3.6. (line 871)
class: See 3.8. (line 1180)
comp6: See 3.7. (line 932)
compbrl: See 3.7. (line 922)
context: See 3.10. (line 1246)
contraction: See 3.7. (line 1039)
correct: See 3.11. (line 1442)
decpoint: See 3.5. (line 854)
digit: See 3.2. (line 521)
display: See 3.12. (line 1489)
endbold: See 3.13. (line 1548)
endcaps: See 3.3. (line 620)
endcomp: See 3.4. (line 840)
endital: See 3.13. (line 1536)
endnum: See 3.7. (line 1117)
endunder: See 3.13. (line 1560)
endword: See 3.7. (line 1077)
exactdots: See 3.7. (line 1086)
firstletterbold: See 3.4. (line 772)
firstletterital: See 3.4. (line 727)
firstletterunder: See 3.4. (line 808)
firstwordbold: See 3.4. (line 752)
firstwordital: See 3.4. (line 703)
firstwordunder: See 3.4. (line 795)
grouping: See 3.2. (line 544)
hyphen: See 3.5. (line 860)
include: See 3.12. (line 1470)
italsign: See 3.13. (line 1528)
joinnum: See 3.7. (line 1122)
joinword: See 3.7. (line 1021)
largesign: See 3.7. (line 984)
lastletterbold: See 3.4. (line 776)
lastletterital: See 3.4. (line 731)
lastletterunder: See 3.4. (line 813)
lastwordboldafter: See 3.4. (line 767)
lastwordboldbefore: See 3.4. (line 758)
lastworditalafter: See 3.4. (line 720)
lastworditalbefore: See 3.4. (line 711)
lastwordunderafter: See 3.4. (line 804)
lastwordunderbefore: See 3.4. (line 799)
lenboldphrase: See 3.4. (line 784)
lenitalphrase: See 3.4. (line 739)
lenunderphrase: See 3.4. (line 822)
letsign: See 3.3. (line 626)
letter: See 3.2. (line 560)
litdigit: See 3.2. (line 577)
literal: See 3.13. (line 1564)
locale: See 3.12. (line 1478)
lowercase: See 3.2. (line 565)
lowword: See 3.7. (line 1030)
math: See 3.2. (line 592)
midendword: See 3.7. (line 1073)
midnum: See 3.7. (line 1111)
midword: See 3.7. (line 1069)
multind: See 3.12. (line 1506)
noback: See 3.7. (line 910)
nocont: See 3.7. (line 942)
nocross: See 3.7. (line 1011)
nofor: See 3.7. (line 917)
noletsign: See 3.3. (line 634)
noletsignafter: See 3.3. (line 651)
noletsignbefore: See 3.3. (line 643)
numsign: See 3.3. (line 659)
partword: See 3.7. (line 1081)
pass2: See 3.10. (line 1246)
pass3: See 3.10. (line 1246)
pass4: See 3.10. (line 1246)
postpunc: See 3.7. (line 1102)
prepunc: See 3.7. (line 1098)
prfword: See 3.7. (line 1057)
punctuation: See 3.2. (line 514)
repeated: See 3.7. (line 967)
replace: See 3.7. (line 949)
repword: See 3.7. (line 975)
sign: See 3.2. (line 584)
singleletterbold: See 3.4. (line 780)
singleletterital: See 3.4. (line 735)
singleletterunder: See 3.4. (line 818)
space: See 3.2. (line 508)
sufword: See 3.7. (line 1053)
swapcc: See 3.9. (line 1225)
swapcd: See 3.9. (line 1213)
swapdd: See 3.9. (line 1218)
syllable: See 3.7. (line 999)
undefined: See 3.12. (line 1482)
undersign: See 3.13. (line 1552)
uplow: See 3.2. (line 527)
uppercase: See 3.2. (line 570)
word: See 3.7. (line 995)
Function Index
**************
lou_backTranslate: See 5.8. (line 1897)
lou_backTranslateString: See 5.7. (line 1875)
lou_charToDots: See 5.12. (line 1969)
lou_compileString: See 5.10. (line 1944)
lou_dotsToChar: See 5.11. (line 1956)
lou_free: See 5.20. (line 2057)
lou_getDataPath: See 5.17. (line 2023)
lou_getTable: See 5.18. (line 2031)
lou_hyphenate: See 5.9. (line 1915)
lou_logEnd: See 5.15. (line 2004)
lou_logFile: See 5.13. (line 1983)
lou_logPrint: See 5.14. (line 1994)
lou_readCharFromFile: See 5.19. (line 2044)
lou_setDataPath: See 5.16. (line 2012)
lou_translate: See 5.6. (line 1822)
lou_translateString: See 5.5. (line 1737)
lou_version: See 5.4. (line 1728)
Program Index
*************
lou_allround: See 2.3. (line 250)
lou_checkhyphens: See 2.5. (line 294)
lou_checktable: See 2.2. (line 232)
lou_debug: See 2.1. (line 134)
lou_translate: See 2.4. (line 268)
