Liblouis User's and Programmer's Manual

3.1 General Notes ¶

Back-translation refers to the process of translating backwards, i.e. from Braille to text. For many years, Liblouis was mainly concerned with forward translation, and so were most of the authors of the translation tables. Today however, Liblouis is being used extensively in conjunction with screen reading programs like NVDA and JAWS for Windows as well as Braille note-takers like BrailleSense from HIMS and BrailleNote from HumanWare. So when writing a translation table for Liblouis, it is indeed relevant to consider how the table will work when used for back-translation, if anything special must be done, or if you want to write separate tables for forward translation and back-translation.

Back-translation is generally harder to do in a computer program than forward translation. Ideally, any text could be translated to Braille and then translated back to text giving exactly the same result as the original. However, many Braille codes omit a lot of information and leaves it to the reader to fill in the missing bits. An example of this is letters with accents. In languages where accents are uncommon, e.g. English, Accented letters are usually just marked with a Braille indicator stating that there is an accent, but not which accent, even though this may be crucial to the meaning of the word or the sentence. Another example of this is when not all capital letters are marked in the Braille code, but only the "important" capital letters. A third example is when a Braille character serves as both a punctuation sign, a math sign, and perhaps even as a contraction, and the Braille code then leaves it up to the reader to use his/her knowledge of the context to decide the meaning of the Braille character.

In some cases, you may need to bend the rules of the Braille code if it is important to create Braille that can be properly back-translated. This may include marking all capital letters instead of just the "important" ones, or perhaps marking a Braille character with an indicator stating that this character should in fact be interpreted as a math sign and not a punctuation or Braille contraction. In some cases, the best solution may be to create two separate sets of tables for forward translation: One set for Braille that must be back-translatable (for use with screen readers and note-takers), and another for good and nice literary Braille (for embossing). But no matter how you bend the Braille code, the back-translation process may not be perfect.

3.2 Back-translation with Liblouis ¶

Back-translation is carried out by the function lou_backTranslateString. Its calling sequence is described in Programming with liblouis. 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.

Most opcodes can be preceded by noback opcode (see noback) or nofor opcode (see nofor), and the correct, context and multi-pass opcodes must be preceded with either noback or nofor. So in most cases, it will be perfectly possible to make one table for translation in both directions, although a separate table for forward and backward translation might be more readable in some cases.

Most of the opcodes associated with pass 1 have two operands, a character operand to the left and a dots operand to the right. During forward translation, these operands are used to replace the characters with the dot pattern according to the conditions of the opcode. The opcode works from left to right. When back-translating, these opcodes work the opposite way. The dot patterns are replaced by the text. The opcodes work from right to left.

On the other hand, the correct, context and multi-pass opcodes have a test part to the left and an action part to the right. These opcodes work from left to right in both translation directions. The test is performed, and if true, the action is executed, i.e. replacing, inserting or deleting characters or dots. This is why a translation direction always has to be specified with these opcodes using noback or nofor.