Using LuaTeX to convert interword glue to spaces and kerns

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Auteur

Graham Douglas

Last Updated

il y a 5 ans

License

Creative Commons CC BY 4.0

Résumé

PDF accessibility requirements stipulate that word breaks must be represented explicitly in the PDF. However, TeX engines do not use space characters to separate words: they use interword glue. This project shows how you can use LuaTeX to replace interword glue with space characters and kerns without affecting the visual appearance of typeset text.

To understand the difference this makes for users of accessibility software, listen to this sound recording made using Adobe Reader DC's Read Out Loud feature. It records the two lines of text in this project being read out loud, before and after converting glue to spaces.

This is a plain TeX file compiled using LuaTeX. It is for experimental use only and not intended to be a full, production-quality solution. Primarily, it is designed to assist with understanding technical issues related to accessible PDFs. The Lua code in this project is based on the earlier Overleaf article Boxes and Glue: A Brief, but Visual, Introduction Using LuaTeX.

Note

This project uses its own simple and very minimal OpenType font loader derived from this code: http://wiki.luatex.org/index.php/Use_a_TrueType_font.

Using LuaTeX to convert interword glue to spaces and kerns

%================================================
% This is a Plain TeX file compiled using LuaTeX.
% It is for experimental use only, designed for simplicity
% and to help with understanding the technical issues.
% It is not intended to be a full, production-quality 
% solution. Feel free to build on this work.
%
% PDF accessibility requirements stipulate that word 
% breaks must be represented explicitly in the PDF.
% TeX engines do not use space characters to separate
% words: they use glue. This project shows how you can
% replace interword glue with spaces and kerns 
% and not affect the typeset output. 
%
% !!!!NOTE!!!!---> 
%
% This project uses its own SIMPLE and very MINIMAL
% OpenType font loader derived from 
% http://wiki.luatex.org/index.php/Use_a_TrueType_font
%================================================

\outputmode=1
\nopagenumbers
\edef\pdfminorversion {\pdfvariable minorversion}
\pdfminorversion=5
\edef\pdfcompresslevel {\pdfvariable compresslevel}
\pdfcompresslevel=0
%====================================================

\directlua{
SPACEVECTORS={}
require("definefont.lua") % **SIMPLE ** OpenType font loader
require("lineparse.lua") % Callback to remove glue
id, error = callback.register("define_font", myfunc)
}
%
\font\foo=Merriweather-Regular.ttf at 9pt
\foo

David Robert Jones (8 January 1947 – 10 January 2016) was known professionally as David Bowie.

% Now activate the callback for the second paragraph. Here we 
% remove interword glue and replace it with space characters.
% Ask Adobe Reader to read-aloud these two paragraphs and you
% will hear the difference!
\directlua{callback.register("post_linebreak_filter", parsepara)}

David Robert Jones (8 January 1947 – 10 January 2016) was known professionally as David Bowie.

\bye