xpdftex

Experiments in emulating pdftex

This is a sketch of configuring luatex to be closer to pdftex to allow legacy pdflatex documents to be processed while still accessing internals via Lua for PDF Tagging, MathML or other reasons.

Some differences in formatting may still exist (different underlying hyphenation mechanism, and some more obscure pdftex extensions are harder to emulate).

Test documents and other documents explicitly testing definitions with \meaning or \ifprimitive can detect the emulation, but most "normal" documents should (eventually) work without error.

luatex is mostly hidden so that packages such as graphics/hyperref etc will choose a pdftex back end. The command \@@!directlua is available as a back door to Lua (and from there you can use tex.enableprimitives to access any luatex functionality).

Sources

Mostly the pdftex primitive emulations are taken from luatex85 (they could be further hidden using Lua definitions).
The 8-bit input/output is taken from luainputenc (via dpc-inputenc-tests github)
The Lua definitions of the pdftex primitives are used from expl3.lua (which is loaded)
shell escape from the shellescape package.

Partially implemented

tex--xet so no \beginL etc (Code by Marcel in xet--tex branch)

NOT DONE

logging (probably just in l3build normalisation)
\eTeXgluestretchorder
pdftex space primitives \pdfadjustinterwordglue,
\pdfmatch

Usage (without installing)

luatex -ini xpdflatex.ini

should make a xpdflatex.fmt format and

luatex \&xpdflatex test-latin-latin1.tex

should process an example document.

Integration into TeXLive

Install the files with l3build install
Create binaries
- on windows make copies from the small (2kb) luahbtex.exe (or luatex.exe if you want to use that engine) in ...\texlive\2022\bin\win32 and call them xpdflatex.exe and xpdflatex-dev.exe.
- on other systems you should be able to make a symbolic link of luahbtex (or luatex) so in .../texlive/2022/bin/* execute ln -s luahbtex xpdflatex and ln -s luahbtex xpdflatex-dev or equvalent commands for your operating system.
Add entries to a fmtutil.cnf in texmf-local/web2c (create it if needed):

A sample fmtutil.cnf is included in the repository

The -dev version is optional, and instead of the engine luahbtex one could use luatex (currently unclear which is better).

Add search paths to a local texmf.cnf, e.g. the one in texlive/2022:

Sample lines are in texmf-sample.cnf
Create the formats with

fmtutil-sys --byfmt xpdflatex
fmtutil-sys --byfmt xpdflatex-dev

The commands xpdflatex and xpdflatex-dev should then be available.