Why can I include tikz in a custom latex format with pdftex and not with luatex?

Question

Consider the following class.cls:

\LoadClass{article}
\RequirePackage{tikz}

and the following test.tex:

\documentclass{class}
\begin{document}
Test 
\end{document}

With pdftex (TeXLive 2016), I do:

pdftex -ini -jobname="test-fmt-pdftex" "&pdflatex" mylatexformat.ltx """test.tex"""
pdftex -fmt=test-fmt-pdftex.fmt test.tex

It works with no problem.

With luatex (TeXLive 2016), I do:

luatex -ini -jobname="test-fmt-luatex" "&lualatex" mylatexformat.ltx """test.tex"""
luatex -fmt=test-fmt-luatex.fmt test.tex

The second line ends up with an error:

! error:  (pdf backend): referenced object has wrong type others; should be obj
!  ==> Fatal error occurred, no output PDF file produced!

I know that I cannot dump in a luatex format lua code or opentype fonts but I think that tikz does not do that. There must be an other reason. Do you have any ideas?

---

I investigated a bit.

As far as I understand, there has been an updated version of pgfsys-luatex.def to comply with the changes made in luatex related to \pdf... primitives. This is one hint I can think of.

There is also this in the luatex manual:

One change involves the so called xforms and ximages. In pdfTEX these are implemented as so called whatsits. But contrary to other whatsits they have dimensions that need to be taken into account when for instance calculating optimal line breaks. In LuaTEX these are now promoted to normal nodes, which simplifies code that needs those dimensions.

But I must admit I don't really understand all that.

Answer 1

pgfsys-luatex.def calls at the end two commands

• \pgfutil@setuppdfresources and
• \pgfutil@addpdfresource@colorspaces.

If you remove these calls your example works. So there is somewhere the source of the problem. The commands call the code in \pgf@sys@setuppdfresources@plain and this set up pdf objects: disabling the obj in \pgfutil@everyby avoids the error too.

Boiling everthing a bit down one can create the following (senseless!!) minimal document:

\documentclass{book}
\usepackage{ifluatex}
\ifluatex
\pdfextension obj  reserveobjnum \edef\myobjnum{\the\numexpr\pdffeedback lastobj\relax}
\else
\pdfobj reserveobjnum \edef\myobjnum{\the\pdflastobj}
\fi
%\makeatother
\begin{document}
\ifluatex
\immediate \pdfextension obj  useobjnum \myobjnum {<</pgfprgb [/Pattern /DeviceRGB]>>}
\else
\immediate \pdfobj  useobjnum \myobjnum {<</pgfprgb [/Pattern /DeviceRGB]>>}
\fi
blub
\end{document}

It compiles fine with pdflatex and lualatex and one can open the pdf in the adobe reader (which means that it is not completly broken). With pdflatex you can put the preamble in a custom format using your compilation steps. But a compilation with luatex -fmt=myfmt.fmt test-utf8 ends with

! error:  (pdf backend): cannot find referenced object
!  ==> Fatal error occurred, no output PDF file produced!

It looks as if luatex doesn't like it if one tries to reserve objects in the format. I can't tell you if this is intended, or some side effect, or even a bug: you will have to ask on the luatex list.

Even if it works with pdftex: I would avoid to put objects in a format anyway (and so not move tikz there).