includegraphics with pdf can create XObject with multiple transparency groups

[lukas-code]

steps to reproduce

Step 1: Obtain a PDF that specifies a transparency group in the page dictionary.

In my case this was a file created by cairo (via Firefox' print to PDF feature), but we can also create one with LaTeX as follows:

% file foo.tex

\RequirePackage{pdfmanagement}
\SetKeys[document/metadata]{
    pdfversion=1.7,
    lang=en-US,
    uncompress,
}

\documentclass{standalone}

\ExplSyntaxOn
\pdfmanagement_add:nnn{Page}{Group}{<<~/S~/Transparency~/CS~/DeviceRGB~/I~true~>>}
\ExplSyntaxOff

\begin{document}
    foo
\end{document}

compile with pdflatex foo.tex to get a foo.pdf with

7 0 obj
<<
  /Type /Page
  % ...
  /Group << /S /Transparency /CS /DeviceRGB /I true >> 
  % ...
>>

Step 2: Include foo.pdf via \includegraphics.

% file bar.tex

\RequirePackage{pdfmanagement}
\SetKeys[document/metadata]{
    pdfversion=1.7,
    lang=en-US,
    uncompress,
}

\documentclass{article}
\usepackage{graphicx}

\begin{document}
    bar

    \includegraphics{foo.pdf}
\end{document}

compile with pdflatex bar.tex (twice) and we get a PDF with an XObject that specifies multiple transparency groups:

5 0 obj
<<
  /Group<</S/Transparency/K false/I false>>
  /Type /XObject
  % ...
  /Group 11 0 R
  % ...
>>
% ...
11 0 obj
<< /S /Transparency /CS /DeviceRGB /I true >>

When using lualatex instead of pdflatex the order of the /Group entries changes.

---

This causes tools like qpdf or verapdf to complain, but for some reason the PDF still validates.

$ qpdf bar.pdf /dev/null
WARNING: bar.pdf (object 5 0, offset 374): dictionary has duplicated key /Group; last occurrence overrides earlier ones
$ ~/verapdf/verapdf bar.pdf
WARNING: Dictionary/Stream contains duplicated key /Group(object key = 5 0 obj, offset = 875)

---

likely regressed in: latex3/latex3@4c017aa

likely related: https://tex.stackexchange.com/questions/755198/latex-verapdf-dictionary-stream-contains-duplicated-key-group https://tex.stackexchange.com/questions/758882/latex-verapdf-warning-duplicated-key-group

[josephwright]

I've moved here as the cod to implement \includegraphics isn't currently using expl3.

[josephwright]

We added the grouping in the first place as otherwise you can't reliably apply a transparency to an included PDF graphic. We could provide a key to disable to grouping, but I don't think we can tell from within the TeX run that there's a group already present: so automation is not realistic.

[davidcarlisle]

Couldn't we in principle add the transparency group in its own object containing the original so you end up with nested transparency groups rather than one dictionary with the Group key duplicated?

[josephwright]

Couldn't we in principle add the transparency group in its own object containing the original so you end up with nested transparency groups rather than one dictionary with the Group key duplicated?

Ah, good idea: I'll investigate.