Incorrect block shading with XeLaTeX

[leo-liu]

The latest release (3.51) and dev version (82101f9cba398faa182df9a7bde80de9cd8851fb) may produce incorrect block shading with XeLaTeX.

It seems caused by pull request #469.

MWE:

\documentclass{beamer}
\usetheme{Warsaw}
\begin{document}

\begin{frame}
\begin{block}{Title}
text
\end{block}

\begin{block}{}
TEXT
\end{block}
\end{frame}

\begin{frame}
\begin{theorem}
text

text

text

text
\end{theorem}
\end{frame}

\end{document}

[josephwright]

I agree that the issue comes from the pull request. Looking at the code there, everything is done 'properly' using pgf interfaces: I don't think the logic in beamer is the issue. Rather, there must be something up in pgf with the operations used (or it could be (x)dvipdfmx). I'll think a little: in a way, this is 'not a beamer bug', but probably the outcome means we'll have to back out.

[ghost]

@josephwright Maybe related discussion: https://github.com/josephwright/beamer/issues/161

[ghost]

Not only xelatex is affected, also the latex->dvi->ps->pdf route, see https://tex.stackexchange.com/questions/448709/beamer-warsaw-how-to-have-a-rounded-shadow

[josephwright]

@samcarter8 Quite possibly. As I say, the problem is that if you look at the code, there is nothing 'low level', it's all \pgf.... So the calculations are right in beamer.

[leo-liu]

AFAIK, PostScript doet not support transparency. I guess that the system layer of pgf for dvipdfmx also has some issue.

Anyway, a possible workaround for beamer is to fallback to old pgf code when pdfTeX/LuaTeX is not the engine.

[josephwright]

@leo-liu PostScript doesn't support transparency, but PDFs out of the process can still show it (see e.g. https://tex.stackexchange.com/questions/172764/opacity-and-transparency, using PSTricks). I agree though that there are issues in pgf here.

I'm not that keen on trying to have entirely separate code paths: I'll probably back out the change for the present, and see if I can track down the underlying issue.

[samcarter]

@u-fischer found a way to get the transparent shadows to work with xelatex by using unique names for the shadings, some code for testing can be found in this branch:

https://github.com/samcarter/beamer/tree/resuscitating_transparent_shadows

(needs the pgf dev version due to the problem with the shifted shadows)

Risk: for each new width or height an additional shading will be created - which at one point will probably cause problems if there are too many (could not find any problems when testing with a couple of thousand randomly sized boxes, no idea how far this will work)

@josephwright @louisstuart96 What do you think? Is this too risky? Otherwise one could also have this as a new box type, transparentshadowbox or something ...

Note to self: test file document.txt

[dbitouze]

@samcarter Would this code fix the issue noticed in the comment of https://tex.stackexchange.com/a/448720/18401?

(Sorry, I cannot test myself because I currently cannot test the pgf dev version.)

[samcarter]

@samcarter Would this code fix the issue noticed in the comment of https://tex.stackexchange.com/a/448720/18401?

This will work for the upcoming pgf version (the one currently distributed in texlive has an issue with the offset of shadings)

[u-fischer]

@dbitouze you need from the pgf-dev actually only this redefinition (and only if you use xelatex/xdvipdfmx):

\makeatletter

\def\pgfutil@insertatbegincurrentpagefrombox#1{%
  \edef\pgf@temp{\the\wd\pgfutil@abb}%
  \global\setbox\pgfutil@abb\hbox{%
    \unhbox\pgfutil@abb%
    \hskip\dimexpr2in-2\hoffset-\pgf@temp\relax% changed
    #1%
    \hskip\dimexpr-2in-2\hoffset\relax% new
  }%
}

[dbitouze]

@u-fischer And what about LuaLaTeX?

[dbitouze]

@u-fischer and @samcarter This redefinition doesn't fix the issue noticed in the comment of https://tex.stackexchange.com/a/448720/18401, both with xelatex and lualatex (not sure whether I should open a separate issue or not):

\documentclass{beamer}
\usepackage{pgf}

% add a background image
\setbeamertemplate{background canvas}{\includegraphics[height=\paperheight]{example-image}}

% use rounded blocks with shadow
\setbeamertemplate{blocks}[rounded][shadow=true]

\makeatletter
\def\pgfutil@insertatbegincurrentpagefrombox#1{%
  \edef\pgf@temp{\the\wd\pgfutil@abb}%
  \global\setbox\pgfutil@abb\hbox{%
    \unhbox\pgfutil@abb%
    \hskip\dimexpr2in-2\hoffset-\pgf@temp\relax% changed
    #1%
    \hskip\dimexpr-2in-2\hoffset\relax% new
  }%
}
\makeatother

\begin{document}
\begin{frame}
  \frametitle{Block with shadow and with background image}

  \begin{block}{Block with shadow}
    Content...
  \end{block}
\end{frame}
\end{document}

[u-fischer]

@dbitouze no, this fix is for displaced shadings, to correct the transparent shadow you need the code @samcarter is just including into beamer.

[samcarter]

@dbitouze Works fine for me with the beamer branch I linked to

[samcarter]

@u-fischer And what about LuaLaTeX?

there was no problem with shadows in lualatex

[dbitouze]

@dbitouze Works fine for me with the beamer branch I linked to

Ooops, sorry.

there was no problem with shadows in lualatex

I guess you mean for displaced shadings, not for correct transparent shadow (the MCE I provided above points out a problem for transparent shadow with lualatex as well).

[samcarter]

I guess you mean for displaced shadings, not for correct transparent shadow (the MCE I provided above points out a problem for transparent shadow with lualatex as well).

Transparent shadows did work in lualatex, but because the code did not work in xelatex, it had to be replaced with the white shadows

[dbitouze]

Transparent shadows did work in lualatex, but because the code did not work in xelatex, it had to be replaced with the white shadows

Too bad! It would have been nice to make use of iftex package and \ifxetex...\fi in order to affect only xe(la)tex.

[samcarter]

@josephwright @louisstuart96 Now that the new pgf version is available, any opinions on https://github.com/josephwright/beamer/compare/master...samcarter:resuscitating_transparent_shadows ? Take the chance and merge this into beamer? Or add it as a new transparent shadows outer theme?

[josephwright]

@samcarter I think we risk it and if necessary roll-back.

[samcarter]

@josephwright Great! I'll push the necessary chances