Figure code may generate wrong LaTeX code (when rendering Rmd to tex) if the fig.cap is not TeX-safe

[s-u]

Reproducible and illustrative example (test.Rmd):

---
title: test
---

```{r out.width="100%", fig.cap = "10%"}
plot(1)
``` 

then render with rmarkdown::render("test.Rmd","pdf_document").

The figure output will be offset and followed with verbatim text \begin{figure} in the PDF document.

Analysis:

When special options are needed in hook_plot_md for LaTeX output (here triggered by out.width), it will call hook_plot_tex and pass its output verbatim into the .md file - here (test.knit.md):

```r
plot(1)
```

\begin{figure}
\includegraphics[width=1\linewidth]{test_files/figure-latex/unnamed-chunk-1-1} \caption{10%}\label{fig:unnamed-chunk-1}
\end{figure}

That works in most cases, because pandoc will detect valid LaTeX and simply copy that content. However, in the above case, the caption is not TeX-safe (the % needs escaping) so pandoc will detect invalid LaTeX and treat it as simple text - escaping all the commands - (from test.tex):

\textbackslash begin\{figure\}
\includegraphics[width=1\linewidth]{test_files/figure-latex/unnamed-chunk-1-1}
\textbackslash caption\{10\%\}\label{fig:unnamed-chunk-1}
\textbackslash end\{figure\}

The cause is really hard to detect. The user may not be aware that the caption contained an unescaped % sign that caused the problem.

Solutions

There are really two related issues with possible solutions:

• fig.cap could possibly escape naked % characters when producing LaTeX code for the figure (partly to make it both html-target and PDF-target consistent). This would at least avoid the arguably most common cause of this problem (which is really hard to debug!).
• hook_plot_md might wrap the LaTeX code from hook_plot_tex in a =latex block to prevent pandoc from falling back to regular text rendering. This will make the LaTeX processing fail, but at least it is clear what happened at that point.

Note that pandoc may fail on complex LaTeX code beyond the simple issue above, so one might argue that using =latex bock is a way to go.

---

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[yihui]

I've adopted the second solution you proposed. Thanks!

[s-u]

Thanks for your prompt reply! Much appreciated! I can confirm that this fixes the issue (LaTeX will correctly fail with "runaway argument" for the above example).

[yihui]

Sorry, I just discovered that the above fix would break some reverse dependencies. I'll investigate and may consider switching to the first solution (but one tricky thing is to avoid escaping % that has already been escaped by the author).

[s-u]

What about gsub("(?<!\\\\)%", "\\\\%", "cap 10% but not 90\\%", perl=TRUE)?

[yihui]

Yes, that's what I was thinking.

[yihui]

Just committed the new fix. Thanks!

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