Add ability to pass flags to dvisvgm when using tikz engine?

[andrewheiss]

Currently , if the tikz engine is used and fig.ext = "svg", dvisvgm is called to convert a DVI version of the TikZ chunk to SVG. The shell command for dvisvgm is hard-coded in the engine function:

knitr/R/engine.R

Line 309 in 55a2df9

if (system2('dvisvgm', c('-o', shQuote(fig2), fig)) != 0)

This makes it impossible to add any additional flags to the command. One situation where this might be useful is when embedding fonts. According to the first FAQ at dvisvgm's list, most web browsers and vector editors struggle with embedded fonts from TikZ output. This causes issues with tikz chunks too. For instance, here's a simple reprex:

tikz-example.Rmd:

---
title: "tikz font fun"
output:
  html_document: default
---

```{r setup, include=FALSE}
# Specify location of libgs.dylib on macOS with MacTeX-2021
Sys.setenv(LIBGS = "/usr/local/share/ghostscript/9.53.3/lib/libgs.dylib.9.53")
```

```{tikz, test-dag, echo=FALSE, cache=TRUE, fig.ext="svg"}
\usetikzlibrary{positioning}
\begin{tikzpicture}[every node/.append style={draw, minimum size=0.5cm}]
\node[draw=none] (X) at (0,0) {$X_{it}$};
\node[draw=none] (Y) at (2,0) {$Y_{it}$};
\node[rectangle] (Z) at (1,1) {$Z$};
\path [-latex] (X) edge (Y);
\draw [-latex] (Z) edge (Y);
\draw [-latex] (Z) edge (X);
\end{tikzpicture}
```

That will knit to HTML and include an SVG file, but the resulting SVG will use placeholder fonts:

If you open the resulting SVG file in Illustrator or Inkscape, it will complain about missing fonts.

If you run dvisvgm on the cached DVI file from the terminal and include --no-fonts, text will be converted to outlines instead of embedded as text, and the resulting SVG will look 99% correct (the conversion to outlines isn't perfect, but it's close enough):

dvisvgm --no-fonts tikz-example_files/figure-html/test-dag-1.dvi

It might be helpful to allow users to add additional options to dvisvgm when using the tikz engine, or perhaps just have a logical flag for embedding fonts or not. Within the eng_tikz() function, converting the tikz chunk to PNG allows for additional options from options$engine.opts$convert.opts:

knitr/R/engine.R

Lines 311 to 315 in 55a2df9

	} else {
	# convert to the desired output-format using magick
	if (ext != 'pdf') magick::image_write(do.call(magick::image_convert, c(
	list(magick::image_read_pdf(fig), ext), options$engine.opts$convert.opts
	)), fig2)

I wonder if a similar approach could be used for SVG output, so users could do something like engine.opts = list(embed_fonts = FALSE) or something similar.

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[andrewheiss]

Or even better, the --font-format=woff option for dvisvgm will embed the fonts in the SVG file as base64-encoded web fonts, which makes the resulting SVG file work universally:

dvisvgm --font-format=woff tikz-example_files/figure-html/test-dag-1.dvi

First part of test-dag-1.svg:

<?xml version='1.0' encoding='UTF-8'?>
<!-- This file was generated by dvisvgm 2.11.1 -->
<svg version='1.1' xmlns='http://www.w3.org/2000/svg' xmlns:xlink='http://www.w3.org/1999/xlink' width='69.540777pt' height='39.78342pt' viewBox='-68.679588 -72.00005 69.540777 39.78342'>
<style type='text/css'>
<![CDATA[@font-face{font-family:cmmi7;src:url(data:application/x-font-woff;base64,d09GRgABAAAAAATMAA0AAAAABkAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABGRlRNAAABMAAAABoAAAAcjlK29U9TLzIAAAFMAAAAQgAAAFZVmV5oY21hcAAAAZAAAABLAAABSgCUCCBjdnQgAAAB3AAAAAQAAAAEACECeWdhc3AAAAHgAAAACAAAAAj//
...

Somehow allowing for the --font-format option in if (system2('dvisvgm', c('-o', shQuote(fig2), fig)) != 0) would be fantastic and would make it so SVG-based tikz files would be fairly universally usable

[andrewheiss]

As an attempted workaround to avoid making any changes to the core knitr code, I tried creating a function for use in fig.process that would take the DVI and rerun it through dvisvgm with the --font-format option, thus overwriting the original non-embedded SVG, but I can never get the dvi2svg() function to actually ever trigger when knitting. This could theoretically fix the issue:

```{r setup, include=FALSE}
# Specify location of libgs.dylib on macOS with MacTeX-2021
Sys.setenv(LIBGS = "/usr/local/share/ghostscript/9.53.3/lib/libgs.dylib.9.53")

dvi2svg <- function(path) {
  path_svg <- xfun::with_ext(path, "svg")
  path_dvi <- xfun::with_ext(path, "dvi")

  if (system2('dvisvgm', c('--font-format=woff', '-o', shQuote(path_svg), shQuote(path_dvi))) != 0)
      stop('Failed to convert ', path_dvi, ' to ', path_svg)
  path_svg
}
```

```{r, engine="tikz", test-dag, echo=FALSE, cache=TRUE, fig.ext="svg", dev="tikz", fig.process=dvi2svg}
\usetikzlibrary{positioning}
\begin{tikzpicture}[every node/.append style={draw, minimum size=0.5cm}]
\node[draw=none] (X) at (0,0) {$X_{it}$};
\node[draw=none] (Y) at (2,0) {$Y_{it}$};
\node[rectangle] (Z) at (1,1) {$Z$};
\path [-latex] (X) edge (Y);
\draw [-latex] (Z) edge (Y);
\draw [-latex] (Z) edge (X);
\end{tikzpicture}
```

[andrewheiss]

Apologies for yet another comment! Here's a working, slightly hacky workaround/solution that uses a chunk hook to rerun dvisvgm on the DVI file that knitr creates. It's roundabout, but it works. It still might be worth adding the ability to include --font-format=woff or --no-fonts in the system2() command.

tikz-example.Rmd:

---
title: "tikz font fun"
output:
  html_document: default
---

```{r setup, include=FALSE}
# Specify location of libgs.dylib on macOS with MacTeX-2021
Sys.setenv(LIBGS = "/usr/local/share/ghostscript/9.53.3/lib/libgs.dylib.9.53")

embed_svg_fonts <- function(before, options, envir) {
  if (!before) {
    paths <- knitr:::get_plot_files()

    knitr:::in_base_dir(
      lapply(paths, function(x) {
        message("Embedding fonts in ", x)
        path_svg <- xfun::with_ext(x, "svg")
        path_dvi <- xfun::with_ext(x, "dvi")
        
        if (system2('dvisvgm', c('--font-format=woff', '-o', shQuote(path_svg), shQuote(path_dvi))) != 0)
          stop('Failed to convert ', path_dvi, ' to ', path_svg)
      })
    )
  }
}

knitr::knit_hooks$set(embed_svg_fonts = embed_svg_fonts)
```

```{tikz, test-dag, echo=FALSE, fig.ext="svg", embed_svg_fonts = TRUE}
\usetikzlibrary{positioning}
\begin{tikzpicture}[every node/.append style={draw, minimum size=0.5cm}]
\node[draw=none] (X) at (0,0) {$X_{it}$};
\node[draw=none] (Y) at (2,0) {$Y_{it}$};
\node[rectangle] (Z) at (1,1) {$Z$};
\path [-latex] (X) edge (Y);
\draw [-latex] (Z) edge (Y);
\draw [-latex] (Z) edge (X);
\end{tikzpicture}
```

[yihui]

Please feel free to submit a PR. It appears simple enough to implement---perhaps add something like options$engine.opts$dvisvgm.opts here? Thanks!

knitr/R/engine.R

Line 309 in 55a2df9

if (system2('dvisvgm', c('-o', shQuote(fig2), fig)) != 0)

[andrewheiss]

Whoa, that was a super easy fix. PR #2039 fixes this issue and now this chunk will automatically embed fonts without needing the knit hook workaround above:

```{tikz example, fig.ext="svg", engine.opts=list(dvisvgm.opts = "--font-format=woff")}
\usetikzlibrary{positioning}
\begin{tikzpicture}
\node (X) at (0,0) {$X_{it}$};
\end{tikzpicture}
```

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