Presentations in (R)markdown

There are many ways to turn a markdown or Rmarkdown document into a presentation. Way too many, and none of them is perfect. I made my first presentation with knitr / Rmarkdown for the tmod package.

After trying various options in knitr, I decided on an approach in which the Rmarkdown document is oblivious of the presentation system and the job of turning it into a presentation is taken up by pandoc. There were several bumps and problems, and I will give now a step – by – step guide.

1. Input file

Let’s start with an example Rmd. In the following, I assume it has been saved under “test.Rmd”.

---
title: "Example presentation"
author: January Weiner 
date: "`r Sys.Date()`"
---

# First part
## Slide 1
Code:

```{r plot1}
plot(1:10, 1:10)
```

## Slide 2
Some maths: $sum_{i=1}^{N}$

# Second part
## Slide 3
... contents ...

2. From Rmarkdown to markdown

I use knitr only to create a markdown file.

Rscript -e 'knitr::knit("test.Rmd")'

This produces the file test.md. With that, knitr’s job is finished, we will not need it anymore.

3. Download reveal.js

I decided for reveal.js. It was easy to work with and adapt to my needs, it had elegant default themes, it has a low footprint and shortcuts. And it has the “2D” layout, meaning that sections (level one headers) are arranged horizontally, while slides within one section are arranged vertically. Pressing “Esc” in a presentation shows the slide overview:

reveal_example

Anyway, download reveal.js and unpack it in the same directory as test.md.

Making the presentation

Use pandoc to create the reveal.js presentation. Note that this is not the final command line; in the following points I will discuss the problems which will influence the final version.

pandoc -s -S -t revealjs --mathjax -o test.html test.md

4. MathJax

On slide 2, we have a bit of maths. The maths is written in a LaTeX-like notation, and there are many ways to turn it into an elegant mathematical equation on the final presentation. I have tried many options with pandoc, and found that only MathJax works properly and without a major hassle. This is why on the previous command line I used the option --mathjax.

However, if you run the above command line, you will notice that on “Slide 2”, the maths doesn’t work, despite using the ‘–mathjax’ option. It would work, though, if we put the file on a server. The reason is that pandoc puts the URL to MathJax in the form ‘src=”//cdn.mathjax…”‘. This assumes the context of how we opened the file. If we opened it from a server, using http or https, this would have worked. If we open it directly in a browser, it uses “file://cdn.mathjax…” which is obviously not on our file system. We have two options.

4.1 External MathJax

Use the command line

pandoc -s -S -t revealjs --mathjax="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML" -o test.html test.md

This works unless we have no Internet access, for example because we show our presentation in another institute, where our laptop cannot connect to the Internet, because then we are screwed.

4.2 Local MathJax

Alternatively, you can download the whole MathJax:

wget https://github.com/mathjax/MathJax/archive/v2.5-latest.zip
unzip v2.5-latest.zip
mv MathJax-2.5-latest/ MathJax

and specify the local installation with the following command line:

pandoc -s -S -t revealjs --mathjax="MathJax/MathJax.js?config=TeX-AMS-MML_HTMLorMML" -o test.html test.md

This works, but our presentation has suddenly over 170 megabytes. Which sucks.

5. 2D layout and section headers

I mentioned previously that reveal.js allows a neat 2D layout, in which slides from one section are arranged vertically, and sections are put next to each other. However, sections with only a title and no contents might be a bit boring, so let us modify the .md file changing the second section as follows:

# Second part

This is the second part, even more interesting.

## Slide 3
... contents ...

You run pandoc again, and…

reveal2

Huh, where is the 2D layout gone? Why are all slides next to each other? Why are all slides from one section all on one single slide?

Pandoc automatically guesses which level header denotes boundaries between slides. It defines “slide level” as “the highest level followed immediately by non-header contents”. After our modification, the top level header (starting with a single #) became the level at which slides are separated. OK, so maybe we try specifying the slide level manually?

pandoc -s -S -t revealjs --mathjax="MathJax/MathJax.js?config=TeX-AMS-MML_HTMLorMML" -o test.html test.md

OK, this works, but… the contents under the first level header (“This is the second part…”) is gone! This is because “Headers above the slide level in the hierarchy create “title slides,” which just contain the section title and help to break the slide show into sections.”

Turns out that there is no way we can have both: 2D with slides divided neatly into sections, and section slides which contain more than just a title. Not if we use pandoc, that is.

6. Modifying the layout

6.1 reveal.js theme

This is the easiest part: pick one of the existing reveal.js themes (I omit the mathjax command line for simplicity sake, do remember to put it back in):

pandoc -s -S -t revealjs -o test.html test.md -V theme=blood

Note that the themes listed on the reveal.js website start with a capital letter, but you must specify a lowercase letter in the above command line.

6.2 Fine tuning the theme

I did not like the sans-serif, capitalized and decorated fonts of the blood theme (shadows on titles, I beg you). Ugly. However, if you know a little CSS (and you’d better learn it!), you can easily adapt it to your needs.

Look up the file reveal.js/css/theme/blood.css for hints and create your own CSS file (let us call it test.css) in the same directory as test.md. In the file below, I reset all the ugly decorations and set two fonts for headers and body, respectively: Garamond for headers, and Quattrocento Sans for body, using the google fonts service:

@import url('http://fonts.googleapis.com/css?family=EB+Garamond');
@import url('http://fonts.googleapis.com/css?family=Quattrocento+Sans');

.reveal {
  font-size: 32px;
  font-family: 'Quattrocento Sans', 'sans-serif'; }

.reveal h1, .reveal h2, .reveal h3, .reveal h4, .reveal h5, .reveal h6 {
  font-family: 'EB Garamond', 'serif';
  font-weight:normal;
  text-transform: none;
  text-shadow: none; }

.reveal h1 { font-size: 2em; }
.reveal h2 { font-size: 1.7em; }
.reveal h3 { font-size: 1.4em; }
.reveal h4 { font-size: 1em; }

Also, as you might notice, I prefer smaller fonts here. We integrate our test.css file with the following option

pandoc -s -S -t revealjs -o test.html test.md -V theme=blood --css test.css

6.3 Adding a logo

You can add a logo (or whatever other background for your slides) by modifying the CSS file test.css. If logo.png is the name of your logo, adding this to your CSS will put it on all your slides in the top left corner:

body {
  background-image: url(logo.png);
  background-repeat: no-repeat;
  background-position:20px 20px;
}

6.4 Better syntax highliting

Pandoc’s syntax highlighting doesn’t look good on a dark background. You can add the following to the “test.css” file to reproduce the Solarized theme.

.reveal pre code { color: #839496; 
  background-color: #2B2B2B; } /* use #FDF6E3 for light background */

.sourceCode .kw { color: #268BD2; }
.sourceCode .dt { color: #268BD2; }
.sourceCode .dv, .sourceCode .bn, .sourceCode .fl { color: #D33682; }
.sourceCode .ch { color: #DC322F; }
.sourceCode .st { color: #2AA198; }
.sourceCode .co { color: #93A1A1; }
.sourceCode .ot { color: #A57800; }
.sourceCode .al { color: #CB4B16; font-weight: bold; }
.sourceCode .fu { color: #268BD2; }
.sourceCode .re { }
.sourceCode .er { color: #D30102; font-weight: bold; }
}

# 7. Creating a PDF of your presentation

Of course you need a PDF for printing and as a backup.

There are two ways for producing PDF from reveal.js. Each one is imperfect. 

## 7.1 Creating PDF using pandoc

Since the `test.md` file is a generic markup, we can turn it into a simple PDF

```bash
pandoc -s -S -o test.pdf test.md

Or even beamer presentation:

pandoc -s -S -t beamer -o test.pdf test.md

Unfortunately, this is not so nice as our presentation, and completely ignores whatever we have put in the CSS.

7.2 Using the reveal.js printing facility and Google Chrome

The second way is interactive only (you cannot create the PDF with a command line). Open the file in google chrome and add ?print-pdf to the file URL, such that the end of the URL reads test.html?print-pdf.

The output looks garbled: the slides overlap. Don’t worry, it’s OK. Open the print dialog (press Ctrl-P), and you will see that now the output is correct. You can save it as PDF or send it to a printer.

8. The final command line

pandoc -s -S -t revealjs --mathjax="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"  -V theme=blood --css test.css -o test.html test.md
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