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Converting MD containing emojis to PDF

Markdown to PDF conversion is quite easy using pandoc. The real problem arises when trying to use unicode emojis. These emojis are becoming a real standard right now, and they are also useful, since not only faces and hands are represented, but also, as one can see below, they can represent common concepts such as packages, folders and files.

However useful it is, it's quite difficult to convert a markdown file containing emoji characters. You must setup special fonts, overcome lack of support by the latex engines, only to have black-and-white emojis.

The Pandoc Emoji Filter present in this repository solves all of these problems: it provides colored emojis by using known emoji SVG repositories such as twemoji and noto-emoji. Some TeX knowledge might be useful though, since there is a template.tex file needed to insert SVG images in some contexts, such as code blocks.

Features

  • Emojis can be used in normal text, like this 🍎, or like the following:

    📦package
    ┣ 📂dir1
    ┃ ┗ 📂subdir
    ┗ 📜file1

  • Codeblocks and Code can be used, like in 📂dir1, or the following code block:

    • even if it is inside a subitem

      📦package
┣ 📂dir1
┃ ┗ 📂subdir
┗ 📜file1

    This requires some messing with latex code, because some packages must be used. Specifically in this case, fvextra provides a Verbatim environment that allows the inclusion of inner commands, needed to display the image of the emoji. Thats why a template.tex must be provided to Pandoc.

  • Literal emojis like :name: are not converted by this filter... but Pandoc can handle this. Use --from markdown+emoji or --from gfm, and also the Pandoc Emoji Filter to get these converted to PDF.

    😄 ✈️

    See Non-pandoc extensions - Pandoc manual

Limitations

  • Pre/Code tags are ignored by Pandoc when converting to PDF. This filter is not supposed to do anything about this, so I won't be doing anything here about it.

    The following code block uses <pre>...</pre> tags. They will not be rendered in the PDF, but are visible in the markdown visualization.

      📦package \
  ┣ 📂dir1 \
  ┃ ┗ 📂subdir \
  ┗ 📜file1

    There should be a code block above... if you are looking at the example.pdf, you'll see nothing above.

Compiling a PDF from this readme.md file

In this project, I experimented with multiple ways to do what I wanted. First, I tried with Javascript emoji converter + Python Pandoc filter. Then, I discovered that Pandoc filters could be made in Javascript, using NodeJs, and made a completely JS solution.

Use the NodeJS solution instead of the Python as it is the one I am currently developing more actively.

Javascript + Python filter

node app.js
pandoc --template="template.tex" -o out.pdf output.md \
    --filter=svg_filter.py

Javascript only filter via NodeJs

First, install JS script dependencies using npm:

npm install

Then, run pandoc passing in the filter file name:

pandoc --template="template.tex" -o out.pdf readme.md \
    --filter=emoji_filter.js

Passing parameters:

pandoc --template="template.tex" -o out.pdf readme.md \
    --filter=emoji_filter.js -M __debug=1 -M emoji=twemoji

Debugging using VSCode:

pandoc --template="template.tex" -o out.pdf readme.md \
    --filter=emoji_filter.js -M __debug=1

Then, in VSCode, attach to Node process... the JS filter code will be waiting, and will only continue execution after the debugger is attached. If you don't attach the debugger, then it will stall in an infinite loop.

Example: compiling this readme.md

This readme.md file was compiled using the following command:

pandoc --template="template.tex" -o example.pdf readme.md \
    --filter=emoji_filter.js -M emoji=noto-emoji --from gfm

This is the resulting PDF: example.pdf

You can also run create-example.sh.

Filter parameters

Parameters are passed to the filter in the form of metadata. Pandoc receives metadata using the --metadata or -M keywords. Also, metadata can be specified inside markdown files, or YAML files via --metadata-file. See Pandoc's manual - Metadata Blocks for more info.

__debug

This is used to debug the filter using NodeJS inspector. VSCode can debug NodeJS instances, even the ones that started without debug parameters, via SIGUSR1 signal.

The problem the this flag solves it that the filter execution must wait for the debugger to be attached, otherwise it finished before the user has the opportunity to debug the code.

Usage: pass -M __debug=1 to pandoc along with other params.

emoji

This is used to select the emoji source. At this moment only two sources are available:

Usage: pass -M emoji=noto-emoji or -M emoji=twemoji to pandoc along with other params.

See Emojipedia for a list of emojis.

Changing emoji cache directory

Just set the environment variable SVG_FILTER_CACHE_DIR.

You may need to use declare -x SVG_FILTER_CACHE_DIR in your script, so that the filter can see the environment variable.

References