attr-color.lua

A Pandoc filter which sets text/background/frame color(s) on Span and Link elements based on custom Pandoc attributes.

Version

This document describes version 2020121418 of the filter.

Usage

pandoc -L attr-color.lua -o document.ltx document.md

pandoc -L attr-color.lua -o document.pdf document.md

pandoc -L attr-color.lua -o document.html document.md

To colorize a Span or Link set one or more of the following custom attributes on the Span or Link:

Attribute	Sets color of
fg	foreground (text)
bg	background
fr	frame

The value of the attributes should be one of

Notation	Description
"#rrggbb"	RGB color in HTML notation
"#rgb"	Short HTML RGB notation
ColorName	xcolor/CSS color name.

A ColorName should be:

• If you are producing LaTeX: a color name from one of the predefined xcolor named color sets (described in the section of the xcolor documentation called Colors By Name) loaded by Pandoc's latex template (unless you want to go through the hassle of injecting your own color definitions into the LaTeX preamble!), such as ForestGreen.

• If you are producing HTML: an HTML or CSS color name.

Configuration

The filter can be configured by setting certain variables (strings, or in one case also a boolean) in the document metadata or in the environment. If both the metadata value and the environment variable are defined the metadata takes precedence.

Meta	Environment	Default	Description
attr_color_format	PDC_ATTR_COLOR_FORMAT	Current output format	Markup format. A format known to the filter.
attr_color_keep_attrs	PDC_ATTR_COLOR_KEEP_ATTRS	'false'	Whether to keep the fg, bg, fr attributes (false/true).
attr_color_fallback_format	PDC_ATTR_COLOR_FALLBACK_FORMAT	'html'	Format to fall back to if (output) format is unknown.

Markup formats vs. Pandoc output formats

The filter "knows", i.e. has code to handle only some Pandoc output formats, currently

• latex
• html

Usually the filter detects whether the current Pandoc output format is a known format and falls back to html otherwise. Unlike latex the html mode does not inject any raw markup into the document, but sets a CSS style attribute on the span or link. This makes it a "safe" default, since Pandoc will just ignore that attribute for output formats which don't use CSS styling.

If you for example want to inject the literal raw LaTeX code into your Markdown document:

[*Foo*]{fg=red}

`\textcolor{red}{`{=latex}*Foo*`\strut}`{=latex}

you can call pandoc like this:

pandoc -L attr-color.lua -M attr_color_format=latex document.md -o md-with-latex.md

Note that if the format is unknown to the filter (i.e. currently anything except latex or html) the filter will fall back on the fallback format as described under Configuration (by default html and if that is also unknown the filter will not do anything.

RGB notation

In the #rrggbb notation each r/g/b stands for an hexadecimal digit 0-9a-z, two digits for each of the red, green and blue component of the color. The #rgb notation is equivalent to an #rrggbb where both digits in each component are identical, so that for example blue, #00f and #0000ff are equivalent.

Unquoted color names

As implied in the table Pandoc generally lets you leave color names, which contain only ASCII letters and digits, unquoted as attribute values, but the RGB formats need to be quoted because of the # character.

Examples

Markdown	LaTeX
[Foo]{fg=red}	\textcolor{red}{Foo\strut}
[Foo]{fg="#f00"}	\textcolor[HTML]{FF0000}{Foo\strut}
[Foo]{fg="#ff0000"}	\textcolor[HTML]{FF0000}{Foo\strut}
[Foo]{fg="#F00"}	\textcolor[HTML]{FF0000}{Foo\strut}
[Foo]{bg=green}	\colorbox{green}{Foo\strut}
[Foo]{bg=yellow fr=blue}	\fcolorbox{blue}{yellow}{Foo\strut}
[Foo]{fg=red bg=yellow fr=blue}	\fcolorbox{blue}{yellow}{\textcolor{red}{Foo\strut}}
[Foo]{fg=red fr=blue}	{\color{blue}\fbox{\textcolor{red}{Foo\strut}}}
[Foo]{fr=red}	{\colorlet{curcolor}{.}\color{red}\fbox{\color{curcolor}Foo\strut}}

Markdown	Markdown/CSS
[Foo]{fg=red}	[Foo]{style="color: red;"}
[Foo]{fg="#f00"}	[Foo]{style="color: #f00;"}
[Foo]{fg="#ff0000"}	[Foo]{style="color: #ff0000;"}
[Foo]{fg="#F00"}	[Foo]{style="color: #F00;"}
[Foo]{bg=green}	[Foo]{style="padding: 0.1em; background-color: green;"}
[Foo]{bg=yellow fr=blue}	[Foo]{style="padding: 0.1em; background-color: yellow; border: 0.01em solid blue;"}
[Foo]{fg=red bg=yellow fr=blue}	[Foo]{style="padding: 0.1em; color: red; background-color: yellow; border: 0.01em solid blue;"}
[Foo]{fg=red fr=blue}	[Foo]{style="padding: 0.1em; color: red; border: 0.01em solid blue;"}
[Foo]{fr=red}	[Foo]{style="padding: 0.1em; border: 0.01em solid red;"}

Markdown	Rendered
[Foo]{fg=red}	Foo
[Foo]{fg="#f00"}	Foo
[Foo]{fg="#ff0000"}	Foo
[Foo]{fg="#F00"}	Foo
[Foo]{bg=green}	Foo
[Foo]{bg=yellow fr=blue}	Foo
[Foo]{fg=red bg=yellow fr=blue}	Foo
[Foo]{fg=red fr=blue}	Foo
[Foo]{fr=red}	Foo

As you can see the LaTeX for frames without background color is a hack. It could certainly be wrapped in a command, but in that case it would be necessary to get that command into the LaTeX preamble, which is non-trivial. Both the filter code and the LaTeX code would be even prettier if the adjustbox package were used, but again that would mean getting the \usepackage{adjustbox} into the preamble.

Links

Note that if you use this filter to color links you should not use the hyperref link coloring feature as triggered by colorlinks and other Pandoc variables since the two methods clash.

Dependencies

• A version of pandoc which supports Lua filters.
• The xcolor package. If you produce standalone LaTeX with pandoc, such as when producing PDF with Pandoc/LaTeX, the latex template loads xcolor automatically, along with the dvipsnames, svgnames and xllnames. Some older versions of pandoc may not be fully supportive, but of course it is a good idea to update to the latest anyway.

Todo

• Add the necessary metadata trickery to at least optionally use adjustbox or at least to inject the code for a \colorfbox macro so that the LaTeX source doesn't look so ugly.

• Add an HTML mode which translates the attributes to inline CSS style attributes.

Bugs

Report them at https://github.com/bpj/pandoc-attr-color/issues

Author and license

This software is Copyright (c) 2020 by Benct Philip Jonsson.

This is free software, licensed under:

The MIT (X11) License

http://www.opensource.org/licenses/mit-license.php