"fj" Ligature in Computer Modern

after having had used STIX2 which looks good but a little too heavy/bold for my taste, I had a dream yesterday night. It was the solution. By using a hack with manual kerning and creation of the fj ligature and extraligfj command and with LuaLaTeX on-the-fly text-replacement I have created a seamless solution.

ProvidesPackage{extralig}
usepackage{xcolor}
usepackage{luacode}
newcommand{extraligfj}{fillap{textcolor{white}{rule[-0.05em]{0.252em}{0.55em}}}kern-0.01emllap{j}kern-0.05emrelax}
begin{luacode}
local function vartosrcvar ( line )
return string.gsub(line, "fj" , "extraligfj{}")
end
luatexbase.add_to_callback( "process_input_buffer",  vartosrcvar, "var_to_srcvar")
end{luacode}

By saving this code as extralig.sty in your .tex file directory and calling it with usepackage{extralig} it will work perfectly!

Obviously because of the Lua code you must compile this with LuaLaTeX

You are not required to use this as a package, simply copy everything into your document in corresponding places and it will work fine too but its much cleaner to have it as a seperate package.

How does this work?

• I define extraligfj, a hack where the i in fi is replaced with a j, making a perfect fj ligature with some manual kerning. (Thanks to this TeX.SX post!)
• With LuaLaTeX, I use code which replaces all instances of fj with extraligfj, which we defined before.
• This is all placed neatly inside of extralig.sty and ready to be seamlessly loaded into our document as a package.

TL;DR Save the code snippet as extralig.sty and place it the same directory as your .tex file and load it with usepackage{extralig}. use LuaLaTeX. This is only guaranteed to work with Computer Modern, no other fonts (they will need manual kerning adjustment). Doing this then fj will automagically be ligatured.

Pros:

• I was expecting there to be some really weird glitches with linebreaks due to the nature of the hacked glyph, but to my surprise, it works perfectly even when broken in lines at f and j
• Only need to compile once because LuaTeX runs it on the fly, which is why its fully seamless, no slowdowns.