LuaLaTeX with OpenType and microtype, create microtype configuration on the fly?

Question

I am using LuaLaTeX (>=2022), OpenType fonts, microtype. Platform is Linux. I have permission to write files in the same directory (or subdirectory) as my main *.tex document. I cannot use --shell-escape, but I can use Lua io.open, write, read, close, as needed.

Although I am using a custom document class, this question should be applicable to standard classes. I have looked through the microtype documentation, and the fontspec documentation, without seeing what I need. I did find the Lua script sidebearings.lua by Philipp Gesang @phi-gamma which is functional, and possibly useful as a component of what I need.

Instead of reading a microtype.cfg file (standard, or one of my own design), I would like to (a) load the font using fontspec, (b) Get the side bearings of a few characters of my choice, (c) Create a custom-microtype.cfg on-the-fly, either in memory or written, (d) Use that configuration when I load microtype. In this regard, I do not load microtype until \AtBeginDocument.

Why I wish to do this: The output PDF will be robot-inspected for minimum margins. If a letter, such as lowercase j, intrudes into the margin, then the file will be rejected. It does not help to set protrusion=false, due to font kerning (which is required). If the document always used one or two fonts, I could manually learn what I need. But the document may use any OpenType fonts, including professional (not free) fonts that I cannot access, because there are other users.

I propose to use sidebearings.lua to discover the numbers, then nullify negative side bearings, using negative protrusion in a custom microtype configuration.

My questions: (1) Is this already available? I searched, but did not find. (2) If not already available, is it worth the effort to do this, or is there some "gotcha" that I do not understand? If it is worth the effort, I will do it myself.

EDIT: Here is a MWE for visualization only. Normally, there would be no black bars, and I would also compensate for the bearings of O.

\documentclass{article} % Compile with lualatex.
\RequirePackage{fontspec}
\setmainfont{Libre Caslon Text} % Already installed.
\begin{document}
% The rules are for visualization.
\noindent\rule{2pt}{10pt}Old Nick was a\\
jolly elf.\par
\noindent\rule{2pt}{10pt}Old Nick was a\\
\strut\hspace{.143em}jolly elf.\par % The .143 is from fontforge.
\end{document}

And here is the result. Note that the first j protrudes to the left of the reference bars. The second j does not. I am trying to do this without such an hspace trick.

EDIT2: Apparently, my request is possible, without needing to write a *.cfg file. Using the sidebearings method from

Accessing side-bearings in LuaTeX

I can do it while the document compiles. After loading microtype, the \SetProtrusion command works in Preamble. It can be used to re-set whatever was in the *cfg file.

Answer 1

Instead of going through microtype it's nicer for this use-case to set the protrusion directly during font loading in a feature. There the protrusion for each character is set and the bounding box (and therefore the side-bearing) are directly available, so there is no need to write any files:

\documentclass{article}
\directlua{
  local function set_protrusion(tfmdata, _, value)
    % The bounding box is available in a dlightly hidden descriptions table 
    local descriptions = tfmdata.shared.rawdata.descriptions
    for i, char in next, tfmdata.characters do
      local bbox = descriptions[i].boundingbox
      % Determine the side bearings from the bounding box and width
      local left_bearing = bbox[1]
      local right_bearing = descriptions[i].width - bbox[3]
      % For negative bearings set protrusion parameters:
      if left_bearing < 0 then
        char.left_protruding = left_bearing
      end
      if right_bearing < 0 then
        char.right_protruding = right_bearing
      end
    end
  end
  % Setup this functionality to be controlled by the `bearing_prot` feature:
  fonts.constructors.features.otf.register{
    name = 'bearing_prot',
    description = 'Use protrusion to drop negative side-bearings',
    manipulators = {
      node = set_protrusion,
    },
  }
}
% And then load a font with this feature.
\usepackage{fontspec}
\setmainfont[RawFeature=bearing_prot]{Libre Caslon Text}
\begin{document}
% We have to enable protrusion since we don't use microtype:
\protrudechars=2
\noindent Old nick was a\par
% Print a rule to see the boundary of the text block
\hrule depth0pt height1pt
\noindent jolly elf.
\end{document}