Pandoc filter - math to png

math2png.lua

--[[ pandoc filter to convert inline math to png images

Sses dvipng to convert inline and display math to png. There are better ways to handle math output,
but some platforms (e.g. Confluence) can only handle images.

Inspirations:

- https://github.com/pandoc/lua-filters/tree/master/math2svg
- The "Building images with tizk" example: https://pandoc.org/lua-filters.html#module-pandoc.template
- Sphinx imgmath from https://github.com/sphinx-doc/sphinx/blob/aee3c0ab75974790adf359a9c5089d1d781a6b21/sphinx/ext/imgmath.py#L1
--]]

local system = require 'pandoc.system'
local path = require 'pandoc.path'

local function latex(texfile)
    -- Arguments from
    local args = { "latex", "-interaction=nonstopmode", texfile }
    os.execute(table.concat(args, " "))
end

local function dvi2png(dvifile, pngfile)
    local args = { "dvipng", "-gamma", "1.5", "-D", "110", "-bg", "Transparent", "-T", "tight", "-z9", "-o",
        pngfile, dvifile }
    os.execute(table.concat(args, " "))
end

local function wrapmath(content, displaymath)
    content = string.gsub(content, "^\n*(.-)\n*$", "%1") or ""
    if displaymath then
        return string.format("\\begin{displaymath}%s\\end{displaymath}", content)
    else
        return string.format("\\begin{math}%s\\end{math}", content)
    end
end

-- values from https://github.com/sphinx-doc/sphinx/blob/master/sphinx/templates/imgmath/template.tex.jinja
local function math_doc_template(content, displaymath, fontsize, baselineskip)
    local template = [[
    \documentclass[12pt]{article}
    \usepackage[utf8]{inputenc}
    \usepackage{amsmath}
    \usepackage{amsthm}
    \usepackage{amssymb}
    \usepackage{amsfonts}
    \usepackage{anyfontsize}
    \usepackage{bm}
    \pagestyle{empty}

    \renewcommand{\vec}[1]{\mathbf{#1}}
    \newcommand{\Normal}{\mathsf{Normal}}
    \newcommand{\Beta}{\mathsf{Normal}}
    \newcommand{\Binomial}{\mathsf{Binomial}}
    \newcommand{\E}{\mathbb{E}}
    \newcommand{\SE}{\mathrm{SE}}
    \newcommand{\SEhat}{\widehat{\mathrm{SE}}}

    \begin{document}
    \fontsize{%d}{%.1f}\selectfont
    %s
    \end{document}
    ]]
    content = wrapmath(content, displaymath)
    fontsize = fontsize or 12                     -- Replace with actual font size
    baselineskip = baselineskip or fontsize * 1.2 -- Replace with actual baseline skip

    local tex = template:format(fontsize, baselineskip, content)
    return tex
end

local function tex2png(src, outfile)
    system.with_temporary_directory("math2png", function(tmpdir)
        system.with_working_directory(tmpdir, function()
            local texfile = "math.tex"
            local dvifile = "math.dvi"
            local pngfile = "math.png"
            local f = io.open(texfile, "w")
            f:write(src)
            f:close()
            latex(texfile)
            dvi2png(dvifile, pngfile)
            os.rename(pngfile, outfile)
        end)
    end)
end

local function file_exists(name)
    local f = io.open(name, 'r')
    if f ~= nil then
        io.close(f)
        return true
    else
        return false
    end
end

function Math(elem)
    -- full latex document from template
    local displaymath = elem.mathtype == "DisplayMath"
    local doc = math_doc_template(elem.text, displaymath)
    -- print(doc)
    -- sha of the doc. using the doc accounts for preamble changes
    -- if the preamble changes, the sha will change and the image will be regenerated
    local doc_sha = pandoc.utils.sha1(doc)
    local filetype = "png"
    local fbasename = doc_sha .. '.' .. filetype
    local dst_dir = path.join({ system.get_working_directory(), "math_files" })
    if not file_exists(dst_dir) then
        system.make_directory(dst_dir, true)
    end
    local fname = path.join({ dst_dir, fbasename })
    if not file_exists(fname) then
        tex2png(doc, fname)
    end
    local cls
    if displaymath then
        cls = "math display"
    else
        cls = "math inline"
    end
    local img = pandoc.Span(pandoc.Image("", fname, "", { ["alt"] = elem.text }), { class = cls })    
    return img
end