[Dev-luatex] Wrong spacing for OpenType math subscripts

15 Mar 2021

      Hi LuaTeX developers,

first of all, thank you very much for all your work on this and the 
great OpenType support and possibility of fixing internals using Lua code!

In the following document, the math subscripts are so close to the 
following characters that they sometimes overlap:

    \documentclass{article}
    \usepackage{unicode-math}

    \def\supp{{\rm supp}}

    \begin{document}

    $$
    I(P_X,P_{Y|X}) = \sum_{(X,Y) \in \supp P_{XY}}
      P_{XY}(X,Y) \log_2 \frac{P_{XY}(X,Y)}{P_X(X) P_Y(Y)}.
    $$

    \end{document}

The problem cannot be fixed by simply adding spacing, e.g. \Uordopenspacing,
because the space is different per character. We need italics correction,
in all circumstances where an italic symbol is last in the subscript and 
something follows the noad. By the manual, Section 7.6.4 this is not 
applied to subscripts, but only the base characters.

Setting \mathitalicsmode=1 to force it changes nothing, because italics 
correction is then only applied in the subscript to the characters 
before the last one. Maybe luatex assumes the last character to be 
followed by nothing, because in the mlist of the subscript follows 
nothing even if after the whole noad follows something.

Setting \mathitalicsmode=1 and adding manual 0-kerns (like \/) helps. 
This is implemented by the following lua code:

  \directlua{
    ty = node.types()

    sub_addkern = function(sub)
      scan_sub_addkern(sub)

      local kern = node.new("kern")

      if ty[sub.id] == "math_char" or ty[sub.id] == "math_char_text" then
        local old = node.new("noad", 0)
        old.nucleus = sub

        sub = node.new("sub_mlist")
        sub.head = old
        node.insert_after(sub.head, old, kern)
      elseif ty[sub.id] == "sub_box" or ty[sub.id] == "sub_mlist" then
        local tail = node.tail(sub.head)
        if not (ty[tail.id] == "kern") then
          node.insert_after(sub.head, tail, kern)
        end
      end
      return sub
    end

    scan_sub_addkern = function(n)
      if n.head then
        n.head = scan_all_sub_addkern(n.head)
      end
      if n.sub then
        n.sub = sub_addkern(n.sub)
      end
      if n.nucleus then
        scan_sub_addkern(n.nucleus)
      end
      if n.num then
        scan_sub_addkern(n.num)
      end
      if n.denom then
        scan_sub_addkern(n.denom)
      end
    end

    scan_all_sub_addkern = function(head)
      for n in node.traverse(head) do
        scan_sub_addkern(n)
      end
      return head
    end

    luatexbase.add_to_callback("pre_mlist_to_hlist_filter", scan_all_sub_addkern,
      "default mlist_to_hlist + italics-corrected subscript")
  }

XeTeX does this right out of the box.

Best,

Johannes

[Dev-luatex] Wrong spacing for OpenType math subscripts

Johannes Rosenberger