[luatex] How to access user-defined kerning

[Hans Hagen]

On 4/20/2018 5:52 AM, Henri Menke wrote:
> Dear list,
> 
> I'm trying to determine the kerning between two letters (here A and V).  For the
> sake of this example I'm inserting negative kerning of -400/1000 em.  However,
> when I examine the kern in the kerning callback this massive enhancement of the
> negative kern doesn't show up but I only see the kerning from the font itself.
> 
> How can I access the user-defined kerns?  MWE is below.

no real answer but there is nreal need to go nuts in your code

local function show_kerns(head)
     local glyph = node.id("glyph")
     for item in node.traverse(head) do
         if item.id == glyph then
             local char = item.char
             if char == 65 then
                 local font = item.font
                 print(font,char)
                 local next     = item.next
                 if next.id == glyph then
                   local nextchar = next.char
                   local nextfont = next.font
                   local data = fonts.hashes.identifiers[font]
                   local kern = 
fonts.handlers.otf.getkern(data,char,nextchar)
                   print(kern)
                 end
             end
         end
     end
end

that said, what you could do is the loop over the to be packed list (so 
later) and then look for a left math node and then start looking at the 
characters and kerns


function foo(head)
     local glyph = node.id("glyph")
     local math  = node.id("math")
     local current = head
     while current do
         if current.id == math then
             current = current.next
             while current do
                 if current.id == math then
                     break
                 elseif current.id == glyph then
                     local char = current.char
                     if char == 0x1D434 then
                         local font = current.font
                         local next = current.next
                         if next.id == glyph then
                             local nextchar = next.char
                             if char == 0x1D449 then
                                 local nextfont = next.font
                                 local data = fonts.hashes.identifiers[font]
                                 local kern = 
fonts.handlers.otf.getkern(data,char,nextchar)
                                 print(char,nextchar,kern)
                             end
                         end
                         current = next
                     else
                         current = current.next
                     end
                 else
                     current = current.next
                 end
             end
         end
         current = current.next
     end
end

depending on the macro package that you need to look at math unicodes 
and even then you might not see a print because there is already a kern

keep in mind that this kind of hackery is macro package dependent so 
your plain test might work out different elsewhere (i haven't tested it 
as i operate in a different environment)


> Cheers, Henri
> 
> ---
> 
> \input luaotfload.sty
> 
> \directlua{
> local function show_kerns(head)
>     for item in node.traverse(head) do
>        if item.id == node.id("glyph") and item.char == 65 then
>           % Get current char and font
>           local nut = node.direct.todirect(item)
>           local char = node.direct.getchar(nut)
>           local font = node.direct.getfont(nut)
>           print(font)
>           % Get next char and font
>           local next = node.next(item)
>           local nextnut = node.direct.todirect(next)
>           local nextchar = node.direct.getchar(nextnut)
>           local nextfont = node.direct.getfont(nextnut)
>           % Determine the kern
>           local data = fonts.hashes.identifiers[font]
>           local kern = fonts.handlers.otf.getkern(data,char,nextchar)
>           print(kern)
>        end
>     end
> end
> %
> luatexbase.add_to_callback("kerning",
>                             function(head)
>                                show_kerns(head)
>                                node.kerning(head)
>                             end,
>                             "show_kerns")
> }
> 
> \directlua{
> fonts.handlers.otf.addfeature{
>     name = "ktest",
>     type = "kern",
>     data = { ["A"] = { ["V"] =  -400 } } % make them overlap for the example
> }
> }
> 
> \font\test="Latin Modern Roman:+ktest"\test
> AV
> 
> \bye
> 


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