files for http://tex.stackexchange.com/q/45549/14066

sidebearings.lua

-----------------------------------------------------------------------
--         FILE:  sidebearings.lua
--        USAGE:  ./sidebearings.lua 
--  DESCRIPTION:  access sidebearings with Luatex
-- REQUIREMENTS:  luaotfload version >= 2.2
--       AUTHOR:  Philipp Gesang (Phg), <phg42.2a@gmail.com>
--      VERSION:  1.0
--      CREATED:  2013-08-27 15:27:20+0200
-----------------------------------------------------------------------
--

packagedata                  = packagedata or { }
packagedata.sidebearings     = { }
local sidebearings           = packagedata.sidebearings

local utfbyte                = utf.byte
local texsprint              = tex.sprint

local get_sidebearings = function (id, char)
  local tfmdata = font.getfont (id)

  if not (tfmdata and tfmdata.shared) then
    return 0, 0
  end

  local descriptions = tfmdata.shared.rawdata.descriptions
  local glyphdata    = descriptions [char]
  if not glyphdata then
    --- font lacks the glyph
    return 0, 0
  end

  local boundingbox   = glyphdata.boundingbox
  local lside         = boundingbox [1] or 0
  local wd            = boundingbox [3] or glyphdata.width
  local rside         = glyphdata.width - wd

  inspect (glyphdata)

  return lside / 100, rside /100
end

local sidebearings = function (id, char, left)
  char = utfbyte (char)
  local lside, rside = get_sidebearings (id, char)
  if left then
    texsprint (tostring (lside), "pt")
  else
    texsprint (tostring (rside), "pt")
  end
end

packagedata.sidebearings.left  = function (char)
  return sidebearings (font.current (), char, true)
end

packagedata.sidebearings.right = function (char)
  return sidebearings (font.current (), char, false)
end

sidebearings.tex

\ifdefined \directlua
  \input luaotfload.sty
  \directlua {require "sidebearings"}

  \def \lsidebearing #1{%
    \directlua {packagedata.sidebearings.left [[#1]]}%
  }

  \def \rsidebearing #1{%
    \directlua {packagedata.sidebearings.right [[#1]]}%
  }

  \font \mainfont = "file:Iwona-Regular.otf"

\else
  \def \lsidebearing #1{\the \XeTeXglyphbounds1 \the \XeTeXcharglyph`#1}
  \def \rsidebearing #1{\the \XeTeXglyphbounds3 \the \XeTeXcharglyph`#1}
  \font \mainfont = "[Iwona-Regular.otf]"
\fi

\mainfont

\def \test #1{[#1] left: \lsidebearing {#1}, right: \rsidebearing {#1}\par}

\test a
\test b
\test x
\test y
\test а
\test б
\test ю
\test я

\bye