In the book The Complete Manual of Typography by James Felici, it is recommended to sometimes alter the spaces between letters in order not to stretch the inter-word spaces too dramatically.

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I have tried using Tracking from microtype package, but I can only set it to the constant value. There is also LetterSpace option in fontspec, but it seems to do pretty the same.

Can this be done in LuaLaTeX?




        expansion   =   false,
        tracking    =   alltext,

    family  =   *}{10} 



  • A related link from the typography perspective: it's worth knowing that typographers' opinions on such letter spacing (and font expansion: basically all the features provided by microtype!) differ. – ShreevatsaR Nov 14 '17 at 15:31
  • @ShreevatsaR AFAIK there is a big controversy around the expansion, but i find alterning the spaces OK. Is this very feature provided by microtype? – Michael Fraiman Nov 14 '17 at 15:41

Although typographers differ in their opinion on letter spacing, when setting justified text in narrow columns we have to settle for some compromise somewhere (which includes considering ragged-right text), and letter spacing seems acceptable. So I am shocked that the microtype package does not seem to provide this feature; after all it is extensively discussed in Hàn Thế Thành's thesis Micro-typographic extensions to the TeX typesetting system. I am still not sure whether I simply missed something in the microtype documentation, though I looked through it several times for this feature.

Anyway, this is easy to do with LuaTeX. There are multiple approaches: for example, after line-breaking (in post_linebreak_filter) we could add the appropriate amount of stretch between nodes (this is the approach discussed in Hàn Thế Thành's thesis, section 6.8). Or else, before line-breaking (in pre_linebreak_filter) we could add stretchable glue nodes. The second approach is implemented in the chickenize package, as \letterspaceadjust. It's just a handful of lines of code, so we can copy it and make changes if we desire (in the code sample below I change only letterspace_glue.stretch from 0.5 pt to 10pt).




The results of the first round of\\
the elections shook the public\\
rigid, as it seemed impossible\\
that a candidate from the far\\
right wing---who indeed had\\
been a Nazi Party member dur-\\
ing his youth---ousted the cen-\\
ter-left candidate. The outcry\\
from the general public was\\
considerable but also considera-\\
bly too late. The run-up to the\\
election had been somnolent---and\\
people learned too late that the\\
unimaginable only seems that\\
way when people fail to use\\
their imagination.}

\noindent The default:



\noindent Best result without letter spacing:
\tolerance=9999 \hfuzz=0.1pt \emergencystretch=\hsize



\noindent With letter spacing:



\noindent Exaggerated letter spacing:
local nodeid   = node.id
local nodenew  = node.new
local nodecopy = node.copy
local nodetraverseid = node.traverse_id
local nodeinsertbefore = node.insert_before

local letterspace_glue   = nodenew(nodeid"glue")
local letterspace_pen    = nodenew(nodeid"penalty")
letterspace_glue.width   = tex.sp"0pt"
letterspace_glue.stretch = tex.sp"10pt"
letterspace_pen.penalty  = 10000
letterspaceadjust = function(head)
  for glyph in nodetraverseid(nodeid"glyph", head) do
    if glyph.prev and (glyph.prev.id == nodeid"glyph" or glyph.prev.id == nodeid"disc" or glyph.prev.id == nodeid"kern") then
      local g = nodecopy(letterspace_glue)
      nodeinsertbefore(head, glyph, g)
      nodeinsertbefore(head, g, nodecopy(letterspace_pen))
  return head




Just for clarity, here is the relevant code again, without loading it from a package. Put the following in a file called letterspacing.lua:

local interletter_glue   = node.new('glue')
interletter_glue.width   = tex.sp(0)
interletter_glue.stretch = tex.sp('0.5 pt') -- This number controls how much the space between letters can stretch.
local interletter_pen    = node.new('penalty')
interletter_pen.penalty  = 10000

add_interletter_glue = function(head)
   -- Adds equivalent of "\penalty10000\hskip 0pt plus 0.5pt" (the above penalty and glue)
   -- before every letter (glyph) that follows a glyph, discretionary (hyphen), or kern node.
   for glyph in node.traverse_id(node.id('glyph'), head) do
      if glyph.prev and (glyph.prev.id == node.id('glyph') or
                         glyph.prev.id == node.id('disc') or
                         glyph.prev.id == node.id('kern')) then
         local g = node.copy(interletter_glue)
         node.insert_before(head, glyph, g)
         node.insert_before(head, g, node.copy(interletter_pen))
   return head
luatexbase.add_to_callback("pre_linebreak_filter", add_interletter_glue, "Allow variable interletter spacing.")

Now you can typeset the following file with lualatex:

\usepackage{lipsum}                                            % For random text
\hsize=10em \frenchspacing              % Just to make the effects easier to see
\tolerance=9999 \emergencystretch=\maxdimen  % Work harder, avoid overfull boxes

\lipsum                                 % This is typeset without letter-spacing

\directlua{dofile('letterspacing.lua')}                 % Turn on letter-spacing

\lipsum                                    % This is typeset with letter-spacing

You can control the amount of allowed stretching by changing the value of interletter_glue.stretch (third line of letterspacing.lua) (though note that by setting \emergencystretch we're explicitly telling TeX to exceed that limit if necessary… however, you usually won't need \emergencystretch with letter-spacing, in many cases where you would have needed it otherwise).

  • I also haven't seen this feature in microtype, which is strange. Where exactly is the tracking controlled in your code? – Michael Fraiman Nov 14 '17 at 20:00
  • @MichaelFraiman Search for letterspace_glue.stretch in the code… I added another version to try to make this clearer (in which search for interletter_glue.stretch). – ShreevatsaR Nov 14 '17 at 20:56
  • microtype doesn't provide this feature because pdftex doesn't: paragraph composition with variable letterspacing was possible in early versions, but this was later dropped, leaving font expansion as the only option to improve justification. – Robert Nov 15 '17 at 4:20
  • @Robert Thank you so much! That explains it… makes sense now. – ShreevatsaR Nov 15 '17 at 4:32
  • Just to note that the output would be improved if the glue were also allowed to shrink. – Robert Nov 17 '17 at 3:33

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