Achieving better kerning in text mode italics

Question

In the font I'm using to set italics, the apostrophe / right single quote is outputting poorly kerned:

The kerning of the first apostrophe in 'n' and the apostrophes in they've and they're is acceptable. But elsewhere it is too close to the following character and too far from the preceding one—consider it's, isn't, the second apostrophe in 'n' and both the right single quotes.

The kerning in the roman font is fine. So, using LuaLaTeX, what is the best way to adjust the kerning for the apostrophes only for the italic font?

It may be the case that the kerning of the font is for whatever reason not being applied. For in the MWE given below, if times.ttf is changed to, say, Helvetica, and timesi.ttf is left the same, the following (much better) output is obtained:

Additionally, if I leave the font files the same but add the following code, adapted from this answer, to the preamble of the MWE:

\usepackage{luacode}

\begin{luacode*}
local function fix_italic_kern(fontdata)
  if fontdata then
    local chars = fontdata.characters
    if chars then
      local ch = chars[39]  -- apostrophe
      if ch then
        if not ch.kerns then
          ch.kerns = { }
        end
        ch.kerns[115] = 1  -- lowercase s
        ch.kerns[116] = 1  -- lowercase t
      end
    end
  end
end
luatexbase.add_to_callback("luaotfload.patch_font",
  fix_italic_kern, "fix_italic_kern")
\end{luacode*}

I obtain nicer output for it's and isn't (though not elsewhere), despite the kern only being 1.

Here Khaled Hosny suggests to use RawFeature={+itlc}, but this has no effect.

What accounts for the inconsistency in how the italics are kerned, and how can I select the kerning that results in better output while having times.ttf as my main font?

Here is the MWE:

\documentclass{minimal}

\usepackage{fontspec}
\setmainfont{times.ttf}[
  ItalicFont = timesi.ttf ,
  Ligatures  = Discretionary ,
]

\begin{document}

`It's time to rock 'n' roll,' he said. \par
`They've said they're coming for us. Isn't that right?' \par
\textit{`It's time to rock 'n' roll,' he said.} \par
\textit{`They've said they're coming for us. Isn't that right?'}

\end{document}

I'm using MacTeX2019 and macOS 10.14.5.

Answer 1

@egreg has shown that the font is dodgy. The version of Times New Roman on my machine is the ancient one that Microsoft released for web compatibility back last Century! And it certainly is hideous. The same bad kerning also occurs for me in Libre Office.

You can manually adjust the kerning for each character pair using some lua code. This is just set by eye, but gives nicer results. I don't think it really looks better with Helvetica as the main font… (I agree it's slightly different in your screenshots, but I can't reproduce this with the fonts on my Linux system.)

MWE

(Update with a loop to set all kerning pairs for Latin 1. I don't know if this is the best way of doing this. However, to my mind you need to tune them all so that they are slightly different, rather than just setting every kern to the same.)

\documentclass{article}
\usepackage{fontspec}
\directlua {
  kerndata = {}
  kerndata["’"] = {}
  for i = 33, 126 do
    kerndata[i] = { ["’"] = -180 } 
    kerndata["’"][i] = 180
  end
  kerndata[","] = { ["’"] = -360 }
  kerndata["."] = { ["’"] = -360 }
  fonts.handlers.otf.addfeature {
    name = "aposkern",
    type = "kern",
    data = kerndata,
  }
}
\setmainfont{times.ttf}[
  ItalicFont = timesi.ttf ,
  ItalicFeatures = {RawFeature=+aposkern},
  Ligatures  = Discretionary ,
]
\begin{document}
`It's time to rock 'n' roll,' he said. \par
`They've said they're coming for us. Isn't that right?' \par
\textit{`It's time to rock 'n' roll,' he said.} \par
\textit{`They've said they're coming for us. Isn't that right?'}
\end{document}

Answer 2

The problem seems to be that the bounding box of the font is incorrect, so the best fix might be to "move" the characters in their bounding box. This can be archived by adding "single" features:

For example, to move the apostrophe left by 180 units, you would use

\documentclass{minimal}
\usepackage{fontspec}
\directlua{
  fonts.handlers.otf.addfeature("fix_times", {
    name = "fix_times",
    type = "single",
    data = {
      [8217] = {-160, 0, 0, 0},
    },
  })
}
\setmainfont{times.ttf}[
  ItalicFeatures = {RawFeature = fix_times,},
  ItalicFont = timesi.ttf,
  Ligatures = Discretionary,
]

\begin{document}
`It's time to rock 'n' roll,` he said.\par
`They've said they're coming for us. Isn't that right?' \par
\itshape `It's time to rock 'n' roll,` he said.\par
`They've said they're coming for us. Isn't that right?' \par
\end{document}

Here {-180, 0, 0, 0} means "move 180 units right, do not move up or down, do not change the width or height.

In contrast to "kern"ing, this does not depend on the next character, so you do not need a loop and this also works if the apostrophe comes next to a non-glyph etc.

Of course, you can still add specific kerning pairs using an additional feature of type "kern":

\documentclass{minimal}
\usepackage{fontspec}
\directlua{
  fonts.handlers.otf.addfeature{
    name = "fix_times",
    type = "single",
    data = {
      ['’'] = {-140, 0, 0, 0},
    },
  }
  fonts.handlers.otf.addfeature{
    name = "aposkern",
    type = "kern",
    data = {
     ['.'] = {['’'] = -180},
     [','] = {['’'] = -180},
    },
  }
}
\setmainfont{times.ttf}[
  ItalicFeatures = {RawFeature = fix_times;aposkern},
  ItalicFont = timesi.ttf,
  Ligatures = Discretionary,
]

\begin{document}
`It's time to rock 'n' roll,' he said.\par
`They've said they're coming for us. Isn't that right?' \par
\itshape `It's time to rock 'n' roll,' he said.\par
`They've said they're coming for us. Isn't that right?' \par
\end{document}