How to manipulate selected letters in LuaTeX?

Question

Please look at the following MWE:

\documentclass[20pt]{extarticle}
\usepackage{unicode-math}
\usepackage{luacode}
%% Works: change width of 
\begin{luacode}
local function newwidth(tfmdata)
tfmdata.shared.rawdata.descriptions[119891].width=3000
end
fonts.constructors.features.otf.register{
    name = 'width',
    description = 'Overwrite width values',
    initializers = {
        base = newwidth,
    },
}
\end{luacode}
%% Does not work: math accent horizontal position of 
\begin{luacode}
    local function newacc(tfmdata)
    tfmdata.shared.rawdata.descriptions[119891].math.accent=900
    end
    fonts.constructors.features.otf.register{
        name = 'acc',
        description = 'Overwrite accent values',
        initializers = {
            base = newacc,
        },
    }
\end{luacode}
%% Does not work: change bounding box of 
\begin{luacode}
    local function newbbox(tfmdata)
    tfmdata.shared.rawdata.descriptions[119891].boundingbox={ -400, 0, 600, 0 }
    end
    fonts.constructors.features.otf.register{
        name = 'bbox',
        description = 'Overwrite boundingbox values',
        initializers = {
            base = newbbox,
        },
    }
\end{luacode}
\setmathfont{Latin Modern Math}[math-style=literal, RawFeature={+width;+bbox;+acc}]
\begin{document}
$$ \hat   $$% Sorry for $$, I usually use Plain TeX.
\end{document}

I changed succesfully width of (unicode 119891, mathematical italic small f), but I cannot change its sidebearings (bounding box) and its horizontal math accent position.

How to do that properly?

Edit: Somewhere I saw tfmdata.shared.rawdata.descriptions and somewhere tfmdata.characters. Could someome explain the difference please? When to use the former, when the latter?

Answer 1

I guess we should start at the end:

Somewhere I saw tfmdata.shared.rawdata.descriptions and somewhere tfmdata.characters. Could someome explain the difference please? When to use the former, when the latter?

Never change anything in tfmdata.shared.rawdata. As the name indicates it's raw data not affected by any selected features and is shared by different TeX level fonts. So if you change it in your custom feature and then load the font once with and once without your feature your change might affect both. Or not. Depending on the order, implementation defined internals about when stuff gets shared or not, the version of all affected components and the phase of the moon. (Ok, the last one is unlikely but I can't guarantee that it can't happen...) Also relying on the rawdata misses anything changed by other features.

So instead things should be changed in tfmdata.characters. Now if you set simething there in initializers it doesn't have much effect since it gets overwritten by the rawdata. The initializers are more for initialization, argument parsing and setting up dynamic processors, actual character manipulation is done in manipulators. Attention: manipulators run, in contrast to initializers, after scaling the font. So you have to scale your changes too.

Therefore changing the width would be better done with

local function newwidth(tfmdata)
  tfmdata.characters[119891].width=3000 * tfmdata.parameters.factor
end
fonts.constructors.features.otf.register{
    name = 'width',
    description = 'Overwrite width values',
    manipulators = {
        base = newwidth,
    },
}

This way you can also change the accent position:

    local function newacc(tfmdata)
      tfmdata.characters[119891].top_accent = 900 * tfmdata.parameters.factor
    end
    fonts.constructors.features.otf.register{
        name = 'acc',
        description = 'Overwrite accent values',
        manipulators = {
            base = newacc,
        },
    }

Overwriting the bounding box on the other hand doesn't really make much sense because the bounding box is a physical property of how the glyph looks. It's informational and isn't used anywhere except to determine the initial height/depth/italic which happens before the initializers, so at the point your code runs it isn't accessed anymore. What are you actually trying to archive by changing it?

If you want to move the glyph horizontally relative to the origin, you can use virtual font commands to apply an offset. This effectively changes the sidebearings:

  local function moveright(tfmdata)
    tfmdata.characters[119891].commands={
      {'right', 900 * tfmdata.parameters.factor},
      {'char', 119891}
    }
  end
  fonts.constructors.features.otf.register{
      name = 'moveright',
      manipulators = {
        base = moveright,
      },
  }

If you want to move the glyph to the left, you still use the 'right' command but just use a negative amount. There is no 'left' command. You can also use 'down' for vertical adjustments.