Re: [NTG-context] MkIV italic correction?

27 May 2009

      Dohyun Kim wrote:
...
Considering current state that we don't know any fonts that has ITLC table,
it would be better than nothing to implement italic correction as follows.
In the following code, "fontdata" is a table returned by the function
"fonts.define.read".
local param = fontdata.parameters
        local italicangle = fontdata.shared.otfdata.metadata.italicangle
        if italicangle and italicangle < 0 then
            local uwidth = fontdata.shared.otfdata.metadata.uwidth or 40
            local factor = fontdata.factor or 655.36
            param.slant = - math.tan(italicangle*math.pi/180) * param.quad
            for i,v in pairs(fontdata.characters) do
                local gl = fontdata.descriptions[i]
                local it = (gl.boundingbox[3] - gl.width + uwidth*0.5) * factor
                if it > 0 then v.italic = it end
            end
        end
there are seleveral solutions:

- extend the font with this info (faster but then it's always there 
which might not be ok as it's an approximation)

- calculate it after loading (which is what you propose)

in the mkiv code we do have a hook for that kind of things so this is 
then what i propose. watch how we don't scale here, we just add an entry 
to the shared data as that's where we hook in; the real implementation 
would look slightly different as an optimization is possible

\starttext

\startluacode
table.insert(fonts.triggers,"itlc")

local function itlc(tfmdata,value)
     if value then
         -- the magic 40 and it formula come from Dohyun Kim
         local fontdata = tfmdata.shared.otfdata or tfmdata.shared.afmdata
         local metadata = fontdata and fontdata.metadata
         if metadata then
             local italicangle = metadata.italicangle
             if italicangle and italicangle ~= 0 then
                 local uwidth = (metadata.uwidth or 40)/2
                 for unicode, d in next, tfmdata.descriptions do
                     local it = d.boundingbox[3] - d.width + uwidth
                     if it ~= 0 then
                         d.italic = it
                     end
                 end
             end
         end
     end
end

fonts.initializers.base.otf.itlc = itlc
fonts.initializers.node.otf.itlc = itlc

fonts.initializers.base.afm.itlc = itlc
fonts.initializers.node.afm.itlc = itlc
\stopluacode

\definedfont[SerifItalic*default at 24pt] test\/test

\definefontfeature[xdefault][default][itlc=yes]

\definedfont[SerifItalic*xdefault at 24pt] test\/test

\stoptext

i could add it to the generic code (although i'm not going to add all 
the other context goodies to the generic code definitely not as long as 
they're experimental)

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