Re: [NTG-context] issue with optical bounds (OpenType)

9 Dec 2018

      On 12/9/2018 1:34 PM, Pablo Rodriguez wrote:
...
On 12/7/18 11:25 PM, Hans Hagen wrote:
...
On 12/7/2018 9:14 PM, Pablo Rodriguez wrote:
...
[...]
The issue is that, although opbd values are used, that results from opbd
and from single placement in addfeature are different (although LuaTeX
reports the same values for lfbd).
because you implement it as kerning and not as opbd (protrusion is not
kerning) ... you can try to change the name to "lfbd" (if you would
enable lfbd as feature for sure you would get funny kerning so opbd
picks up from those instead)
If protrusion has to apply other values than the ones besides opbd, the
values for optical bounds aren’t reliable for accurate glyph
positioning. Sorry, but otherwise there is no need for optical margins
in OpenType (standard protrusion would be all that is needed).
Kerning might be selectively applied, such as in:
\startluacode
     fonts.handlers.otf.addfeature {
         name = "leftbounds",
         type = "single",
         data = {
             Ἠ = { -212, 0, -212, 0 },
             Ὧ = { -196, 0, -196, 0 },
         }
     }
     \stopluacode
     \definefontfeature[default][default][protrusion=quality]
     \definefontfeature[leftbounds][leftbounds=yes]
     \setupfirstline
        [alternative=word, n=1, style={\feature[+][leftbounds]}]
     \showframe
     \setuplayout[page]
     \definefontfamily[mainface][rm][Typographica Old Standard]
     \definefontfamily[mainface][tt][Latin Modern Mono]
     \setupbodyfont[mainface, 300pt]
     \setupalign[hanging]
     \startbuffer
         «a\par
         «b\par
         Ω\par
         Ὧ\par
         Ἠ\par
     \stopbuffer
     \starttext
         \setupparagraphintro[each][\setfirstline]
         \getbuffer
     \stoptext
At least to get Greek hanging diacritics, I think this is the way to go.
It would be extremely useful to enable a \setuplineintro for verse.
It would be also useful to have a function that substracts the width of
two given glyphs. (This is exactly the amount to get hanging diacritics
properly kerned.)
you confuse mechanism:
- protrusion only happen at the start and end of the line
- kerning (like single does) happens everywhere

so, you need to extend "lfbd" and not use some own kerning feature 
"leftbounds" which applyu it everywhere while "lfbd" is treated special 
i.e. sets the protrusion values (when "opbd" is enabled) ... no other 
way to do this

Hans

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