[NTG-context] modifying kerning breaks opentype ligatures (and other features)

mf massifr at fastwebnet.it
Mon Dec 10 16:26:53 CET 2018


Hello list,
i noticed that setting a different kerning breaks some features of 
opentype fonts; that is true, for example, for ligatures and fractions 
(frac feature).

It looks like a modified kerning inserts something between two adjacent 
character, making them no more adjacent.
That way the lookup of opentype tables fails.

"f", "l"            => "fl" ligature glyph

"f", [kerning], "l" => distinct "f" and "l" glyphs

"1", "/", "4" => "ΒΌ" glyph (with "frac" feature switched on)

"1", [kerning], "/", [kerning], "4" => distinct "1", "/", "4" glyphs 
(always with "frac" feature switched on)

MWE:

\definecharacterkerning[narrow][factor=-.015]
\definecharacterkerning[wide][factor=.015]

\definefontfeature[frac][frac=yes]

\starttext
\feature[+][frac]%
Some ligatures: float, finance, affine; a fraction: 1/4.

\setcharacterkerning[narrow]Some ligatures: float, finance, affine; a 
fraction: 1/4.

\setcharacterkerning[wide]Some ligatures: float, finance, affine; a 
fraction: 1/4.

\stoptext

In my XML to PDF workflow, fractions are marked up, so it's easy to 
switch the kerning off only for them.

Instead i have no solution for ligatures: parts of text with a modified 
kerning loose ligatures, and i end up with the same words, once with 
ligatures, once without, even in the same page.
Is there a workaround for this?

Massimiliano




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